Gene Expression and Control

1
Gene Expression and Control

• Chapter 10

2
Ricin and Your Ribosomes

• Ricin is a potential weapon of bioterrorism
• Derived from castor beans
• Binds to ribosomes
• Disrupts protein synthesis

3
Steps from DNA to Proteins

• Same two steps produce all proteins
• 1) DNA is transcribed to form RNA
• Occurs in the nucleus
• RNA moves into cytoplasm
• 2) RNA is translated to form polypeptide chains,
  which fold to form proteins

4
Three Classes of RNAs

• Messenger RNA
• Carries protein-building instruction
• Ribosomal RNA
• Major component of ribosomes
• Transfer RNA
• Delivers amino acids to ribosomes

5
A Nucleotide Subunit of RNA
uracil (base)
phosphate group
ribose (sugar)
6
Base Pairing during Transcription
DNA
G C A U
RNA
G C A T
DNA
C G T A
C G T A
DNA
base pairing in DNA replication
base pairing in transcription
7
Transcription

• Like DNA replication
• Nucleotides added in one direction
• Unlike DNA replication
• Only small stretch is template
• RNA polymerase catalyzesnucleotide addition
• Product is a single strand of RNA

8
Promoter

• A base sequence in the DNA that signals the start
  of a gene
• For transcription to occur, RNA polymerase must
  first bind to a promoter

9
Gene Transcription
DNA to be transcribed unwinds
transcribed DNA winds up again
mRNA transcript
RNA polymerase
10
Adding Nucleotides
5
3
growing RNA transcript
DNA
direction of transcription
5
3
11
Transcript Modification
unit of transcription in a DNA strand
3
5
exon
intron
exon
exon
intron
transcription into pre-mRNA
poly-A tail
cap
5
3
5
3
mature mRNA transcript
12
Genetic Code

• Set of 64 base triplets
• Codons
• 61 specify amino acids
• 3 stop translation

13
tRNA Structure
codon in mRNA
anticodon
amino-acid attachment site
amino acid
OH
14
Ribosomes
tunnel
small ribosomal subunit
large ribosomal subunit
intact ribosome
15
Three Stages of Translation

• Initiation
• Elongation
• Termination

16
Initiation

• Initiator tRNA binds to small ribosomal subunit
• Small subunit/tRNA complex attaches to mRNA and
  moves along it to an AUG start codon
• Large ribosomal subunit joins complex

17
Binding Sites
binding site for mRNA
A (second binding site for tRNA)
P (first binding site for tRNA)
18
Elongation

• mRNA passes through ribosomal subunits
• tRNAs deliver amino acids to the ribosomal
  binding site in the order specified by the mRNA
• Peptide bonds form between the amino acids and
  the polypeptide chain grows

19
Elongation
20
Termination

• A stop codon moves into place
• No tRNA with anticodon
• Release factors bind to the ribosome
• mRNA and polypeptide are released

mRNA
new polypeptide chain
21
What Happens to the New Polypeptides?

• Some just enter the cytoplasm
• Many enter the endoplasmic reticulum and move
  through the cytomembrane system where they are
  modified

22
Overview
transcription
mRNA
rRNA
tRNA
mature mRNA transcripts
ribosomal subunits
mature tRNA
translation
23
Gene Mutations

• Base-pair substitutions
• Insertions
• Deletions

24
Base-Pair Substitution
a base substitution within the triplet (red)
original base triplet in a DNA strand
During replication, proofreading enzymes make a
substitution.
possible outcomes
or
original, unmutated sequence
a gene mutation
25
Frameshift Mutations

• Insertion
• Extra base added into gene region
• Deletion
• Base removed from gene region
• Both shift the reading frame
• Result in altered amino acid sequence

26
Frameshift Mutation
mRNA
parental DNA
amino acids
arginine
glycine
tyrosine
tryptophan
asparagine
altered mRNA
DNA with base insertion
altered amino- acid sequence
arginine
glycine
leucine
glutamate
leucine
27
Transposons

• DNA segments that move spontaneously about the
  genome
• When they insert into a gene region, they usually
  inactivate that gene

28
Mutations

• Each gene has a characteristicmutation rate
• Natural and synthetic chemicals, and radiation,
  can increase mutation rate
• Only mutations that arise in germ cells can be
  passed on to next generation
• Important evolutionary consequences

29
Mutagens

• Ionizing radiation (x rays)
• Nonionizing radiation (UV)
• Natural and synthetic chemicals

30
Gene Control

• Which genes are expressed in a cell depends upon
• Type of cell
• Internal chemical conditions
• External signals
• Built-in control systems

31
Mechanisms of Gene Control

• Controls related to transcription
• Transcript-processing controls
• Controls over translation
• Post-translation controls

32
Regulatory Proteins

• Can exert control over gene expression through
  interactions with
• DNA
• RNA
• New polypeptide chains
• Final proteins

33
Control Mechanisms

• Negative control
• Regulatory proteins slow down or curtail gene
  activity
• Positive control
• Regulatory proteins promote or enhance gene
  activities

34
Chemical Modifications

• Methylation of DNA can inactivate genes
• Acetylation of histones allows DNA unpacking and
  transcription

35
Gene Control in Prokaryotes

• No nucleus separates DNA from ribosomes in
  cytoplasm
• When nutrient supply is high, transcription is
  fast
• Translation occurs even before mRNA transcripts
  are finished

36
The Lactose Operon
operator
regulatory gene
gene 1
gene 2
gene 3
operator
transcription, translation
promoter
lactose operon
repressor protein
37
Low Lactose

• Repressor binds to operator
• Binding blocks promoter
• Transcription is blocked

38
High Lactose
allolactose
lactose
mRNA
RNA polymerase
gene 1
operator
operator
promoter
39
Controls in Eukaryotic Cells

• Controls of transcription
• Transcript-processing controls
• Controls over translation
• Controls following translation

40
Most Genes Are Turned Off

• Cells of a multicelled organism rarely use more
  than 510 percent of their genes at any given
  time
• The remaining genes are selectively expressed
• Cell differentiation starts in the embryo

41
Homeotic Genes

• Occur in all eukaryotes
• Master genes that control development of body
  parts
• Encode homeodomains (regulatory proteins)
• Homeobox sequence can bind to promoters and
  enhancers

42
X Chromosome Inactivation

• One X inactivated in each cell of female
• Creates a mosaic for X chromosomes
• Dosage compensation