Chapter 12 and 13: Transcription and Translation

1
Chapter 12 and 13 Transcription and
Translation Lecture 12 October 28, 2003
Whats due? CH6 and CH10 problem set (if you
havent all ready turned it in) CH 11 problem set
2
Review Molecular Basis of Genetics, so far
Structure
DNA as the genetic material
Griffith transforming principle
Avery, MacLeod and McCarty - DNA was the
transforming principle
Hershey and Chase - DNA was the genetic material
Composed of nucleotides deoxyribose
phosphate group nitrogenous base
Strands are antiparallel and complementary A
T C - G
3
Review Molecular Basis of Genetics, so far
Replication
Mode of DNA Replication
Meselson and Stahl heavy and light
nitrogen isotopes
Origin of replication
Bi-directional
Roles of each polymerase (prokaryotes) DNA
polymerase I - primer removal, gap-filling
synthesis DNA polymerase II - DNA repair
DNA polymerase III - main
replication enzyme
At least six DNA polymerases in eukaryotes
4
Review A Coherent Model of DNA Replication
Helicases unwind helix (DnaA, B and C)
SSBPs prevent closure
DNA gyrase reduces tension
Association of
core polymerase with template Primase
synthesizes short RNA primer
DNA synthesis (DNA pol III)
Primer removal and replacement with DNA
(DNA pol I) Ligase closes up the gaps b/w
Okazaki fragments
5
Gene Expression Transcription and Translation
Gene expression mechanism by which hereditary
factors are coded for and expressed (to cause a
gene to manifest its effects in the phenotype or
the detectable effect of a gene)
Gene unit of inheritance which occupies a
specific chromosomal location
KSM A DNA sequence that produces a functional
RNA molecule
Type of RNA Encodes Copies/genome
mRNA Functional protein Single or few
tRNA Molecule needed for translation Few
rRNA Component of ribosomes Many
Non (protein) coding RNAs
6
Gene Expression
Protein coding gene - A DNA sequence coding for a
single polypeptide
Gene expression mechanism by which hereditary
factors are coded for and expressed
Genes control inherited variation via DNA,
RNA and protein
Transcription transfer of genetic information
from DNA via synthesis of RNA
Translation the formation of a protein,
directed by an mRNA in association with a ribosome
7
Gene A Molecular Description
In eukaryotes introns and exons
Non-coding regions contains nucleotide sequence
that will get transcribed BUT not translated
Regulatory regions sequence involved in the
control of expression of a given gene, usually
involves interaction with another molecule
8
Gene A Molecular Description
Only one of the two strands encodes the mRNA for
a given gene Template strand coding strand
sense strand template for transcription Non-te
mplate strand nonsense strand RNA transcript
is exactly the same as the non-sense strand
Given that RNA polymerase synthesizes RNA in a 5
to 3 direction, which strand is the template
strand?
Transcript will always look like the non-sense
strand
9
Transcription
Transcription the process by which RNA
molecules are synthesized on a DNA template
RNA polymerase enzyme that copies template
strand to build an RNA molecule
-synthesis in 5 to 3 direction
nucleotides added to 3-OH
growing strand has base complementarity
to template strand unlike DNA pol, no primer
required
reminder RNA contains ribose, phosphate group
and A, C, G and U (not T)
RNA polymerase (from E. coli )
10
Transcription
Factor something that cycles on and off core
complexes
Multiple types of sigma factors in bacterial
cells - regulation
Promotors - sequence involved in the control of
expression of a given gene, site where RNA
polymerase binds
Serve three different functions 1. ON/OFF
switch 2. Speed switch 3.
Alignment
11
Transcription in Eukaryotes
RNA polymerases RNA polymerase I rRNA (18S,
28S) RNA polymerase II mRNA RNA
polymerase III small RNAs ( tRNA, 5S rRNA,
snRNAs)
Eukaryotic promotors
Enhancers
12
Transcription in Eukaryotes
Generalized Transcription Factorsgroup of
proteins that bind the -25 region
Transcription Factor for RNA polymerase II
TFIIA, TFIIB, etc.
TFIIs not enough! Need factors that bind -80
and enhancers
1. ON/OFF switch -25 region 2. Speed
switch enhancers
Elongation very similar in prokaryotes and
eukaryotes
Termination -Transcription stops - Polymerase and
RNA are released from DNA - DNA rehybdridizes
13
RNA processing in Eukaryotes
Immature RNA mature RNA
Addition of a cap at 5 end- guanyltransferase
makes mRNA more stable, required for translation
Addition of a poly A tail poly A polymerase
mRNA stability, translation
Introns spliced out by spliceosome machinery