Download Document

Chromosome structure and chemical
modifications can affect gene expression
• DNA packing
© 2015 Pearson Education, Inc.
Chromosome structure and chemical
modifications can affect gene expression
• Methylation- Chemical modification of DNA bases
or histone proteins can result in epigenetic
inheritance
© 2015 Pearson Education, Inc.
Chromosome structure and chemical
modifications can affect gene expression
 X inactivation
Early Embryo
Adult
Two cell populations
Cell division
and random
X
Orange
chromosomes X chromosome Active X
fur
inactivation Inactive X
Allele for
Allele for
orange fur black fur
© 2015 Pearson Education, Inc.
Inactive X
Active X
Black fur
The Control of Gene Expression





Each cell in the human contains all the genetic
material for the growth and development of a
human.
Some of these genes will be need to be
expressed all the time.
These are the genes that are involved in of vital
biochemical processes such as respiration.
Other genes are not expressed all the time.
They are switched on an off at need.
Operons
An operon is a group
of genes that are
transcribed at the
same time.
 They usually control
an important
biochemical process.
 They are only found
in prokaryotes.

Different ways to Regulate Metabolism
Feedback inhibition
block transcription
Tryptophan operon
Repressor is inactive alone
Lactose operon
Repressor is active alone
Lactose absent
Repressor
protein
DNA
I
O
Regulator
gene
Operator
site
RNA
polymerase
Blocked
z
y
lac operon
a
Lactose present
DNA
O
I
z
y
Promotor site
Activator
protein steadies
the RNA
polymerase
a
Transcription
DNA
I
O
z
y
Promotor site
a
Complex assemblies of proteins control
EUKARYOTIC transcription
• In eukaryotes, activator proteins seem to be more important
than repressors. Thus, in multicellular eukaryotes, the
default state for most genes seems to be off.
• A typical plant or animal cell needs to turn on and
transcribe only a small percentage of its genes.
• Eukaryotic RNA polymerase requires the assistance of
proteins called transcription factors.
• RNA polymerase then attaches to the promoter, and
transcription begins.
© 2015 Pearson Education, Inc.
Animation: Initiation of Transcription
© 2015 Pearson Education, Inc.
Enhancers
Promoter
Gene
DNA
Activator
proteins
Transcription
factors
Other
proteins
DNA-bending
protein
RNA polymerase
Bending
of DNA
Transcription
© 2015 Pearson Education, Inc.
Eukaryotic RNA may be spliced in
more than one way
• Alternative RNA splicing
• produces different mRNAs from the same transcript
and
• results in the production of more than one
polypeptide from the same gene.
• In humans, more than 90% of protein-coding
genes appear to undergo alternate splicing.
© 2015 Pearson Education, Inc.
Animation: RNA Processing
© 2015 Pearson Education, Inc.
Small RNAs play multiple roles in controlling
gene expression
• Only about 1.5% of the human genome codes for
proteins. (This is also true of many other
multicellular eukaryotes.)
• Another small fraction of DNA consists of genes for
ribosomal RNA and transfer RNA.
• A flood of recent data suggests that a significant
amount of the remaining genome is transcribed
into functioning but non-protein-coding RNAs,
including a variety of small RNAs.
© 2015 Pearson Education, Inc.
Small RNAs play multiple roles in controlling
gene expression
• microRNAs (miRNAs) can bind to complementary
sequences on mRNA molecules either
• degrading the target mRNA or
• blocking its translation.
• RNA interference (RNAi) is the use of miRNA to
artificially control gene expression by injecting
miRNAs into a cell to turn off a specific gene
sequence.
© 2015 Pearson Education, Inc.
The flow of genetic information from a chromosome
to a protein is controlled at several points, just as the
flow of water through pipes is controlled by valves.
DNA unpacking
Chromosome
NUCLEUS
Gene
DNA
Transcription
Exon
Splicing
Addition of a
cap and tail
Flow through
nuclear envelope
mRNA in
nucleus
Cap
Intron
RNA
transcript
Tail
mRNA in
cytoplasm
Breakdown
of mRNA
Broken-down mRNA
Translation
Polypeptide
Cleavage,
modification,
activation
Active
protein
Breakdown
of protein
Amino
acids
© 2015 Pearson Education, Inc.
CYTOPLASM