Transcript protein synthesis notes

What do all of these have in
common?
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Collagen
Melanin
Hemoglobin
Lactase
Immunoglobulins
Actin & Myosin
 They are ALL types of Proteins that do “work” that
contribute to our genetic traits
Protein Synthesis
DNA  RNA Proteins
Honors Biology
Ms. Pagodin
Review…
 Summarize the structure and function of genes
 Describe the function of ribosomes
 Differentiate between DNA and genes
 Describe the structure and function of DNA
 State the base pairing rules
Compare RNA & DNA
RNA (Ribonucleic Acid)
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Single Strand of Nucleotides
5 C sugar is ribose
Uses the N base uracil (U) instead of thymine (T)
3 Types:
 Messenger RNA (mRNA)
 An RNA copy of the gene
 Carries and delivers genetic info from nucleus to ribosome
 Ribosomal RNA (rRNA)
 Components of a ribosome
 Site of translation
 Transfer RNA (tRNA)
 Acts as an interpreter
 Translates mRNA into amino acid sequences
 All 3 types of RNA are essential for processing information
from DNA to proteins.. Gene Expression or Protein
Synthesis
Gene Expression
 Organisms traits are determined by
proteins
 Proteins are assembled according to
genes on DNA
 DNA can not leave the nucleus, but
proteins are made in ribosomes,
therefore need an intermediate
messenger… RNA
 2 stages:
 Transcription – copying DNA info to
mRNA (nucleus)
 Translation – mRNA used to build
protein (cytoplasm)
Overview
Transcription
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RNA polymerase binds to promoter region of DNA
 Promoter region – specific sequence of DNA that serves as a START
signal
DNA unwinds and 2 strands separate
 only 1 side is used as a template
RNA polymerase reads each nucleotide on the 3’ end and pairs it with a
complimentary RNA nucleotide
 Same base pairing rules except “U” pairs with “A”
 RNA dangles off the enzyme like a tail
Proceeds at 60 nt/sec until RNA polymerase reaches a specific STOP
sequence
RNA is released as a free transcript
mRNA Processing
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Introns are cut out before
mRNA leaves the nucleus
mRNA is a copy of exons (coding)
and introns (non-coding) regions
Alternative splicing Introns allow for evolutionary
flexibility, genes to shuffle, and
limits effects of mutations
Add a 5’ cap
Binds to ribosome
Add a 3’ Poly-A tail
100-300 adenine ribonucleotides
Determines how long mRNA will
last in the cytoplasm
Compare Transcription to DNA
Replication
 Only part of the DNA strand is unwound and used as a
template
 The enzyme RNA polymerase adds ribonucleotides
 Results in a single RNA strand
The Genetic Code
 Instructions for building a protein are
written as codons on mRNA
 Codons – 3 nt that code for a specific
a.a.
 Codon chart - a.a. and stop signals
that are coded by each of 64 possible
sequences of mRNA codons
 Highly Conserved (Universal) – the
genetic code is the same in ALL
organisms…significance?
 Ex. GUC codes for the a.a. valine in
bacteria, dogs, lizards, humans, etc
 Reading the codon chart
Translation
 tRNA – one loop has 3 nt sequence called an anticodon
 Anticodon – 3nt complimentary to codon on mRNA
 Enables tRNA to temporarily H-bond to mRNA
 No tRNA w/anticodons for STOP codons UAG, UAA, UGA
 tRNA also carries the a.a. that corresponds to CODON
 Ribosomes
 1,00’s in cytoplasm
 2 rRNA subunits (large and small) bind together to form
ribosome
 3 Binding Sites
 A site – where tRNA anticodon binds to complimentary codon of
mRNA
 P site – holds tRNA w/ growing polypeptide chain
 E site – tRNA exits, leaving a.a. in the “P” site
Translation: Initiation
 Initiator tRNA w/ anticodon
UAC binds to small
ribosomal subunit
 mRNA start codon binds to
tRNA anticodon and finally
a large ribosomal subunit
binds to the initiation
complex
Translation: Elongation
Translation: Termination
Translation: Assembling the
Protein
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mRNA binds to small rRNA subunit w/start codon,
AUG, in the “P” site
tRNA w/ anticodon UAC and carrying a.a.
methionine binds to start codon
The next codon, in “A” site, binds w/
complimentary tRNA (carrying the corresponding
a.a.)
Enzyme forms a peptide bond between adjacent
a.a.
tRNA in “P” site now exits via “E” site and is
recycled
tRNA in the “A” site moves to the “P” site w/
growing polypeptide chain, mRNA moves w/it,
therefore a new codon is in the “A" site
Process continues until it reaches a STOP codon at
the end of the mRNA, there is no anticodon
W/nothing in the “A” site, the ribosome is
disassembled and the newly made polypeptide is
released
Protein Synthesis
Mutations
 Mutation – any change in an organism’s genetic
material
 Causes
 Mutagens – environmental agents that cause mutations
after exposure
 X-rays, UV rays, chemicals
 Carcinogens – mutagens that lead to cancer
 Asbestos, benzene, tobacco
Types of Mutations
 Chromosomal Mutations
 Alterations in chromosome structure
 Deletion, duplication, inversion, translocation
 Point Mutations
 Just one or a few nt changed in a gene
 Substitution – one nt is replaced by a different nt
 Ex. UGU  UGC (no effect b/c both code for cysteine)
 UGU  UGA (early STOP codon)
 Frameshift mutations
 Mutations that cause a gene to be read in the wrong 3 nt sequence
 Insertions – one or m ore nt added to gene
 Ex. AAU CGC UUU
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AGA UCG CUU U
 Deletions – one or more nt deleted from gene
 Ex. AAU CGC UUU
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AUC GCU UU
Note
* If mutation occurs in an intron it will have no effect
*if reading frame is displaced 3 nt, the mutation may have no effect
Prokaryotic Gene Regulation
 Prokaryotic Cells – genes are unbroken set of nt
 Operon
 controls gene expression in prokaryotes
 Cluster of genes that code for proteins w/related
functions
Lac Operon
 Lac Operon – genes for lactose digesting
enzyme
 Only want lactose digesting enzymes when
lactose is present…or else energy is being
wasted transcribing genes
 Operator – acts like an on/off switch
 If no molecule is bound to operator, then
the gene is “ON” and RNA polymerase can
move across
 When a repressor protein binds to the
operator, it blocks the RNA polymerase
from transcribing, genes are “OFF”
 Repressor can be removed by inducer (ex.
allolactose), now gene is turned ‘ON”
Trp Operon
 Trp Operon – genes
for making
tryptophan
 E.coli would
typically get trp
from environment,
therefore gene only
needs to be turned
on when trp is not
present
Eukaryotic Gene Regulation
 No operons…b/c genes w/similar functions are scattered among
different chromosomes
 Multicellular organisms have different types of cells, all somatic
cells contain the same DNA…but what makes them different is
which genes are turned on/off
 Ex. Every cell has hemoglobin genes, but only turned “ON” in rbc
 Transcription takes place at uncoiled regions of chromosome
 RNA polymerase cannot bind w/o transcription factors
 Transcription factors are signaled by 20 messengers that bind to
the enhancer site to turn “ON” the gene