Download DNA & THE GENETIC CODE (protein synthesis)

A sequence of bases on a
DNA molecule which codes
for a sequence of amino
acids in a polypeptide chain
A sequence of 3 DNA bases
in a gene which code for a
single amino acid
4 Bases =
Examples
ACGT
AGC
CTA
A code that starts
at one end of the gene
and reads each codon
independently of the next
AUGAGCACCGAG
amino acid 1
amino acid 2
amino acid 3
amino acid 4
More than one base triplet
can code for a particular
amino acid
If the order of the bases is
important there are 64
different ways of arranging
four bases in a triplet code.
HOWEVER only 20 different
amino acids have to be coded
for.
• Some codes are used as
initiation codons to indicate
the beginning of a gene
• Some are stop codons and
indicate where a gene
terminates
• Others are used as alternative
codons for some amino acids
This involves 2 processes
TRANSCRIPTION
Copying a gene in the nucleus to make mRNA
translation
Using the mRNA to make proteins in the
cytoplasm
tRNA
Ribosome
mRNA
transcription
translation
DNA
POLYPEPTIDE
IN NUCLEUS
ENERGY
(ATP)
IN CYTOPLASM
DNA cannot move into the
cytoplasm where
polypeptides are produced.
This reduces the possibility
of damage to the DNA.
The DNA code is transcribed
(copied) into a single stranded
messenger RNA (mRNA)
molecule in the nucleus.
The process is controlled by
the enzyme RNA polymerase.
Only one of the DNA strands
of the double helix carries
the genetic code.
This is called the coding or
sense strand.
This sense strand acts as a
template for mRNA
production.
DNA helicase separates the
2 strands by causing
hydrogen bonds to break
between the complementary
base pairs in the gene and
the DNA to unwind.
RNA polymerase attaches to
the initiation code of the
gene and moves along the
template adding free RNA
nucleotides to the exposed
DNA nucleotides
Free complementary
RNA nucleotides in the nucleoplasm
attach to the exposed DNA bases.
adenine
thymine
pairs with
on the RNA
on the DNA
uracil
on the RNA
adenine
pairs with on the DNA
whilst
guanine always pairs with cytosine
RNA polymerase joins the sugarphosphate backbone of the mRNA by
condensation reactions.
The completed mRNA molecule passes
through the nuclear pores in the
nuclear membrane into the cytoplasm.
There are only 2 copies of each gene in
the nucleus, but transcription allows
many copies of mRNA to be available
for translation and protein synthesis.
During transcription the whole
gene is copied into mRNA.
However genes contain long
sequences of non-coding DNA
called introns.
These are not required to make
the protein therefore before
the mRNA is used, these
sequences are removed.
Part of DNA / gene to be copied
DNA template
double stranded
INTRON
(noncoding)
EXON
transcription
mRNA
after
Transcription
single stranded
modification
Modified mRNA
used by ribosome
to make protein
• interactive transcription
• Carries amino acids to the ribosomes
during translation
• tRNA is single stranded
• Folded back in itself to form a clover
shape, held by hydrogen bonds
• Specific amino acids are attached to
one end
• A 3 base anticodon at the other end is
complementary to a specific mRNA
codon
Put the sentences in the correct order and
then either rewrite or cut and paste
• This is an enzyme which copies RNA into
DNA
• Transcription involves unpairing of bases
in one region of the DNA helix (the gene
being read), followed by the synthesis of a
strand of mRNA.
• This carries a triplet code sequence
complementary to the template strand of
DNA (i.e. DNA acts as a template)
• mRNA acts as a messenger to carry
information to the ribosome in the
cytoplasm.
• Translation involves decoding/reading the
triplet message on mRNA.
• Each codon, 3 bases, has a natural
complementary sequence of 3 bases,
called the anticodon.
• This set of 3 bases is attached to a
specific tRNA molecule that carries and
transfers a specific amino acid.
• The specific amino acid is determined by
the sequence of the triplet code and
anticodon.
• The complementary triplets on mRNA and
tRNA pair together in ribosomal sites
(peptidyl & aminoacyl).
• Ribosomes are situated in the cytoplasm,
hence the need for mRNA & tRNA.
• When 2 amino acids sit side by side, at the
two sites on the ribosome, a condensation
reaction occurs and a peptide bond will
form between them.