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DNA vs RNA
Transcription=RNA sequence is produced from
a DNA template:
• RNA polymerase separates the DNA strands and
makes a complementary RNA copy from one DNA
strand.
• Once the RNA sequence has been synthesized,
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 and once
made, the mRNA moves to the cytoplasm.
Three main types of RNA are predominantly
made:
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
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 has the following features:
It is universal - every living thing uses the same
code (there are only a few rare and minor
exceptions)
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)
Translation= protein synthesis where mRNA is
translated into a sequence of amino acids in a
polypeptide chain
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 with codons to
provide complementary base pairing (e.g. UAC will align
with AUG)
• Each tRNA molecule carries a specific amino
acid (according to the genetic code)
• The ribosome moves along the mRNA
molecule synthezising a polypeptide chain
until it reaches a stop codon, at this point
translation stops and the polypeptide chain is
released
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