Transcript BIO 101: Transcription and Translation

Gene Expression :
Transcription and Translation
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
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.
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
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
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 bound to amino acid

3 steps: initiation, elongation, termination
tRNA

A clover-shaped RNA
molecule
Structure of
tRNA


3’ end of tRNA
binds to
specific amino
acids
Anti-codon on
tRNA
complements
mRNA codon
tRNA Synthesis



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

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

Ribosome catalyses dehydration synthesis
reaction between amino acids in P site and A site

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

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 protein
Polyribosomes

Many ribosomes
may
simultaneously
translate from a
single mRNA
Gene
Expression:
Overview