Download Genetic flow of information

Genes of cancer
• Cancer is a disease of abnormal cells
• Cancer cells proliferate in an uncontrolled
fashion
• The causes of cancer are quite diverse but
the central role is played by DNA
mutations in development of cancer
Chemicals
spontaneous
Oncogenes
radiation
Mutation
Cancer
Tumour
suppressor
genes
infection
Errors in DNA
replication
Cancer related mutations affect the same
two classes of genes : Oncogenes and
tumour suppressor genes
• Oncogenes are defined as genes whose presence can lead to
cancer. They arise by mutation from normal genes (proto
oncogenes) that code for proteins involved in stimulating cell
proliferation and survival. By producing abnormal forms or excessive
quantities of these proteins, oncogenes contribute to the
uncontrolled proliferation and survival of cancer cells.
• In contrast to oncogenes which are abnormal genes, tumour
suppressor genes are normal genes whose deletion or loss
function can likewise lead to cancer. Tumour suppressor gene
produce proteins that either directly or indirectly exert a restraining
influence on cell proliferation and survival. The loss of such proteins
can therefore allow cell proliferation and survival to evade normal
restraints and controls.
Genetic flow of information
• Chromosomes within our cells have
roughly 30,000genes.
• Each gene codes for a RNA molecule that
is either directly used or used as a guide
for the formation of protein
• DNA (store) to RNA (working form ) to
protein (product)
THE FLOW OF GENETIC INFORMATION
DNA
1
RNA
2
PROTEIN
3
DNA
1. REPLICATION
2. TRANSCRIPTION
3. TRANSLATION
(DNA SYNTHESIS)
(RNA SYNTHESIS)
(PROTEIN SYNTHESIS)
RNA polynucleotide chain
• 2’ -OH makes
3’, 5’ phosphodiester
bond unstable
DNA polynucleotide chain
05_06_compl_pairs.jpg
01_06.jpg
Transcription
• The process in which a particular section
of DNA (genes) are used to produce RNA
is known as transcription
• Goal of transcription is to make an RNA
copy of a gene.
• Only a small percentage of genes are
actually being used to make RNA at a
particular time in a particular cell
The transcription process is tightly
regulated in normal cells.
• Genes must be transcribed at the correct
time.
• The RNA produced from the gene must be
made in correct amount
• Only the required gene must be
transcribed.
• Turning transcription off is just as
important as turning it on.
The steps of transcription
• A transcription factor recognises the start site (promoter)
of a gene to be transcribed
• The enzyme that makes RNA (RNA polymerase) binds to
transcription factor and recognises the start region
• The enzyme proceeds down the DNA making a copy
until the end is reached.
• The enzyme falls of and RNA is released. This copying
process may be repeated numerous times
• If the RNA is one that codes for protein it will leave the
nucleus to enter the cytosol
Transcription and promoter elements for RNA polymerase II
+1
transcription
element
TE
P
exon
transcription unit
exon
promoter
Promoter (DNA sequence upstream of a gene)
• determines start site (+1) for transcription initiation
• located immediately upstream of the start site
• allows basal (low level) transcription
Transcription element (DNA sequence that regulates the gene)
• determines frequency or efficiency of transcription
• located upstream, downstream, or within genes
• can be very close to or thousands of base pairs from a gene
• includes
enhancers (increase transcription rate)
silencers (decrease transcription rate)
response elements (target sequences for signaling molecules)
• genes can have numerous transcription elements
Proteins regulating eukaryotic mRNA synthesis
General transcription factors
• TFIID (a multisubunit protein) binds to the TATA box
to begin the assembly of the transcription apparatus
• the TATA binding protein (TBP) directly binds the TATA box
• TBP associated factors (TAFs) bind to TBP
• TFIIA, TFIIB, TFIIE, TFIIF, TFIIH1, TFIIJ assemble with TFIID
RNA polymerase II binds the promoter region via the TFII’s
Transcription factors binding to other promoter elements and
transcription elements interact with proteins at the promoter
and further stabilize (or inhibit) formation of a functional
preinitiation complex
1TFIIH
is also involved in phosphorylation of RNA polymerase II, DNA repair
(Cockayne syndrome mutations), and cell cycle regulation
Initiation of transcription and promoter clearance
E
B
F
TFIID
H
initiation
TBP
J
+1
RNA pol II
CTD
P
P
P
• RNA pol II is phosphorylated by TFIIH on the carboxy terminal
domain (CTD), releasing it from the preinitiation complex and
allowing it to initiate RNA synthesis and move down the gene
Transcription (elongation)
/antisense strand
Transcription (termination)
RNA polymerase falls off
terminator
Coding strand 5’TACGCTGCCCAAGCA
Template strand 3’ATGCGACGGGTTCGT
RNA sequence
Animation:
http://www.phschool.com/science/biology_place/biocoach/images/transcription/tcani.gif
Post-transcriptional
modifications
Transcription factors
• The inappropriate activity of transcription factors has
been identified in almost all types of known cancers,
Some examples of transcription factors that malfunction
in human cancers.
• P53- The protein that the p53 codes for is important
because it controls the transcription of genes involved in
causing cells to divide.
• Rb- The protein product of this gene works by blocking
other transcription factors thus preventing transcription of
key genes required for cell division to progress.
• The oestrogen receptor (ER) This protein binds
oestrogen and the combination acts as transcription
factor to turn on genes that enable target cells to divide.
What is translation
• After hnRNA production through the process of transcription, it is
processed in the nucleus to produce mRNA which is then released
into cytosol.
• The mRNA is then recognised by the ribosomal subunits and the
message is read by the ribosome to produce a protein. The
information for the direction of protein synthesis is encoded in
nucleotide sequence that makes up mRNA. Groups of three
nucleotides (codons) are read by ribosomes and lead to the
insertion of a particular amino acid in growing peptide.
• After the protein is formed it is folded to perform its function in the
cell The proper folding, transportation, activity and eventual
destruction of protein are all highly regulated processes. The genes
that control these processes are often found to be damaged and
malfunction in cancer cells.
Messenger RNA (mRNA)
5’
Cap
m7Gppp
5’ untranslated region
initiation
codon
AUG
translated (coding) region
UGA
3’ untranslated region
AAUAAA
termination
codon
(AAAA)n 3’
poly(A)
tail
Protein translation: summary
Elongation
Initiation
Termination
http://www.phschool.com/science/biology_place/biocoach/translation/init.html
Reading frame
• reading frame is determined by the AUG initiation codon
• every subsequent triplet is read as a codon until reaching a stop codon
...AGAGCGGA.AUG.GCA.GAG.UGG.CUA.AGC.AUG.UCG.UGA.UCGAAUAAA...
MET.ALA.GLU.TRP.LEU.SER.MET.SER
• a frameshift mutation
...AGAGCGGA.AUG.GCA.GA .UGG.CUA.AGC.AUG.UCG.UGA.UCGAAUAAA...
• the new reading frame results in the wrong amino acid sequence and
the formation of a truncated protein
...AGAGCGGA.AUG.GCA.GAU.GGC.UAA.GCAUGUCGUGAUCGAAUAAA...
MET.ALA.ASP.GLY
Cell division and mitosis
• For mitosis to take place the following must occur;
• The genetic material , the DNA in chromosomes , must
be faithfully copied. This occurs via a process known as
replication
• The organelle, such as mitochondria , must be
distributed so that each daughter cell receives adequate
amount to function
• The cytoplasm of the cell must be physically separated
into two different cells.
• Many features of cancer cells are due to defects in the
genes that control cell division.
The mammalian cell cycle
DNA synthesis and
histone synthesis
Rapid growth and
preparation for
DNA synthesis
S
phase
G0
Quiescent cells
G1
phase
G2
M
phase
Mitosis
phase
Growth and
preparation for
cell division
Overview of the major events in
mitosis
Interphase
prophase
metaphase
anaphase
telophase
In case of DNA damage or failure of critical processes
P53 stimulates induction of inhibitory
proteins that halt DNA replication
Defects in p53 are associated with a
variety of cancers
DNA damage repair or initiation of programmed cell death (apoptosis)
Chromosomes and genes
DNA synthesis
• Occurs in the S-phase
• Every chromosome is copied with high fidelity.
• In this process double stranded DNA is unwound and each
individual strand is used as a template for the production of
complimentary strand.
• Errors may occur during replication that lead to changes in the
nucleotide sequence of the chromosomes. If these changes occur
within genes they can alter function of the cell. Human cells have
evolved several mechanism to correct errors of this type but they are
not perfect.
• These mistakes can lead to mutated genes.
• Accumulation of mutations can lead to the development of cancer
• There are several cancers types that are associated specifically with
breakdown of repair processes.
DNA replication is semi-conservative
Parental DNA strands
Each of the parental strands serves as a
template for a daughter strand
Daughter DNA strands
Daughter strand
Parent strand 1
5’GATCCTAGGTACTGACCTTGC3’
Parent strand 2
3’CTAGGATCCATGACTGGAACG5’
Daughter strand
Features of DNA Replication
• DNA replication is semiconservative
– Each strand of template DNA is being copied.
• DNA replication is bidirectional
– Bidirectional replication involves two replication forks,
which move in opposite directions
• DNA replication is semidiscontinuous
– The leading strand copies continuously
– The lagging strand copies in segments (Okazaki
fragments) which must be joined
Mechanisms of Repair
• Mutations that occur during DNA replication are repaired
when possible by proofreading by the DNA polymerases
• Mutations that are not repaired by proofreading are repaired
by mismatched (post-replication) repair followed by
excision repair
• Mutations that occur spontaneously and in response to
mutagens at any time are repaired by excision repaired (base
excision or nucleotide excision)
Mismatched (post-replication) repair
• the parental DNA strands are
methylated on certain
adenine bases
CH3
5’
3’
CH3
• the mutations are repaired
by excision repair mechanisms
• after repair, the newly
replicated strand is methylated
CH3
• mutations on the newly
replicated strand are
identified by scanning
for mismatches prior to
methylation of the newly
replicated DNA
CH3
Some common type of DNA
damage
• Depurination involves loss of the base
adenine or guanine caused by hydrolysis
of the bond linking it to DNA chain
• Deamination involves the removal of an
amino group by hydrolysis
• Pyrmidine dimers are created by an
environmental mutagen, the UV radiation
in sunlight
Deamination of cytosine can be repaired
Deamination of 5-methylcytosine cannot be repaired
More than 30% of all single base changes that have been detected
as a cause of genetic disease have occurred at 5’-mCG-3’ sites
Excision repair (base or nucleotide)
deamination
ATGCUGCATTGA
TACGGCGTAACT
uracil DNA glycosylase
ATGC GCATTGA
TACGGCGTAACT
repair nucleases
AT
GCATTGA
TACGGCGTAACT
DNA polymerase b
ATGCCGCATTGA
TACGGCGTAACT
DNA ligase
thymine dimer
ATGCUGCATTGATAG
TACGGCGTAACTATC
excinuclease
AT (~30 nucleotides) AG
TACGGCGTAACTATC
DNA polymerase b
ATGCCGCATTGATAG
TACGGCGTAACTATC
DNA ligase
ATGCCGCATTGA
TACGGCGTAACT
ATGCCGCATTGATAG
TACGGCGTAACTATC
Base excision repair
Nucleotide excision repair
Defects in DNA repair or replication
• Xeroderma pigmentosum
• Ataxia telangiectasia
• Fanconi anemia
• Bloom syndrome
100
human
• Cockayne syndrome
elephant
Life span
cow
10
Correlation between DNA repair
activity in fibroblast cells from
various mammalian species and
the life span of the organism
hamster
rat
mouse
shrew
1
DNA repair activity
The control of cell division.
• Is the DNA fully replicated?
• Is the DNA damaged?
• Are there enough nutrients to support cell
growth
• If these checks fail normal cells will stop
dividing
• Cancer cells do not obey these rules and
will continue to grow and divide.
18_17_arrest_checkpt.jpg
The control of cell division
• Cells divide in response to external signals
• What are the signals that make cells stop
dividing
• A lack of positive external signals
• Contact inhibition
• Cellular Senescence
18_23_01_mitogens.jpg
18_23_02_mitogens.jpg
Cell division in cancer cells
• Cancer cells can divide without
appropriate external signals
• Cancer cells do not exhibit contact
inhibition
• Cancer cells divide without receiving the
‘all clear’ signal.