Central dogma of genetics

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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.
The DNA strand is converted into an
RNA, or what is called mRNA , strand in a
process called
transcription

2.
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].

Fig
1
:
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.



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)











Adapted from

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
.





tRNA

Anti
-
codon

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
MC
1370565
/