gene expression [autosaved].pptx
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Kevin White
Present
Moham
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Whats a gene?
gene (jn)n.
A hereditary unit consisting of a sequence of DNA
that occupies a specific location on a chromosome
and determines a particular characteristic in an
organism. Genes undergo mutation when their
DNA sequence changes.
[German Gen, from gen-, begetting, in Greek words
(such as genos, race, offspring).
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And so the gene is
A segment of DNA
It is inherited
It is transcribed
It may be translated
It has regulatory andother functional regions
it has certain coordinateand sequence
genes in different organimay be homologous
its sequence may beoptimized for transcrip
its sequence may beoptimized for translatio
the structure and sequeof these regions may havsomething in common
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Where are the genes located?
Genes are located on the
chromosomes.
Every species has a differentnumber of chromosomes.
There are two types ofchromosomes: autosomes and
sex chromosomes
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Genes are located on the chromosomes which arefound in the nucleus of a cell.
When a cell is undergoing cell reproduction, the
chromosomes are visible. Chromosomes appearwhen the chromatin condenses and become visible
Most of the time (90%) the genetic material in theform of chromatin.
A genome is the complete genetic information
contained in an individual.
(gene + chromosome)
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Gene Expression
Gene expressionis the process bywhich a genes information is converted intothe structures and functions of a cell by aprocess of producing a biologically functionalmolecule of either protein or RNA (geneproduct) is made.
Gene expression is assumed to becontrolled at various points in the sequenceleading to protein synthesis.
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Gene Structure
Eukaryotic gene structure: Mosteukaryotic genes in contrast to typicalbacterial genes, the coding sequence(exons) are interrupted by noncodingDNA (introns).
The gene must have
( Exon; start signals; stop sregulatory control elemen
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The Central Dogma
Proposed by Francis Crick 1958
DNA holds the coded hereditary information in thenucleus
This code is expressed at the ribosome during protesynthesis in the cytoplasm
The protein produced by the genetic information is
what is influenced by natural selection If a protein is modified it cannot influence the gene
that codes for it
Therefore there is one way flow of information:
DNA RNA Protein
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The central dogma of molecular biology
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Overview of Gene ExpressionAn organism may contain many types of somatic cells, each with distinct shape and function. How
they all have the same genome. The genes in a genome do not have any effect on cellular functio
they are "expressed". Different types of cells express different sets of genes, thereby exhibiting
shapes and functions.
Figure. Essential
steps involved in the
expression of protein
genes.
Gene expression" means the
production of a protein or a fun
RNA from its gene. Several ste
required:
Transcription: A DNA strand is
the template to synthesize a RN
strand, which is called the prim
transcript.
RNA processing: This step invo
modifications of the primary tra
to generate a mature mRNA (fo
protein genes) or a functional t
rRNA.
For RNA genes (tRNA and rRNA
expression is complete after a
functional tRNA or rRNA is
generated. However, protein g
require additional steps:
Nuclear transport: mRNA has t
transported from the nucleus to
cytoplasm for protein synthesis
Protein synthesis: In the cytop
mRNA binds to ribosomes, whic
synthesize a polypeptide based
sequence of mRNA.
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TRANSCRIPTION
TRANSLATION
DNA
mRNA
Ribosome
Polypeptide
(a) Bacterial cell
Nuclearenvelope
TRANSCRIPTION
RNA PROCESSINGPre-mRNA
DNA
mRNA
TRANSLATION Ribosome
Polypeptide
(b) Eukaryotic cell
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Transcription
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Transcription: The synthesis of a strand of mR(and other RNAs)
Uses an enzyme RNA polymerase Proceeds in the same direction as replication (5 to 3
Forms a complementary strand of mRNA
It begins at a promotor site which signals thebeginning of gene is not much further down the
molecule (about 20 to 30 nucleotides) After the end of the gene is reached there is a
terminator sequence that tells RNA polymerase tostop transcribing
NB Terminator sequence terminator codon.
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Process of Transcription
Transcription starts with RNA polymerase binding to the promoter.
This binding only occurs under some conditions: when the gene is on.Various other proteins (transcription factors) help RNA polymerase bind the promoter. Other DNA sequences further upstream from the promotare also involved.
Once it is bound to the promoter, RNA polymerase unwinds a small sectiof the DNA and uses it as a template to synthesize an exact RNA copy ofthe DNA strand.
The DNA strand used as a template is the coding strand; the other strais the non-coding strand. Notice that the RNA is made from 5 end to 3end, so the coding strand is actually read from 3 to 5.
RNA polymerase proceeds down the DNA, synthesizing the RNA copy.
In prokaryotes, each RNA ends at a specific terminator sequence. Ineukaryotes transcription doesnt have a definite end point; the RNA isgiven a definitive termination point during RNA processing.
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Transcription Bubble
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RNA Processing (Pre-mRNA mRNA)
Capping Splicing
Addition of poly A tail
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The primary product of RNA transcription; thehnRNAs contain both intronic and exonic sequences
These hnRNAs are processed in the nucleus to give
mature mRNAs that are transported to the cytoplasmwhere to participate in protein synthesis.
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RNA Processing
Capping
The cap structure is added to the 5' of the newly transcribed mRNAin the nucleus prior to processing and subsequent transport of the mmolecule to the cytoplasm.
Splicing:Step by step removal of pre mRNA and joining of remaining exons
place on a special structure called spliceosomes.
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RNA Processing
Addition of poly A tail:Synthesis of the poly (A) tail involves cleavage of its 3' end and th
addition of about 40- 200 adenine residues to form a poly (A) tail.
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Transcription plan
Transcription
DNA
messenger
RNA
Gene
Nucleus
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The Genetic Code
Translation of nucleic acids into am
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The words of the DNA language are triplets of bacalled codons
The codons in a gene specify the amino acid sequence of apolypeptide
Translation of nucleic acids into amacids
Codons: Triplets of Bases
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Codons: Triplets of Bases
The flow of information from gene to protein isbased on a triplet code: a series of nonoverlappin
three-nucleotide words These triplets are the smallest units of uniform
length that can code for all the amino acids
Example: AGT at a particular position on a DNAstrand results in the placement of the amino acidserine at the corresponding position of thepolypeptide to be produced
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During transcription, one of the two DNA strandscalled the template strand provides a template fo
ordering the sequence of nucleotides in an RNAtranscript
During translation, the mRNA base triplets, called
codons, are read in the 5 to 3 direction
Each codon specifies the addition of one of 20amino acids
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Features of the Code all 64 codons have been assigned
61 code for amino acids
3 (UAA, UAG, and UGA) serve as termination signals
AUG also serves as an initiation signal (also Met)
only Trp and Met have one codon each
more than one triplet can code for the same amino acid; Leuand Arg, for example, are each coded for by six triplets
(degenerate) the third base is irrelevant for Leu, Val, Ser, Pro, Thr, Ala, Gly,
Arg
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Features of the Code
for the 15 amino acids coded for by 2, 3, or 4 triplets, it is only the
of the codon that varies. Gly, for example, is coded for by GGA, GGand GGU
the code is almost universal: it the same in viruses, prokaryotes, aeukaryotes; the only exceptions are some codons in mitochondria
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DNA molecule
Gene 1
Gene 2
Gene 3
DNA strand
TRANSCRIPTION
RNA
Polypeptide
TRANSLATION
Codon
Amino acid
Virtually all organisms share the same genetic cod
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U C A G
U
C
A
G
UUU
UUC
UUA
UUG
CUU
CUC
CUA
CUG
AUU
AUC
AUAAUG
GUU
GUC
GUA
GUG
phe
leu
leu
ile
met (start)
val
UCU
UCC
UCA
UCG
CCU
CCC
CCA
CCG
ACU
ACC
ACAACG
GCU
GCC
GCA
GCG
ser
pro
thr
ala
UAU
UAC
UAA
UAG
CAU
CAC
CAA
CAG
AAU
AAC
AAGAAA
GAU
GAC
GAA
GAG
tyr
stop
stop
his
gln
asn
lys
asp
glu
UGU
UGC
UGA
UGG
CGU
CGC
CGA
CGG
AGU
AGC
AGAAGG
GGU
GGC
GGA
GGG
cys
stop
trp
arg
ser
arg
gly
FirstBase
Second Base
y g g
unity of life
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Translation
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Translation
Translation is the process by which ribosomes read the genetic
message in the mRNA and produce a protein product accordinthe message's instruction.
l tid
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polypeptides
Codons
tRNAmolecules
mRNA
Growingpolypeptide
Largesubunit
Smallsubunit
mRNA
mRNA
bindingsite
P site A site
P A
Growingpolypeptide
Next a
to be polype
An initiation codon marks th
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t at o codo a s tstart of an mRNA message
End
Start of genetic message
AUG = methionine
mRNA a specific tRNA and the ribosome subunits
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mRNA, a specific tRNA, and the ribosome subunitsassemble during initiation
1
Initiator tRNA
mRNA
Startcodon Small ribosomal
subunit
2
P site
Largeribososubun
A site
Amino acid
Polypeptide
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1 Codon recognition
Anticodon
AsiteP site
Polypeptide
2 Peptide bond
formation
3 Translocation
Newpeptide
bond
mRNAmovement
mRNA
Stopcodon
Summary of TRANSCRIPTION
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Summary oftranscription andtranslation 1Stage mRNA
transcribed from
DNA template.
Anticodon
DNA
mRNA
RNApolymerase
TRANSLATION
Enzyme
Amino acid
tRNA
Initiator
tRNA Largeribosomalsubunit
Smallribosomalsubunit
mRNA
StartCodon
2Stage Each am
acid attaches to itsproper tRNA with thelp of a specificenzyme and ATP.
3Stage Initiatiopolypeptide synt
The mRNA, the fi
tRNA, and theribosomal subuncome together.
TRANSCRIPTION
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4Stage ElongationGrowingpolypeptide
Codons
5Stage Termination
mRNA
Newpeptidebondforming
Stop Codon
The ribosome recognizesa stop codon. The poly-peptide is terminated andreleased.
A succession of tRNAs
add their amino acids tothe polypeptide chain asthe mRNA is movedthrough the ribosome,one codon at a time.
Polypeptide
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