Transcript Central dogma of genetics

Central dogma of genetics
Lecture 4
The Central dogma of Genetics: conversion of DNA to
Proteins
• When a gene is “Expressed”
[activated] it undergoes two
main process /steps:
1.
2.
The primary/template strand of
dsDNA is converted into an
RNA strand, or what is called
mRNA (messanger RNA), in a
process called transcription
The mRNA strand is then
converted into an amino acid
chain in a process called
translation.
• The basic process is illustrated
in Fig 1
Fig1: adaped from klug p. 241
Step 1: Transcription
•
The RNA polymerase {shown in yellow), that performs
the conversion, unwinds the double helix, moves in the
5’ to 3’ direction of the coding/primary strand,
•
It uses the antisense strand to produce a
complementary mRNA strand. Remember the
complements are: A <-> T and G<->C
•
This mRNA is a complement of the complement of the
primary strand; in other words:
if ATA is the primary sequence; its DNA compliment is
TAT,
However in mRNA a T (thymine) is replaced by U
(uracil)
•
Adapted from
•
•
Therefore the TAT on the complimentary strand
becomes AUA on the mRNA strand
•
The net effect is the mRNA is an exact copy of the
primary DNA strand but T being replaced by U
DNA version of genetic code table
• In order for a DNA sequence to be converted
to amino acid sequence and subsequently a
protein a code is used.
• The code uses sets of 3, triplets, of
nucleotides. Often called a codon.
• The different codons use the following table to
be translated into their equivalent amino acid.
• The following slide show the code for the DNA
and RNA conversion tables.
The genetic code
• RNA conversion table
DNA conversion table
Adapted from Ref [1] p. 247
What is the amino acid of the DNA sequence transcribed above: TAT
Translation mRNA -> AA
• The translation occurs in a component
of the cell called the ribosome.
• Translation consists of three phases:
• Initiation, elongation and termination
• Stage 1 Initiation:
1. The ribosome attaches to the mRNA
strand and moves to the initiation
codon, AUG (ATG on the DNA strand)
2. Then, another version of RNA called,
transfer tRNA attaches to the AUG of
mRNA
• The tRNA has two essential elements:
an anti-codon, e.g. UAC, and an
attached amino acid, e.g. methonine.
Initiation of translation
Anti-codon
tRNA
Amino acid:
methonine
Adapted from chapter 12 Klug
Translation mRNA -> AA
• Elongation:
• The large subunit of the
ribosome attaches to the
strand
• The ribosome has three
chambers; going from left to
right:
– The E site where tRNAs to be
discarded.
– The P site: holds the tRNA for
that is attached to the
“current” codon.
– The A site: holds the tRNA for
the next codon in the mRNA
strand: e.g. UUC
Adapted from chapter 12 Klug
Translation mRNA -> AA
• Elongation continued:
• The amino acid(s) on the P
site join to the amino acid
on the A site.
• The ribosome then moves
down the mRNA to the next
codon ;e.g. GGU.
• In the process
1. The tRNA in the E site is
discarded
2. the tRNA in the P site
moves to the E site
3. The tRNA and its amino
acid chain in the A site
moves to the P site.
Elongation “stage 2”
Adapted from chapter 12 Klug
Translation mRNA -> AA
• Elongation continued:
• A new tRNA molecule
with the correct “anticodon” moves into the
empty A chamber.
• A peptide bond is
formed between the
amino acids in the P
and A chamber and
step 2 of elongation
beings again.
Elongation “stage 3”
Adapted from chapter 12 Klug
Translation mRNA -> AA
• Termination :
• The ribosome continues to
move down the mRNA strand
until it reaches a termination
codon; e.g. UGA.
• There is no tRNA
corresponding to this codon so
the “following” chamber
remains empty.
• No peptide bond is formed so
when the ribosome moves
again it causes the amino acid
chain to break of its tRNA and
so release the amino acid
chain.
Adapted from chapter 12 Klug
Animation of the translation process
• Translation Animation
• Translation Animation 2
• Translation animation 3
Exam Question
• Describe, using suitable examples, the steps
involved in the “Central Dogma “ of genetics:
converting a DNA strand into its
corresponding amino acid chain.
References
• [2] http://www.di.uq.edu.au/sparqtransctrans
accessed on the 30/09/2011
• [5]
http://www.ncbi.nlm.nih.gov/pmc/articles/P
MC1370565/