Chapter 7 Genes and Protein Synthesis

Report
Chapter 7
GENES AND PROTEIN SYNTHESIS
ONE GENE-ONE POLYPEPTIDE HYPOTHESIS
DNA contains all of our
hereditary information
 Genes are located in our
DNA
 ~25,000 genes in our
DNA (46 chromosomes)
 Each Gene codes for a
specific polypeptide

MAIN IDEA

Central Dogma
 Francis
Crick (1956)
OVERALL
PROCESS
Gene 1
Gene 3
DNA molecule

Transcription
 DNA

Gene 2
to RNA
Translation
of
amino acids
into
polypeptide
 Using RNA
DNA strand
 Assembly
TRANSCRIPTION
RNA
Codon
TRANSLATION
Polypeptide
Amino acid
KEY TERMS

RNA transcription

Initiation, Elongation, Termination
TATA box
 Introns, Exons
 mRNA, tRNA, rRNA
 Translation
 Ribosome
 Codon
 Amino Acids
 Polypeptide

DNA
RNA
Double stranded
Single stranded
Adenine pairs with Thymine
Adenine pairs with Uracil
Guanine pairs with Cytosine
Guanine pairs with Cytosine
Deoxyribose sugar
Ribose sugar
DNA TO PROTEIN
Protein is made of amino acid sequences
 20 amino acids
 How does DNA code for amino acid?

GENETIC CODE

Codon
 Three
letter code
 5’ to 3’ order
 Start codon
 Stop codon
AA are represented
by more than one
codon
 61 codons that
specify AA

AMINO ACIDS

Abbreviated
 Three
letters
RNA polymerase
TRANSCRIPTION
DNA to RNA
 Occurs in nucleus
 Three process

 Initiation
DNA of gene
Promoter
DNA
Initiation
Terminator
DNA
Elongation
 Elongation
 Termination
Termination
Growing
RNA
Completed RNA
RNA
polymerase
INITIATION
RNA polymerase
binds to DNA
 Binds at promoter
region

 TATA
box
RNA polymerase
unwinds DNA
 Transcription unit

 Part
of gene that is
transcribed
INITIATION
Transcription factors bind
to specific regions of
promoter
 Provide a substrate for
RNA polymerase to bind
beginning transcription
 Forms transcription
initiation complex

ELONGATION

RNA molecule is built

RNA polymerase
Primer not needed
 5’ to 3’
 3’ to 5’ DNA is template
strand
 Coding strand



Produces mRNA


DNA strand that is not
copied
Messenger RNA
DNA double helix
reforms
TERMINATION
RNA polymerase recognizes a termination
sequence – AAAAAAA
 Nuclear proteins bind to string of UUUUUU on
RNA
 mRNA molecule releases from template strand

POST-TRANSCRIPTIONAL MODIFICATIONS
Pre-mRNA undergoes
modifications before it
leaves the nucleus
 Poly(A) tail

Poly-A polymerase
 Protects from RNA
digesting enzymes in
cytosol


5’ cap
7 G’s
 Initial attachment site for
mRNA’s to ribosomes


Removal of introns
SPLICING THE PRE-MRNA

DNA comprised of
 Exons
– sequence of
DNA or RNA that
codes for a gene
 Introns – non-coding
sequence of DNA or
RNA

Spliceosome
 Enzyme
that removes
introns from mRNA
SPLICING PROCESS

Spliceosome contains a handful of small
ribonucleoproteins
 snRNP’s

(snurps)
snRNP’s bind to specific regions on introns
ALTERNATIVE SPLICING
Increases number and variety of proteins
encoded by a single gene
 ~25,000 genes produce ~100,000 proteins

TRANSLATION
mRNA to protein
 Ribosomes read
codons
 tRNA assists
ribosome to
assemble amino
acids into
polypeptide chain
 Takes place in
cytoplasm

TRNA

Contains
 triplet
anticodon
 amino acid attachment
site

Are there 61 tRNA’s to
read 61 codons?
TRNA: WOBBLE HYPOTHESIS



First two nucleotides of codon for a specific AA is always
precise
Flexibility with third nucleotide
Aminoacylation – process of adding an AA to a tRNA


Forming aminoacyl-tRNA molecule
Catalyzed by 20 different aminoacyl-tRNA synthetase enzymes
RIBOSOMES
Translate mRNA chains into amino acids
 Made up of two different sized parts

 Ribosomal
subunits (rRNA)
Ribosomes bring together mRNA with
aminoacyl-tRNAs
 Three sites

A
site - aminoacyl
 P site – peptidyl
 E site - exit
Amino acid
TRANSLATION
PROCESS
Polypeptide
A
site
P site
Anticodon
mRNA

1
Three stages
 Initiation
 Elongation
Codon recognition
mRNA
movement
 Termination
Stop
codon
New
peptide
bond
3
Translocation
2
Peptide bond
formation
INITIATION


Ribosomal subunits associate with mRNA
Met-tRNA (methionine)





Forms complex with ribosomal subunits
Complex binds to 5’cap and scans for start codon (AUG) –
known as scanning
Large ribosomal subunit binds to complete ribosome
Met-tRNA is in P-site
Reading frame
is established
to correctly
read codons
ELONGATION
Amino acids
are added to
grow a
polypeptide
chain
 A, P, and E
sites operate
 4 Steps

TERMINATION

A site arrives at a stop codon on mRNA

UAA, UAG, UGA
Protein release factor binds to A site releasing
polypeptide chain
 Ribosomal subunits, tRNA release and detach from
mRNA

POLYSOME
b
a
Red object = ?
What molecules are present in this
photo?
PROKARYOTIC RNA TRANSCRIPTION/TRANSLATION
Throughout cell
 Single type of RNA
polymerase transcribes
all types of genes
 No introns
 mRNA ready to be
translated into protein
 mRNA is translated by
ribosomes in the cytosol
as it is being transcribed

REVIEW
What is a gene?
 Where is it located?
 What is the main
function of a gene?
 Do we need our genes
“on” all the time?
 How do we turn genes
“on” or “off”?

REGULATING GENE EXPRESSION



Proteins are not required by
all cells at all times
Regulated
Eukaryotes – 4 ways





Transcriptional (as mRNA is
being synthesized)
Post-transcriptional (as mRNA
is being processed)
Translational (as proteins are
made)
Post-translational (after
protein has been made)
Prokaryotes


lacOperon
trpOperon
TRANSCRIPTIONAL REGULATION
Most common
 DNA wrapped around histones keep gene promoters
inactive
 Activator molecule is used (2 ways)

Signals a protein remodelling complex which loosen the
histones exposing promoter
 Signals an enzyme that adds an acetyl group to histones
exposing promoter region

TRANSCRIPTIONAL REGULATION

Methylation
Methyl groups are added to the cytosine bases in the
promoter of a gene (transcription initiation complex)
 Inhibits transcription – silencing
 Genes are placed “on hold” until they are needed
 E.g. hemoglobin

AGOUTI MICE
POST TRANSCRIPTIONAL REGULATION

Pre-mRNA processing


Alternative splicing
Rate of mRNA degradation

Masking proteins – translation
does not occur
 Embryonic
development
Hormones - directly or
indirectly affect rate
 Casein – milk protein in
mammary gland
 When casein is needed,
prolactin is produced
extending lifespan of casein
mRNA

TRANSLATIONAL REGULATION
Occurs during protein synthesis by a ribosome
 Changes in length of poly(A) tail

 Enzymes
add or delete adenines
 Increases or decreases time required to translate
mRNA into protein
 Environmental
cues
POST-TRANSLATIONAL REGULATION

Processing
 Removes
sections of protein to make it active
 Cell regulates this process (hormones)

Chemical modification
 Chemical
groups are added or deleted
 Puts the protein “on hold”

Degradation
 Proteins
tagged with ubiquitin are degraded
 Amino acids are recycled for protein synthesis
PROKARYOTIC REGULATION

lacOperon

Regulates the production of lactose metabolizing proteins
PROKARYOTIC REGULATION

trpOperon

Regulates the expression of tryptophan enzymes
CANCER
Lack regulatory mechanisms
 Mutations in genetic code (mutagens)

 Probability
increases over lifetime
 Radiation, smoking, chemicals

Mutations are passed on to daughter cells
 Can
lead to a mass of undifferentiated cells (tumor)
 Benign and malignant

Oncogenes
 Mutated
genes that once served to stimulate cell
growth
 Cause undifferentiated cell division
CANCER
GENETIC MUTATIONS

Positive and negative
 Natural
selection – evolution
 Cancer –death

Small-Scale – single base pair
 Point
mutations
 Substitution,

insertion/deletion, inversion
Large-Scale – multiple base pairs
SMALL-SCALE MUTATIONS

Four groups
 Missense,
 Lactose,
 SNPs
nonsense, silent, frameshift
sickle cell anemia
– single nucleotide polymorphisms
 Caused
by point mutations
MISSENSE MUTATION
Change of a single base pair or group of base pairs
 Results in the code for a different amino acid
 Protein will have different sequence and structure
and may be non-functional or function differently

NONSENSE MUTATION
Change in single base pair or group of base
pairs
 Results in premature stop codon
 Protein will not be able to function

SILENT MUTATION
Change in one or more base pairs
 Does not affect functioning of a gene
 Mutated DNA sequence codes for same amino
acid
 Protein is not altered

FRAMESHIFT MUTATION
One or more nucleotides are inserted/deleted from a
DNA sequence
 Reading frame of codons shifts resulting in multiple
missense and/or nonsense effects
 Any deletion or insertion of base pairs in multiples of
3 does not cause frameshift

LARGE-SCALE MUTATIONS

Multiple
nucleotides, entire
genes, whole
regions of
chromosomes
LARGE-SCALE MUTATIONS

Amplification – gene
duplication
 Entire
genes are copied
to multiple regions of
chromosomes
LARGE-SCALE MUTATIONS

Large-scale deletions
 Entire
coding regions of DNA are removed
 Muscular
Dystrophy
LARGE-SCALE MUTATIONS

Chromosomal translocation
 Entire
genes or groups of genes are moved from
one chromosome to another
 Enhance, disrupt expression of gene
LARGE-SCALE MUTATIONS

Inversion
 Portion
of a DNA molecule reverses its direction in the
genome
 No direct result but reversal could occur in the middle
of a coding sequence compromising the gene
LARGE-SCALE MUTATIONS

Trinucleotide repeat expansion
 Increases
number of repeats in genetic code
 CAG CAG CAG CAG CAG CAG CAG CAG
 Huntingtons
disease
CAUSES OF GENETIC MUTATIONS

Spontaneous mutations
 Inaccurate

DNA replication
Induced mutations
 Caused
by environmental agent – mutagen
 Directly alter DNA – entering cell nucleus
 Chemicals, radiation
CHEMICAL MUTAGENS

Modify individual nucleotides
 Nucleotides
resemble other base pairs
 Confuses replication machinery – inaccurate copying
 Nitrous

acid
Mimicking DNA nucleotides
 Ethidium
bromide – insert itself into DNA
RADIATION - LOW ENERGY
UV B rays
 Non-homologous end joining

Bonds form between adjacent nucleotides along DNA
strand
 Form kinks in backbone
 Skin cancer

RADIATION – HIGH ENERGY
Ionizing radiation – x-ray, gamma rays
 Strip molecules of electrons
 Break bonds within DNA



Delete portions of chromosomes
Development of tumors
MUTATION IN PROKARYOTES
DNA is mostly coding sequences
 Mutation is harmful – superbugs

GENOMES AND GENE ORGANIZATION

Human Body
 22
autosomal chromosomes
 1 pair of each sex chromosome (XX, YY)
GENOMES AND GENE ORGANIZATION

Components
 VNTR’s–variable
number tandem repeats (microsatellites)
 Sequences of long repeating base pairs
 TAGTAGTAGTAGTAG
 LINEs
– long interspersed nuclear elements
 SINEs – short interspersed nuclear elements
 Transposons – small sequences of DNA that move
about the genome and insert themselves into different
chromosomes
 Pseudogene – code is similar to gene but is unable to
code for protein
VIRUSES
Not alive but can replicate themselves
 Contain

 DNA
or RNA
 Capsid – protein coat
 Envelope – cell membrane
VIRUS

4000 species of virus have been classified
REPLICATION

DNA
 Transcription

and translation
RNA (retrovirus)
 Uses
reverse transcriptase – enzyme
 Uses cells parts to make a single strand of DNA and
then makes a complementary strand from that copy
 Integrase – incorporates into our genetic code
VIRUS AS VECTORS

Transduction
 Using
a virus vector to insert DNA into a cell or
bacterium

similar documents