Biology is the study of life - and life on Earth is extremely diverse, from microscopic single celled organisms, to enormous multicellular mammals. To make things a little easier life is 'classified' into five kingdoms and each kingdom can be defined by its own characteristic kind of cell.

So, what are cells?

"Cells are units of life - the smallest units that can be considered alive". 

 

All living organisms are made of cells: - 

 

When it comes to classifying cells, the biggest division is between the cells of the prokaryotic kingdom  (Bacteria) and those of the other four kingdoms (Animals, Plants, Fungi and Protoctista), which are all Eukaryotic cells.

All cells from each of these 5 kingdoms has its fundamental differences.

(Like whether it has a cell wall or not, if it has a cell wall what is it composed of?

Does the cell contain organelles?

if so which organelles? etc...). 

 

You will have to be able to compare and contrast cells, their structures functions and organelles.

Before moving onto the video lessons covering concepts such as the Eukaryotic cell cycle, mitosis, meiosis and what it means to be diploid... Let's run through the generalised structure of a Eukaryotic cell and the structure and function of organelles. 

 

Prokaryotic cells are smaller and simpler than eukaryotic cells, and you can find out more about their structure, function and division on the prokaryotic cells page.

What is Cytoplasm? 

(Cyto = cell; plasm = colourless fluid). Cytoplasm is the colourless solution within the cell (enclosed by the plasma membrane). Cytoplasm is the fluid which contains enzymes for important metabolic reactions to take place; for example important biochemical reactions such as glycolysis (part of cellular respiration) take place in they cytoplasm of cells. Also contained in the cytoplasm are sugars, salts, amino acids, nucleotides… just about everything needed for the cell to function. 

 

When discussing cytoplasm, you’ll most likely come across the term cytosol - which is sometimes used synonymously with cytoplasm, - however this is incorrect!

 

Cytoplasm and Cytosol are not the same!

Cytosol is essentially just the liquid part of the cytoplasm - i.e. cytosol is the intracellular fluid that contains organic molecules and cytoskeleton filaments, water and salts. It is probably easiest to think of cytosol as the fluid part of cells without the organelles. Whereas cytoplasm is the fluid part of cells that contains organelles and where many biochemical reactions take place.

 

What is a cell Nucleus?

The Nucleus (found in eukaryotic cells) is the largest organelle. The Nucleus is surrounded by a nuclear envelope - a double membrane which contains nuclear pores. Nuclear pores are ‘holes’ within the nuclear membrane. The nuclear pores contain proteins which control the exit of substances from the nucleus, e.g. mRNA and ribosomes.

 

Since the Nucleus is an organelle, with its own nuclear membrane separating it from the cytoplasm of the cell, the fluid part of the nucleus has its own name -it is called nucleoplasm. The nucleoplasm is full of substance called chromatin - a DNA / protein complex which contains the genes of the organism. During cell division chromatin becomes condensed - it becomes shorter and thickens into discrete observable chromosomes. 

 

Also identifiable within the nucleuses is darker region of chromatin - this is called the nucleolus and is involved in making ribosomes.

 

What are Ribosomes?

Your textbook answer to this should always be “Ribosomes are the sites of protein synthesis”. 

Ribosomes are the smallest and most numerous cellular organelles and are the sites of protein synthesis, and are often found in groups called polysomes. Ribosomes are composed of protein and rRNA (ribosomal RNA) and as previously discussed are manufactured in the nucleolus of the nucleus. 

 

Ribosomes can be found in the cell either ‘free floating’ in the cytoplasm, where they make proteins for the cell's own use. Or ribosomes can be found attached to rER (the Rough Endoplasmic reticulum - so called because of its association with ribosomes). When attached to the rER ribosomes make proteins which are exported from the cell. 

 

Ribosomes are composed of 2 subunits, a larger and smaller subunit. The larger subunit is further organised into sites (known as E, P and A) these sites are where tRNA (transfer RNA ‘docks’ when transferring and connecting amino acids when building polypeptides (during protein synthesis this part of the process is known as Translation). The smaller subunit is the binding site for mRNA (messenger RNA) which contains the codons that complementary base pair with the anticodons located on tRNA. Later in your A-Level biology you’ll have to apply the structure and function of ribosomes and their role in protein synthesis (translation).

 

All eukaryotic ribosomes are the larger, "80S", type, whereas prokaryotes, mitochondria and chloroplasts contain the smaller “70S” type.

 

What is Rough Endoplasmic Reticulum (rER)?

Rough Endoplasmic Reticulum is similar to sER (smooth endoplasmic reticulum), however in appearance rER looks ‘studded’ and ‘rough’ due its association with numerous ribosomes. It is the ribosomes that give rER its ‘rough’ appearance. The associated ribosomes are the sites of proteins synthesis and as the polypeptides are made, the proteins are processed in the rER (the proteins are enzymatically modified, adding carbohydrates to the polypeptide chain for example). Once modified the proteins can be “packaged” in the Golgi ready to be exported from the cell.

 

What is Smooth Endoplasmic Reticulum (sER)?

Smooth Endoplasmic Reticulum (sER) is a series of folded membrane channels which are involved in synthesising and transporting of materials, primarily lipids, needed by the cell.

 

What is the Golgi Apparatus (Golgi Body)?

Golgi is another series of flattened membrane vesicles, formed from the endoplasmic reticulum. However, the role of Golgi bodies is to transport proteins from the rER to the cell membrane for export. Parts of the rER containing proteins fuse with one side of the Golgi body (the part of the Glogi that is closely associated with rER is known as the “Cis face” of the Golgi), whilst at the other side (called the “Trans face”) small Golgi vesicles are released. The Golgi vesicles, which appear to “bud off” from the Golgi move towards the cell membrane. At the cell membrane these small Golgi vesicles fuse with the cell membrane and release their contents via exocytosis.

 

What are Lysosomes?

Lysosomes are small membrane-bound vesicles which are formed in the rER. Lysosomes contain digestive enzymes their function is to break down unwanted chemicals, toxins, organelles and even whole cells, so that the materials can be recycled by the cell. Lysosomes also fuse with vacuoles and digest the contents contained with them.

 

What are Vacuoles? 

Vacuoles are membrane-bound organelles which resemble little ‘sacs’ containing water, dilute solutions of salts and other solutes. Most cells can have small vacuoles that are formed as required, but plant cells typically have one large permanent vacuole that can fill most of the cell. These large permeant vacuoles found in plant cells are filled with cell sap (water, sugars, and mineral salts). They are very important in keeping the cell turgid. Some unicellular protoctists have feeding vacuoles for digesting food, or contractile vacuoles for expelling water.

 

What is the Cytoskeleton? 

A cells cytoskeleton is a network of protein fibres which extend throughout all eukaryotic cells. The Cytoskeleton is used for support, transport and motility. Attached to the cell membrane, the cytoskeleton provides the cell with structural integrity, helping maintain the overall shape and structure of the cell. Additionally, the cytoskeleton plays an important role holding organelles in position. The cytoskeleton is a ‘network’ of protein fibres, of which there are 3: -

1) Microfilaments,

2) Intermediate filaments and

3) Microtubules.

 

Each of these protein fibres has a corresponding motor protein that can move along the protein fibre carrying cargo such as organelles, chromosomes or other cytoskeleton fibres. 

 

It is these cytoskeletal motor proteins which are responsible for cellular actions such as: - 

 - chromosome movement during mitosis.

 - cytoplasm cleavage during cell division.

 - cilia and flagella movements, 

 - cell crawling (e.g. amoeba movement due to pseudopodia) and

 - muscle contraction in animals.

 - cytoplasmic streaming (cytoplasmic streaming is the movement or flow of cytosol in cells - particularly in in plant cells - to allow for the transport of molecules around the cell).

 

What are centrioles? 

Centrioles are a pair of short microtubules involved in cell division. Before cell division the

centriole replicates itself and the two centrioles move to opposite poles (opposite ends) of the cell. Here at each pole of the cell the centrioles initiate spindle formation which organises and separates the chromosomes during cell division. (see mitosis for more detail).

 

What are Mitochondria?

Mitochondria are “sausage-shaped organelles” typically about 8μm long. Within the mitochondria is the “space” enclosed by the inner membrane - this “space” is called the mitochondrial matrix, and it is here within the mitochondrial matrix where 70S ribosomes and small circular strands of mitochondrial DNA(mtDNA) can be found. Structurally mitochondria are surrounded by a double membrane: the outer membrane is relatively simple and permeable. Whilst the inner membrane is slightly more complex. This highly folded inner membrane, forms the inner membrane structures called cristae. Cristae of course provide the inner mitochondria membrane with a large surface area - and it is here where ATP synthesis occurs. Mitochondria are where aerobic respiration takes place in all eukaryotic cells.

 

What are Chloroplasts?

Chloroplasts are where photosynthesis takes place. Thus, chloroplasts are only found in photosynthetic organisms (e.g. plants and algae). Similar to mitochondria, chloroplasts are enclosed by a double membrane. However, chloroplasts also have a third membrane called the thylakoid membrane. The thylakoid membrane is folded into thylakoid disks, which are then stacked into structures knows as grana. The thylakoid membrane contains chlorophyll and other photosynthetic pigments arranged in photosystems, and is the site of photosynthesis and ATP synthesis. The space between the inner membrane and the thylakoid is called the stroma. Chloroplasts also contain starch grains, ribosomes (70S) and circular DNA.

 

What is a Cell Membrane?

The cell membrane (aka. the Plasma Membrane or phospholipid bilayer) is a thin, flexible layer around the outside of all cells. Cell membranes are composed of phospholipids and proteins. Cell membranes separate the contents of the cell (intracellular environment) from the outside (extracellular environment). Cell membranes regulate, that is they control the entry and exit of materials. The cell membrane (i.e. The fluid mosaic model) will be discussed in greater detail later.

 

What is a Cell Wall?

Cell walls are a thick layer consisting of a network of fibres - outside of the cell membrane. Cell walls are there to provide strength and rigidity, and whilst the cell wall gives a cell strength it is still freely permeable to solutes (unlike cell membranes). 

 

Plant cell walls are made mainly of cellulose, (plant cell walls can also contain hemicellulose, pectin, lignin and other polysaccharides). Plant cell walls are built up in three layers called the primary cell wall, the secondary cell wall and the middle lamella. Channels through plant cell walls called plasmodesmata link the cytoplasm of adjacent cells. 

 

Fungal cell walls are made of chitin (pronounced Kai-Tin - Not Shittin’ as many of my students liked to say!). Chitin is a complex of fibrous polysaccharides and also forms the exoskeleton of arthropods.

 

Animal cells DO NOT  have a cell wall (though they do have a layer of carbohydrate outside the cell membrane called glycocalyx).

What is Microvilli?

Microvilli are small finger-like extensions of the cell membrane found in certain cells, for example epithelial cells of the intestine. Microvilli increase the surface area for absorption of materials. They are just visible under the light microscope and are described a as a “brush border.”

 

What is Undulipodium? 

Undulipodium is a long flexible tail present in some eukaryotic cells. The Undulipodium is used for motility. A undulipodium is an extension of the cytoplasm, surrounded by the cell membrane, and is full of microtubules and motor proteins so is capable of complex swimming movements. 

 

Cilia are identical in structure to undulipodia, but are much smaller and there are usually very many of them - cilia typically resemble short microscopic hairlike structures which vibrate allowing for movement (e.g. in ciliated protists) or “beating” and “vibrating” creating a flow / propulsion in the surrounding fluid - e.g. in the respiratory tract where they help filter out microorganisms, dust and other particles breathed in from the air.

The Cell Membrane - The Fluid Mosaic Model.

 

The cell membrane / plasma membrane surrounds all living cells. It cannot be overemphasised just how important cell membranes are, since they control how substances can move into and out of cells. Cell membranes are also responsible for many other things too. Membranes that surround the nucleus and other organelles (e.g. mitochondria) are almost identical to the cell membrane, and are composed of phospholipids, proteins and carbohydrates arranged what is commonly known as the fluid mosaic model (conceived in 1972 by Nicolson and Singer).

 

The phospholipids that from cell membranes arrange in a particular way, due to their amphiphilic nature,  remember from the phospholipids lesson how phospholipids form cell membranes. The phospholipids form a bilayer the with their polar (hydrophilic) phosphate heads facing outwards, and their non-polar (hydrophobic) fatty acid tails facing each other in the middle of the phospholipid bilayer. It is the hydrophobic fatty acid tails that form a “layer” in the middle of the bilayer which acts as a barrier to all but the smallest molecules. It is this 'barrier' which effectively isolates the two sides of the cell membrane.

 

Cell membranes can have phospholipids with different fatty acid tails too,  which affects the rigidity / flexibility of the membrane. Animal cell membranes also contain cholesterol which can link to the fatty acids and provide stability, thus, strengthening the cell membrane.

 

The phospholipid bilayer is thin, flexible and in constant motion - the phospholipids move in a range of ways, rotating in situ, swapping positions with neighbours and even “jumping” from one side of the membrane to the other - in a manoeuvre called the “flip flop”. It is of course this “fluidity” of the phospholipid bilayer that gave rise its “fluid” name in Singer and Nicholson's "fluid mosaic model".

 

The “mosaic” part is simply due to the fact that embedded within the phospholipid bilayer are proteins. The proteins "float" in the phospholipid membrane forming channels and receptors for molecules that warrant entry or exit from cells, and don’t forget the carbohydrates that extend out from the proteins too. These carbohydrates have functional parts to play as cell surface receptors, and other roles which we’ll get to shortly.

 

The proteins embedded within the phospholipid bilayer usually span from one side of the phospholipid bilayer to the other (integral proteins), but can also sit on one of the surfaces of cell membranes (peripheral proteins). The proteins can slide around the membrane, moving very quickly and colliding with one another, but can never flip from one side to the other (like phospholipids can!) Cell membrane proteins have hydrophilic regions in contact with the water on the outside of cell membranes (these hydrophilic regions are composed of hydrophilic amino acids). The proteins also have hydrophobic regions (composed of hydrophobic amino acids) in contact with the fatty chains which form the inside part of the cell membrane. Proteins make up approximately 50% of the mass of cell membranes and are responsible for most of the membrane's functional properties (i.e. regulation / transport of substances into and out of the cell).

 

Proteins that span the membrane are usually involved in transporting substances across the membrane.

 

Proteins on the inside surface of cell membranes are often attached to the cytoskeleton and are involved in maintaining the cell's shape and aid in cell motility. Some of these proteins maybe enzymes that catalyse specific reactions.

 

Proteins on the outside surface of cell membranes typically act as receptors -

These proteins  have specific binding sites where hormones or other molecules bind - triggering cellular events. These proteins may also be involved in cell signalling, cell recognition and enzyme activities - like in digestive processes, e.g. maltase is a membrane bound enzyme that hydrolyses the disaccharide maltose into two alpha-glucose molecules.

 

Carbohydrates are not just digested either… some carbohydrates play functional roles and are found on the outer surface of all eukaryotic cell membranes. These carbohydrates are attached to the cell membrane proteins and sometimes directly to the phospholipids. Proteins with carbohydrates attached have specific names - they are called glycoproteins, (“glyco” referring to the carbohydrate portion and “protein” referring to, well the protein part)… 

 

Phospholipids with carbohydrates attached also have special names - these are called glycolipids. (the same convention of naming applies - “glyco” meaning the carbohydrate bit and “lipid” meaning the phospholipid part. 

 

The carbohydrates that associate with cell membranes are short polysaccharides composed of a variety of different monosaccharides. Together they form a cell “coat” or glycocalyx outside the cell membrane (once again naming conventions remain consistent here too - the “glyco” meaning carbohydrate and “calyx” is of Greek origin meaning “case” or “enclosure”. So, the glycocalyx is involved in protection and cell recognition - Antigens, for example the ABO antigens on blood cells are usually cell-surface glycoproteins.

 

The important thing to remember is that cell membranes are more complex that just a simple lipid bilayer allowing things into and out of cells. Rather, cell membranes are “Fluid” - that is to say they are in constant motion and they are “mosaic” - that is, cell membranes are embeddedwith a variety of proteins, enzymes, glycoproteins and glycolipids - all playing essential roles in cell regulation cell recognition, cell signalling, and many many more important cellular events.

Genetics

 

Genetics is the study of heredity (the passing on of characteristics from one generation to the next). Genetics is concerned with a few key questions, but primarily the question 

"Why do organisms look almost, but not exactly, like their parents?" 

 

Genetics can be subdivided into 3 main areas: -

 

1. Mendelian Genetics (Classical Genetics). Mendelian Genetics (pioneered by Gregor Mendel: 1822-1884) is the study of heredity at the whole organism level by investigating how particular characteristics are inherited - it is in classical genetics where you learn all about Punnet Squares and investigating monohybrid and dihybrid inheritance. You can recap simple punnet squares and their application in population genetics in the Hardy-Weinberg lessons.

 

2. Molecular Genetics (Molecular Biology), is the study of heredity at the molecular level, thus, molecular genetics if primarily concerned with the structure, function and properties of DNA and nucleic acids. Molecular genetics also includes genetic engineering, molecular cloning, biotechnology and a whole host of cool cell biology stuff - like PCR and Genetic Fingerprinting.

 

3. Population Genetics, is the study of genetic differences within species and between species, including how species evolve by natural selection. Population genetics utilises statistical models (like the Hardy-Weinberg equilibrium) to investigate how characteristics evolve through populations.

 

In a previous section we learned all about nucleic acids: - DNA, RNA, mRNA, tRNA and also learned a bit about they key players involved in the discovery of DNA, important figures such as Crick, Watson, Franklin, Wilkins, and Chargaff…

 

in this section we move on from the structure of nucleic acids and take a look at how DNA forms chromosomes, the eukaryotic cell cycle, Mitosis, the meaning of diploidmeiosis, and gene mutations.

 

DNA molecules are pretty big polynucleotides… So, in order to fit into the cell, DNA exists in shorter lengths and each length of DNA is tightly wrapped up with histone proteins to form a complex called chromatin. During most of the life of a cell the chromatin is dispersed throughout the nucleus and cannot be seen with a light microscope. However, parts of the chromatin will unwind so that genes on the DNA can be transcribed. 

 

Just before cell division DNA is replicated, and more histone proteins are synthesised, so there

is temporarily twice the normal amount of chromatin. Following replication the chromatin then

coils up even tighter to form short fat bundles called chromosomes. 

 

Chromosomes are about 100,000 times shorter than fully stretched DNA, and therefore 100,000 times thicker, so chromosomes are “large” enough to be seen under a light microscope. 

 

Chromosome have a sort of “X” shape - this is because a chromosome contains two replicated copies of DNA - with each of ‘arms’ of the “X” being identical. Each “arm” of a chromosome is called a chromatid, and chromatids are joined together at the centromere. 

 

DNA molecules extend form one end of a chromosome to the other, and genes are distributed along the DNA. Each gene has its own specific position on a chromosome, and this position is

known as the locus (loci) of the gene.

Some Key terms: -

Chromatin: - DNA + histones at any stage of the cell cycle.

Chromosome: - Compact, short and thick “X” shaped form of chromatin. Chromosomes are condensed forms of chromatin and are therefore visible during mitosis.

Chromatid: -  single arm of an “X” shaped chromosome.

 

A Level Biology - Chromosome Structure and Key Terminology (i.e. Gene, Allele, Locus...)

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Chromosome Structure and Key Terminology (i.e. Gene, Allele, Locus) A3 PDF for A Level Biology

A Level Biology - The Eukaryotic Cell Cycle

 

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Cell_Cycle_Wheelie.Ed._Basic_T.£9.99.jpg
The Eukaryotic Cell Cycle Revision Notes Poster A3 PDF for A Level Biology

The cell cycle

 

There are two ways in which a cell can divide:

1. Mitosis.

2. Meiosis.

 

Mitosis is 'normal' cell division in which the DNA replicates and each daughter cell has an identical copy of the original DNA (unless a gene mutation has taken place).

 

Organisms grow and repair by Mitosis.

 

Whereas Meiosis is a more complex type of cell division with 2 essential things to understand:

  1. The (diploid) chromosomal number is halved 

  2. Genes are randomly assorted (i.e. ‘shuffled’) so that each daughter cell contains different combinations of alleles.

 

Meiosis plays an essential role in the sexual reproduction for most species (but not all!). Meiosis is the process that produces haploid gametes (Sperm and Ova).

A Level Biology - Mitosis

 

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Mitosis Revision Notes with Knowledge Check Questions and Answers PDF for A- Level Biology
Mitosis Revision Notes Poster A3 PDF for A Level Biology

A Level Biology - Practical Skills: Identifying the stages of Mitosis and Calculating the Mitotic Index

 

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 Practical Skills Identifying the stages of Mitosis and Calculating the Mitotic Index A3 PDF for A Level Biology

A Level Biology - Genetics:

Defining Diploid - What is meant by 2n?

 

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Somatic cells are diploid, Humans are Diploid - Defining Diploid A3 Poster PDF for A Level Biology

00:00 Intro Screen / Learning Outcomes

00:24 Somatic Cells are Diploid

00:49 Why 2n?

01:05 Humans are diploid organisms

01:42 Human Body cells contain 2n chromosomes.

02:06 Grizzly bears are diploid too...

2:22 Great White Sharks are diploid...

02:30 Lions are diploid...

02:36 ...and Mosquitos are diploid!

03:00 Why do you need to know any of this?

03:36 Which means Haploid means Half...

Most cells in the human body contain 2 sets of chromosomes – they are said to be diploid. The gametessperm and ova – are haploid, (they contain only 1 set of chromosomes).

 

Fertilisation restores the diploid number and the new, genetically unique, organism grows and develops according to the genes it has inherited from both parents.

This A Level Biology Revision Lesson Explains the term Diploid as expected to be understood and defined for  ★ ALL A Level Biology Specifications 

The term diploid appears in many places in ALL specifications. It’s just a term you must understand and be able to define. Additionally, Students should be able to use the expression 2n to calculate the possible number of different combinations of chromosomes.

A Level Biology - Meiosis

 

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This A Level Biology Revision Lesson Explains Meiosis as expected to be understood, identified and described. ALL A Level Biology Specifications.

 

ALL specifications: -

★ AQA ★ CIE ★ Edexcel (Biology A – Salters-Nuffield) ★ Edexcel (Biology B) ★ OCR (Biology A) ★ OCR (Biology B) ★ WJEC ★ IB ★ BTEC

Being able to describe Meiosis and identify the stages of meiosis is fundamental for ALL specifications. 

A Level Biology - Genetics: Gene Mutations, Deletions and Substitutions

 

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00:00 Intro | Learning Outcomes

00:43 Mistakes Happen...

01:21 Deletions and Substitutions.

01:37 Fame shift Mutation

04:06 The Degenerate Code.

05:04 Gene Mutation: Substitution.

Gene mutations deletions and substitutions A3 Poster PDF for A Level Biology

A gene mutation occurs when a change in the sequence of nitrogenous bases occurs, i.e. when a gene is copied incorrectly. This can result in one or more different amino acids being incorporated into the polypeptide (protein). E.g. if a codon reads AAA and is copied incorrectly so that it was read as AAC, then the amino acid argenine would be coded for in place of the amino acid lysine. 

 

There are different types of gene mutation that can occur.

  • Substitution mutation

  • Deletion mutation

  • Addition mutation

A Substitution mutation is where the wrong nitrogenous base is inserted or "substituted" in place of the correct nitrogenous base.

 

Lets, consider the following nitrogenous base sequence: -

AAT CGG CCC GTA

This will be transcribed into mRNA as: -

UUA GCC GGG CAU 

and then translated into the amino acid sequence: -

leucine–alanine–glycine–histidine

 

However, if just one base is substituted, so that the DNA sequence now reads: -

AAT CAG CCC GTA

The second codon will now be transcribed as GUC which translates into the amino acid valine (replacing the amino acid alanine).

 

This change to the primary structure of the protein may or may not affect the way the protein functions.

 

But If the new amino acid caused a significant change to the tertiary and/or quaternary structure (i.e. the final conformational shape of the protein) then the protein may not be able to function properly.

A deletion mutation involves the loss of a nitrogenous base, which results in a frame shift mutation. This can also change most of the codons after the deletion.

Addition mutation is when an extra nitrogenous base is added into the sequence. All the bases that follow in the sequence are moved along (known as a frame shift mutation). 

 

For example: - An addition mutation (where a cytosine bases is added) could cause the following sequence: -

 

AAT CGG CCC GTA 

 

to become: -

 

AAC TCG GCC CGT A 

 

This will cause most of the amino acids coded for by codons at or after the addition to change.

 

Gene mutations can have one of three consequences.

1. The mutation may be lethal. The new amino acid causes the protein to be significantly different resulting in that protein not being able to function properly in the organism. This change could be something like the active site of an enzyme being wrong shape so it cannot combine with its substrate. This could have greater consequences for the organism wherby the enzyme cannot play its part in a sequence of metabolic reactions (like the sequence of events in glycolysis, for example). Mutations that cause frame shifts i.e. additions and deletions, are more likely to be lethal than substitutions because they change more codons, and therefore more amino acids. A protein is far more likely to function properly with just one amino acid change than with a whole new sequence of amino acids.

2. The mutation may have no effect. The protein may still function despite the new amino acid because the change does not alter the tertiary structure or the shape of the part of the protein which interacts with other chemicals. Also, because some amino acid are coded for by several different codons, (Remember the degenerate code) a mutation may not result in a change in the amino acid.

3. The mutation may be beneficial. The new amino acid may alter the protein in such a way that it works in a different way (helping the organism in some way). This will confer a selective advantage on the organism. 

 

The commonly considered example (for A-Level biology and GCSE biology) is the beneficial mutation of the peppered moth (Biston betularia). In the UK before the Industrial Revolution, the 'peppered' (that is the 'speckled') variety of Biston betularia was common.

The  'peppered' markings provided camouflage on lichen covered rocks and trees. However, during  Industrial Revolution soot pollution was widespread resulting in the peppered moth  being more visible and easily seen and therefore eaten by predators. A random mutation in a gene (that is say a mutation not caused by the soot) resulted in a beneficial mutation that resulted in the production of a black (or melanic) variety of Biston betularia. The mutant moths were better camouflaged on the dark/black sooty surfaces. This gave the melanic variety of Biston betularia a selective advantage. The long term result: The melanic moths became more common in sooty areas than their 'peppered' coloured relatives.

What causes mutations?

Mutations typically occur by chance, that is randomly in the DNA, However, the rate of mutation can be greatly increased by mutagenic agents. sucha as: -

  • ultraviolet light

  • X-rays

  • a and b radiation

  • Chemicals  (e.g. mustard gas and cigarette smoke).

 

Mutations can occur in somatic (body) cells, or in sex cellssperm or ova.

On the whole though, organisms accumulate somatic cell mutations throughout their lives, as such mutations are not be passed on to their offspring.

Only sex cell (or germ line) mutations are passed on to their offsping.

If the genes that control cell division mutate, a tumour may develop as mitosis  takes place in an uncontrolled manner. Tumours (abnormal growth of cells/tissues) may be benign or Malignant.

 
Check Your Exam Specification

AQA Specification Reference: - 3.2 Cells: nucleus (containing chromosomes, consisting of protein-bound, linear DNA). 3.4.1 DNA, genes and chromosomes. In the nucleus of eukaryotic cells, DNA molecules are very long, linear and associated with proteins, called histones. Together a DNA molecule and its associated proteins form a chromosome. A gene occupies a fixed position, called a locus, on a particular DNA molecule.

 

CIE Specification Reference: -  5 The mitotic cell cycle: describe the structure of a chromosome, limited to DNA, histone proteins, chromatids, centromere and telomeres.

 

Edexcel (Biology A – Salters-Nuffield) Specification Reference: - Topic 3: Voice of the Genome. 3.8 i) Know that a locus (plural = loci) is the location of genes on a chromosome. ii) Understand the linkage of genes on a chromosome.

 

Edexcel (Biology B) Specification Reference: - Topic 2: Cells, Viruses and Reproduction of Living Things; Topic 7: Modern Genetics; Topic 8: Origins of Genetic Variation.

 

OCR (Biology A) Specification Reference: -  Terminology is expected knowledge throughout. topics: -2.1 Foundations in biology; 2.1.6 Cell division, cell diversity and cellular organisation & Module 6 – Genetics, evolution and ecosystems).

 

OCR (Biology B) Specification Reference: - Terminology is expected knowledge throughout.: 3.1.1 The developing cell: cell division and cell differentiation; 3.1.2 The developing individual: meiosis, growth and development; 3.3.1 The cellular basis of cancer and treatment; Module 5: Genetics, control and homeostasis - e.g. To include the correct usage of the terms gene, allele (gene variant), locus, phenotype, genotype, dominant and recessive, heterozygous and homozygous and codominant.

 

WJEC Specification Reference: - Terminology is expected knowledge throughout: i.e. Core Concepts 5. Nucleic acids and their functions. CONTINUITY OF LIFE: 2. Genetic information is copied and passed on to daughter cells. 5. Inheritance: (a) alleles as different forms of the same gene. 7. Application of reproduction and genetics.

AQA Specification Reference: - 3.2.2 All cells arise from other cells. Eukaryotic cells that do retain the ability to divide show a cell cycle. DNA replication occurs during the interphase of the cell cycle. Mitosis is the part of the cell cycle in which a eukaryotic cell divides to produce two daughter cells, each with the identical copies of DNA produced by the parent cell during DNA replication. Division of the cytoplasm (cytokinesis) usually occurs, producing two new cells.

 

CIE Specification Reference: - 5. The mitotic cell cycle: When body cells reach a certain size they divide into two. Nuclear division occurs first, followed by division of the cytoplasm. The mitotic cell cycle of eukaryotes involves DNA replication followed by nuclear division. This ensures the genetic uniformity of all daughter cells. Candidates will be expected to use the knowledge gained in this section to solve problems in familiar and unfamiliar contexts. c) outline the cell cycle, including interphase (growth in G1 and G2 phases and DNA replication in S phase), mitosis and cytokinesis.

 

Edexcel (Biology A – Salters-Nuffield) Specification Reference: - Topic 3: Voice of the Genome. 3.10 Understand the role of mitosis and the cell cycle in producing identical daughter cells for growth.

 

Edexcel (Biology B) Specification Reference: - Topic 2: Cells, Viruses and Reproduction of Living Things. 2.3 Eukaryotic cell cycle and division. i Know that the cell cycle is a regulated process in which cells divide into two identical daughter cells, and that this process consists of three main stages: interphase, mitosis and cytokinesis. ii Understand what happens to genetic material during the cell cycle, including the stages of mitosis.

 

OCR (Biology A) Specification Reference: - Module 2: Foundations in biology. 2.1.6 Cell division, cell diversity and cellular organisation. Learners should be able to demonstrate and apply their knowledge and understanding of: the cell cycle - To include the processes taking place during interphase (G1, S and G2), mitosis and cytokinesis, leading to genetically identical cells.

 

OCR (Biology B) Specification Reference: - Module 3: Cell division, development and disease control Learners should be able to demonstrate and apply their knowledge and understanding of: the cell cycle - To include the processes taking place during interphase (G1, S and G2), mitosis and cytokinesis, leading to genetically identical cells.

 

WJEC Specification Reference: - 2. Genetic information is copied and passed on to daughter cells. This topic covers cell division. During the cell cycle, genetic information is copied and passed on to daughter cells.

AQA Specification Reference: - 3.2 Cells: 3.2.2 All cells arise from other cells. Mitosis is the part of the cell cycle in which a eukaryotic cell divides to produce two daughter cells, each with the identical copies of DNA produced by the parent cell during DNA replication. The behaviour of chromosomes during interphase, prophase, metaphase, anaphase and telophase of mitosis. The role of spindle fibres attached to centromeres in the separation of chromatids. Division of the cytoplasm (cytokinesis) usually occurs, producing two new cells.

 

CIE Specification Reference: - 5 The mitotic cell cycle. Explain the importance of mitosis in the production of genetically identical cells, growth, cell replacement, repair of tissues and asexual reproduction. 5.2 Chromosome behaviour in mitosis: describe, with the aid of photomicrographs and diagrams, the behaviour of chromosomes in plant and animal cells during the mitotic cell cycle and the associated behaviour of the nuclear envelope, cell surface membrane and the spindle (names of the main stages of mitosis are expected).

 

Edexcel (Biology A – Salters-Nuffield) Specification Reference: - Topic 3: Voice of the Genome. 3.10 Understand the role of mitosis and the cell cycle in producing identical daughter cells for growth and asexual reproduction.

 

Edexcel (Biology B) Specification Reference: - Topic 2: Cells, Viruses and Reproduction of Living Things. 2.3 Eukaryotic cell cycle and division. i Know that the cell cycle is a regulated process in which cells divide into two identical daughter cells, and that this process consists of three main stages: interphase, mitosis and cytokinesis. ii Understand what happens to genetic material during the cell cycle, including the stages of mitosis. iii Understand how mitosis contributes to growth, repair and asexual reproduction.

 

OCR (Biology A) Specification Reference: - 2.1.6 Cell division, cell diversity and cellular organisation. The main stages of mitosis. To include the changes in the nuclear envelope, chromosomes, chromatids, centromere, centrioles, spindle fibres and cell membrane.

 

OCR (Biology B) Specification Reference: - 3.1.1 The developing cell: cell division and cell differentiation . The main stages of mitosis. To include the changes in the nuclear envelope, chromosomes, chromatids, centromere, centrioles, spindle fibres and cell membrane.

 

WJEC Specification Reference: - Continuity of Life. 2. Genetic information is copied and passed on to daughter cells. This topic covers cell division. During the cell cycle, genetic information is copied and passed on to daughter cells. (a) interphase and the main stages of mitosis (b) the significance of mitosis as a process in which daughter cells are provided with identical copies of genes and the process of cytokinesis.

AQA Specification Reference: -  Required practical 2: Preparation of stained squashes of cells from plant root tips; set-up and use of an optical microscope to identify the stages of mitosis in these stained squashes and calculation of a mitotic index.

 

CIE Specification Reference: - 4.2 Paper 3. Advanced Practical Skills 1 and Advanced Practical Skills 2.

 

Edexcel (Biology A – Salters-Nuffield) Specification Reference: - Practical skills: In carrying out practical activities, students will be expected to use their knowledge and understanding to pose scientific questions which can be investigated through experimental activities. Such activities will enable students to collect data, analyse it for correlations and causal relationships, and to develop solutions to the questions posed.

 

Edexcel (Biology B) Specification Reference: - Practical skills: In carrying out practical activities, students will be expected to use their knowledge and understanding to pose scientific questions which can be investigated through experimental activities. Such activities will enable students to collect data, analyse it for correlations and causal relationships, and to develop solutions to the questions posed.

 

OCR (Biology A) Specification Reference: - Module 1: Development of practical skills in biology. 1.1 Practical skills assessed in a written examination. Practical skills are embedded throughout all the content of this specification. Learners will be required to develop a range of practical skills throughout their course in preparation for the written examinations. 1.1.3 Analysis: 1.2 Practical skills assessed in the practical endorsement. 1.2.1 Practical skills.

 

OCR (Biology B) Specification Reference: - Module 1: Development of practical skills in biology. 1.1 Practical skills assessed in a written examination. Practical skills are embedded throughout all the content of this specification. Learners will be required to develop a range of practical skills throughout their course in preparation for the written examinations. 1.1.3 Analysis: 1.2 Practical skills assessed in the practical endorsement. 1.2.1 Practical skills.

 

WJEC Specification Reference: - PRACTICAL TECHNIQUE REQUIREMENTS: Use of light microscope at high power and low power, including use of a graticule. 5. produce scientific drawing from observation with annotations. Core concepts 2: Scientific drawing of living cells, Preparation and drawing of cells of root tip. MATHEMATICAL REQUIREMENTS AND EXEMPLIFICATION. Recognise and make use of appropriate units in calculations. Handling data. 

AQA Specification Reference: - 3.4.3 Genetic diversity can arise as a result of mutation. Gene mutations involve a change in the base sequence of chromosomes. They can arise spontaneously during DNA replication and include base deletion and base substitution. Due to the degenerate nature of the genetic code, not all base substitutions cause a change in the sequence of encoded amino acids. 3.8.1 Alteration of the sequence of bases in DNA can alter the structure of proteins (A-level only). Gene mutations might arise during DNA replication. They include addition, deletion, substitution, inversion, duplication and translocation of bases.

 

CIE Specification Reference: - 6 Nucleic acids and protein synthesis: 6.2 Protein synthesis. b) state that a gene mutation is a change in the sequence of nucleotides that may result in an altered polypeptide. 16 Inherited change: 16.2 The roles of genes in determining the phenotype. e) explain that gene mutation occurs by substitution, deletion and insertion of base pairs in DNA and outline how such mutations may affect the phenotype.

 

★ Edexcel (Biology A – Salters-Nuffield) Specification Reference: - N/A.

★ Edexcel (Biology B) Specification Reference: - N/A.

 

OCR (Biology A) Specification Reference: - 6.1 Genetics and evolution. Types of gene mutations and their possible effects on protein production and function. To include substitution, insertion or deletion of one or more nucleotides. The possible effects of these gene mutations (i.e. beneficial, neutral or harmful).

 

OCR (Biology B) Specification Reference: - Module 5: Genetics, control and homeostasis 5.1.1 Patterns of inheritance. B) gene mutations (To include cystic fibrosis, sickle cell anaemia, phenylketonuria (PKU) and Huntington’s disease).

 

WJEC Specification Reference: - Continuity of Life 5. Inheritance (f) gene mutation as illustrated by sickle cell anaemia.

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