A Level Biology - Genetics: Genome, Proteome and Protein Synthesis (summary)
Transcription is when the DNA code on a chromosome is copied into a molecule called messenger RNA (mRNA). The mRNA molecules are effectively mobile copies of genes, since the role of mRNA is to carry the genetic code out of the nucleus and to the site of translation: - the ribosomes in the cytoplasm.
Translation is when the genetic code carried in the mRNA molecule is used to build a polypeptide.
The process of Transcription
Most cells in the human body contain two complete sets of genes. However, only a few genes expressed in any particular cell. For example, every cell in the human body contains two copies of the gene which codes for insulin, but only certain cells in the pancreas actually use the gene to make insulin.
The steps in transcription
Step 1. The two strands of DNA unwind along the length of the gene. This event is catalysed by enzymes.
Step 2. The enzyme RNA polymerase moves along one side of the DNA molecule – the sense strand containing the genetic code. RNA polymerase catalyses the assembly of a mRNA molecule by the addition of complementary nucleotides. Note. When an RNA molecule is synthesised, the base thymine is replaced by uracil, so the base pairing in RNA is always A with U and C with G.
Step 3. The mRNA molecule dissociates from the gene and is now able to move out of the nucleus and on to the site of translation.
Transfer RNA (tRNA) is the molecule that transfers amino acids to ribosomes during translation. At one end of the tRNA molecule is the anticodon which complementary base pairs to the codon on the mRNA At the other end of the tRNA molecule is the amino acid specified by the codon.
Ribosomes (the sites of protein synthesis) are small dense organelles composed of 2 subunits that hold together all the components needed for translation to take place.
Step 1. The mRNA attaches itself to a ribosome.
Step 2. The codons are 'translated': e.g. if a codon reads UUU, (for the amino acid phenylalanine), a tRNA molecule with the anticodon AAA will attach, carrying the phenylalanine amino acid at the other end. (Remember the tRNA anticodon is complementary to the mRNA codon).
Step 3. Each subsequent codon is translated in the same way, and each successive amino acid is held alongside its neighbour, with peptide bonds which are formed via condensation reactions between each of the amino acids).
This process is repeated: the mRNA molecule moves along the ribosome until a stop codon is encountered, translation stops and the primary sequence of the polypeptide has been constructed.
A Level Biology - Genetics: Transcription and RNA Splicing
A-Level Biology: Transcription Exam Style Q and A
What is the Question asking?
It's asking you to describe the process of translation.
What are the key words? Underline the key points in the questions like this: -
What do you know from the Question?
The question tells you what mRNA does and where it is made! (in the Nucleus!) so it is Expected that you can go onto describe in detail the process of Transcription! Its worth 6 marks - so you need a well written answer with 6 key facts about how transcription takes place.
★ THE ANSWER ★
Transcription is the process by which mRNA is made from a DNA template and takes place in the nucleus of a cell. The enzyme helicase breaks hydrogen bonds between complementary base pairs of the DNA helix, exposing the DNA strands. One of the DNA strands acts as a template for the attachment of RNA nucleotides, which also form complementary base pairs (A-U and G-C) joined by the enzyme RNA polymerase. Following elongation of the newly synthesised pre-RNA molecule spicing takes place. An enzyme called a spicersome removes the non-coding regions of the pre-RNA – sections called introns. Whilst the expressed segments are ligated to form the mRNA strand which can now exist the nucleus and is used during translation.
★ NOTE ★
Examiners mark schemes will indicate key words and ideas expected in your answer.
Be aware! Marks can easily be lost if your answer does not include the correct terminology or does not demonstrate an appropriate level of understanding.
Examiners Mark scheme: -
1. Helicase; Breaks hydrogen bonds; 2. Only one DNA strand acts as template; 3. RNA nucleotides attracted to exposed bases.
4. (Attraction) according to base pairing rule; 5. RNA polymerase joins (RNA) nucleotides together;
6. Pre-mRNA spliced to remove introns.
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