Central dogma of genetics
Lecture 4
The Central dogma of Genetics: conversion of DNA to
Proteins
• In the field of genetics when gene is
“Expressed” [activated] it undergoes two
main process or steps:
1.
2.
The DNA strand is converted into an
RNA, or what is called mRNA , strand in a
process called transcription
The mRNA is then converted into an
amino acid chain in a process called
translation.
• The process in Prokaryotic cells such as
bacteria is illustrated in Fig 1
• In Eukaryotic cells there are some
additional steps that were discovered
after the central dogma was proposed.
These will be discuss later in the course.
• Essentially when the gene is expressed it
goes from the Genotype [DNA level] to
the Phenotype [physical manifestation].
Fig1: adaped from klug p. 241
Step 1: Transcription
•
The DNA is double stranded or is in the form of a
double helix. One strand goes from 5’ to 3’ and the
other strand goes in the 3’ to 5’ direction
•
The 5’ to 3’ is the coding strand (sense strand) and the
3’ to 5’ strand is the complimentary template strand.
(antisense strand)
•
The transcription process always goes in the 5’ to 3’
direction. [however this does not always mean its on
the same strand or the strands can reverse roles]
The RNA polymerase (the molecule/ element/enzyme)
that performs the conversion unwinds the helix and
moves in the 5’ to 3’ direction of the primary strand.
It uses the antisense strand to produce a
complementary mRNA strand.
•
Adapted from
•
•
This mRNA is a complement of the complement of the
primary strand;
•
Messenger RNA (mRNA) is an exact copy of the coding (
sense strand ) sequence apart from Thymine (T) is
replaced by Uracil (U)
Translation mRNA -> AA
• The translation occurs in a element
of the cell called the ribosome.
• Translation consists of three phases:
• Initiation, elongation and termination
• Initiation:
• The ribosome attaches to the mRNA
and moves to the initiation codon,
AUG
• 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.
Anti-codon
tRNA
Amino acid:
methonine
Adapted from chapter 12 Klug
Translation mRNA -> AA
• Elongation:
• The ribosome then moves down
the mRNA in 5’to 3’ to the next
codon; e.g. UUC.
• The ribosome has three
chambers; going from left to
right:
– the tRNA to be discarded.
– the tRNA for the “current” codon,:
e.g. AUG
– the tRNA for the following codon
in the mRNA: e.g. UUC
• A peptide bond is formed
between the amino acids in the
“current” and “following”
chambers.
Adapted from chapter 12 Klug
Translation mRNA -> AA
• Elongation continued:
• The ribosome then moves
down the mRNA to the next
codon;e.g. GGU.
• The “current” tRNA is
moved into the discard
chamber
• the “following” tRNA is
moved to the current
chamber.
• This tRNA has a chain of
amino acids [in this case
two] attached to it.
Adapted from chapter 12 Klug
Translation mRNA -> AA
• Elongation continued:
• The process continues
with the next tRNA
moving into the empty
“following” chamber
• A peptide bond is formed
with the amino acid of
the new tRNA and the
current peptide chain.
• The ribosome then moves
to the next codon
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/