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3.5 Transcription and Translation
3.5.1 Compare the structure of DNA and RNA
3.5.2 Outline DNA transcription in terms of the formation of an RNA strand
complementary to the DNA strand by RNA polymerase
Transcription is the process by which an RNA sequence is produced from a DNA
template:
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RNA polymerase separates the DNA strands and synthesises a complementary
RNA copy from one of the DNA strands
It does this by covalently bonding ribonucleoside triphosphates that align
opposite their exposed complementary partner (using the energy from the
cleavage of the additional phosphate groups to join them together)
Once the RNA sequence has been synthesised, RNA polymerase will detach
from the DNA molecule and the double helix will reform
The sequence of DNA that is transcribed into RNA is called a gene
Transcription occurs in the nucleus (where the DNA is) and, once made, the
mRNA moves to the cytoplasm (where translation can occur)
Three main types of RNA are predominantly made:
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Messenger RNA (mRNA): A transcript copy of a gene used to encode a
polypeptide
Transfer RNA (tRNA): A clover leaf shaped sequence that carries an amino acid
Ribosomal RNA (rRNA): A primary component of ribosomes
3.5.3 Describe the genetic code in terms of codons comprised of triplets of bases
The genetic code is the set of rules by which information encoded in mRNA sequences
is converted into proteins (amino acid sequences) by living cells
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Codons are a triplet of bases which encodes a particular amino acid
As there are four bases, there are 64 different codon combinations (4 x 4 x 4 =
64)
The order of the codons determines the amino acid sequence for a protein
The coding region always starts with a START codon (AUG) and terminates with
a STOP codon
The Genetic Code
The genetic code has the following features:
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It is universal - every living thing uses the same code (there are only a few rare
and minor exceptions)
It is degenerate - there are only 20 amino acids but 64 codons, so more than
one codon may code for the same amino acid (this allows for silent mutations
whereby a change in the DNA sequence does not affect the polypeptide
sequence)
3.5.4 Explain the process of translation, leading to polypeptide formation
Translation is the process of protein synthesis in which the genetic information encoded
in mRNA is translated into a sequence of amino acids in a polypeptide chain
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Ribosomes bind to mRNA in the cell's cytoplasm and move along the mRNA
molecule in a 5' - 3' direction until it reaches a start codon (AUG)
Anticodons on tRNA molecules align opposite appropriate codons according to
complementary base pairing (e.g. UAC will align with AUG)
Each tRNA molecule carries a specific amino acid (according to the genetic code)
Ribosomes catalyse the formation of peptide bonds between adjacent amino
acids (via a condensation reaction)
The ribosome moves along the mRNA molecule synthesising a polypeptide chain
until it reaches a stop codon, at this point translation stops and the polypeptide
chain is released
The Process of Translation
3.5.5 Explain the relationship between one gene and one polypeptide
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A gene is a sequence of DNA which encodes a polypeptide sequence
A gene sequence is converted into a polypeptide sequence via the processes of
transcription (making an mRNA transcript) and translation (polypeptide synthesis)
Translation uses tRNA molecules and ribosomes to join amino acids into a
polypeptide chain according to the mRNA sequence (as read in codons)
The universality of the genetic code means all organisms show the same
relationship between genes and polypeptides (indicating a common ancestry and
allowing for transgenic techniques to be employed)
Some proteins may consist of a number of polypeptide chains and thus need
multiple genes (e.g. haemoglobin consists of four polypeptide subunits encoded
by two different genes)
When a gene is mutated it may lead to the synthesis of a defective polypeptide,
hence affecting protein function
The 'One Gene - One Polypeptide' Rule
There are two exceptions to the 'one gene - one polypeptide' rule:
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Genes encoding for tRNA and rRNA do not code for polypeptide sequences (only
mRNA sequences code for polypeptides)
A single gene may code for multiple polypeptides if alternative splicing occurs
(the removal of exons as well as introns)