Transcript BIO 101: Transcription and Translation

Gene Expression :
Transcription and Translation
3.4 & 7.3
How Are Different Types of Cells Created and Maintained?
By differential gene expression.
The same genetic information is in all 100 trillion cells of any
one person. Different cells use the same blueprint in different
ways.
How?
In essence, the control of gene
expression occurs by regulating
the flow of information from DNA
to protein.
The “Central Dogma” of Molecular Genetics
Transcription Translation
DNA
RNA
Protein
RNA processing
Trait
Review

DNA
Made up of 4
different
nucleotides

RNA
Made up of 4
different
nucleotides

Proteins
Made up of 20
different
amino acids
Gene Expression in
Prokaryotes vs. Eukaryotes

Transcription
and translation
may occur
simultaneously

Transcription and translation are
separated in time and space
One Gene - One Polypeptide
Hypothesis



Theory: one gene codes for one polypeptide
Some proteins are composed of a number of
polypeptide chains. In this theory each chain has
its own gene.
However, eukaryotic genes are much more
complex and this is not always the case!
• Some genes control the expression of other
genes
• Some genes code for RNA which do not
produce polypeptides
The Genetic Code


Problem: How do only 4 different nucleotides
code for the 20 amino acids that make up
proteins?
Solution: Each group of 3 nucleotides codes
for a different amino acid. These 3 nuclotide
units are called codons. 1 codon codes for 1
amino acid.
DNA
RNA
C
G
A
U
T
A
Amino Acid
Valine
Reading Frame
The 3-nucleotide
units (codons)
must be read in
the correct
reading frame


Start codons
determine
the reading
frame
The Genetic Code Dictionary



There are
multiple codons
for each amino
acid
AUG is always
the start
codon
UAA, UGA, and
UAG are stop
codons
Characteristics of the Genetic Code

The genetic code is degenerate
• More than one codon codes for an
amino acid

The genetic code is universal
(almost)
• Almost all organisms on Earth use the
same code.
Transcription



DNA is used to make a strand of RNA
called the primary transcript (pre-mRNA)
The pre-mRNA is
further processed to
create the finished
mRNA
mRNA exits the
nucleus to be
translated
DNA has two strands
1) Anti-sense strand
(Template strand) (Runs
3’ 5’) – the strand of
DNA that is transcribed.
Has the complementary
genetic code of the
mRNA. (Runs 3’ 5’)
2) Sense strand (coding
strand) (runs 5’  3’) –
the strand of DNA that is
not transcribed. It has the
same genetic code as the
mRNA (except U instead
of T).
Transcription

3 main steps
1. Initiation
2. Elongation
3. Termination
Transcription Initiation


RNA polymerase binds to DNA at a region
called the promoter
RNA polymerase unwinds the DNA and adds
nucleotides in the 5’ → 3’ direction
Transcription Elongation

RNA polymerase moves along the DNA strand,
adding 60 nucleotides/sec

DNA strands
rejoin after
polymerase
passes by
Transcription Termination



Polymerase stops
when it reaches a
DNA sequence called
the terminator
The mRNA has
been completely
transcribed
In eukaryotes, this
is pre-mRNA and
must be further
processed
mRNA Processing


In eukaryotes, pre-mRNA must be further processed to
mRNA before it leaves the nucleus
Guanine is added to 5’
end, forming the 5’ cap


100’s of adenines are added to
3’ end, forming the poly-A tail
Non-coding regions of RNA are spliced out


Intron (non-coding sequences) are cut out by
spliceosomes. Leaving only Exons (Coding sequences)
making up the mRNA that leaves the nucleus.
Alternative splicing patterns means one gene can make
more than one protein
mRNA Splicing
Translation




The process in which mRNA is used to make
proteins
Occurs in the cytoplasm using ribosomes
Requires tRNA (transfer RNA) bound to an
amino acid
3 steps: initiation, elongation, termination
Structure of tRNA



A clover-shaped RNA molecule
Bottom loop has an anti-codon
complementary to the mRNA codon
3’ end has an aa attachment site
with the sequence “ACC” (CCA read
from 5’ 3’)
Amino acid
attachment



RNA is made in the
nucleus
Amino acids float
free in the
cytoplasm
Aminoacyl-tRNA
synthase joins
each amino acid to
the appropriate
tRNA
Ribosomes



3 tRNA
binding
sites
2 subunits
Composed of
proteins and rRNA
Translation: Initiation

mRNA, tRNA and small
ribosomal subunit bind
with the P site at the start
codon (AUG = Met)

Large subunit
binds using
energy from GTP
Translation: Elongation
•mRNA is read 3 nucleotides at a time (Codons)
•tRNA brings corresponding amino acid into the A site of the
ribosome
Transition Elongation





Ribosome catalyses
dehydration synthesis
reaction between aa’s in
P site and A site forming
a peptide bond between
aa’s
Growing polypetpide now
attached to tRNA in A
site
Ribosome moves forward
one codon
Free tRNA in P site exits
out the back of ribosome
(out of E site)
tRNA (with polypeptide)
moves into P site
Translation: Termination


Elongation continues until reaching a stop codon
Release factor binds and hydrolyzes the bond
between the last tRNA and its a.a., freeing the
new polypeptide chain
Polyribosomes

Many ribosomes
may
simultaneously
translate from a
single mRNA
Gene
Expression:
Overview