Genetics

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 Squaresand 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 diploid, meiosis, 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.

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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.

Most cells in the human body contain 2 sets of chromosomes – they are said to be diploid. The gametes – sperm 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.

The term Diploid is standard terminology, 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.

Meiosis as expected to be understood, identified and described by ALL A Level Biology Specifications. Being able to describe Meiosis and identify the stages of meiosis is fundamental for ALL specifications.