Transcript RNA and Protein Synthesis

Chapter 12
RNA and Protein Synthesis
RNA
• Objectives:
– How does RNA differ from DNA?
– How does the cell make RNA?
• Define:
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RNA
Messenger RNA
Ribosomal RNA
Transfer RNA
Transcription
RNA polymerase
Promoter
Intron
exon
I. The Role of RNA
• Double-helix structure  DNA
copied by separating 2 strands,
then use base pairing to make
a new complementary strand
– Structure did not explain how a
gene worked
– Explained by discovery of RNA
• RNA – nucleic acid – involved
in putting genetic code into
action
A. Comparing RNA and DNA
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DNA & RNA – nucleotides
made of 5-carbon sugar,
phosphate group, nitrogenous
base
3 important differences:
1. Sugar in RNA is ribose instead of
deoxyribose
2. RNA is single-stranded instead of
double-stranded
3. RNA contains uracil in place of
thymine
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Differences make it easy for
enzymes to tell the DNA and
RNA apart
• DNA = master plan –
stays safely in nucleus
• RNA = blue prints – go to
protein-building sites in
cytoplasm (ribosomes)
B. Functions of RNA
• Disposable copy of a
segment of DNA
• Working facsimile of a single
gene
• Most only have 1 job =
protein synthesis
• Controls assembly of amino
acids into proteins
– 3 types of RNA & each has
specific job in making proteins
1. Messenger RNA
• Genes contain
instructions for
assembling amino
acids into proteins
• Messenger RNA
carries copies of
instructions from DNA
to other parts of cell
2. Ribosomal RNA
• Proteins assembled on
ribosomes
– Ribosomes composed of 2
subunits
– Subunits made up of several
ribosomal RNA molecules
and as many as 80 different
proteins
• Ribosomal RNA forms an
important part of both
subunits of ribosome
3. Transfer RNA
• Transfers each amino
acid to ribosome as it is
specified by coded
messages in mRNA
• Carries amino acids to
ribosomes
• Matches amino acids to
coded mRNA message
II. RNA Synthesis
• Cells invest large amounts of raw material
and energy into making RNA molecules
• If we understand how cells do this, we can
understand more about how genes work
A. Transcription
• Transcription – segments of
DNA serve as templates to
produce complementary RNA
molecules
• Prokaryotes – RNA synthesis
and protein synthesis occur in
cytoplasm
• Eukaryotes – RNA produced
in nucleus  moves to
cytoplasm to produce protein
Transcription in Eukaryotes
• Requires RNA polymerase
(Enzyme) –binds to DNA &
separates DNA strands
• Uses one strand of DNA as
a template to assemble
complementary strand of
RNA
• Ability to copy a single
strand of DNA into RNA
makes it possible for a single
gene to produce hundreds or
thousands of RNA
molecules
B. Promoters
• RNA polymerase doesn’t bind just
anywhere on DNA
• RNA polymerase only binds to
promoters
• Promoters – regions of DNA that
have specific base sequences
– Are signals to DNA molecule that show
RNA polymerase exactly where to begin
making RNA
– Similar signals in DNA cause
transcription to stop when new RNA
molecules is completed
C. RNA Editing
• Sometimes require editing before ready to be read
(like a 1st, rough draft)
• Pre-mRNA molecules have bits and pieces cut out
of them before they go into action
• Introns – portions cut out and discarded
– Taken out of pre-mRNA while still in nucleus
• Exons – remaining pieces
– Spliced back together to form final mRNA
READING DNA TO MAKING A
PROTEIN
• DNA : TTA GCG AGC GTT (base Triplets) holds
instructions
• mRNA: AAU
CGC UCG CAA (Codons) each
codon codes for an amino acid that builds a protein.
• tRNA: UUA GCG AGC GUU (anti-codons) these
are the tRNA letters that go out and find the correct
amino acid for the protein
Take DNA and Transcribe it into
mRNA and tRNA
• DNA Strand:
• mRNA:
• tRNA:
TAC GGC TAT ACT
Exit Ticket
• What are the 3 types of RNA?
• What are the 3 major differences between
RNA and DNA?
Do Now
• What are the three major differences
between DNA and RNA?
• What are the 3 types of RNA and what job
does each complete in the body?
• Transcribe the following DNA sequence:
– ATAGCTGATCGA
Section 13.2
Ribosomes and Protein Synthesis
• Objectives:
– What is the genetic code, and how is it read?
– What role does the ribosome play in assembling
proteins?
– What is the “central dogma” of molecular biology?
• Define:
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Polypeptide
Genetic code
Codon
Translation
Anticodon
Gene expression
I. The Genetic Code
• Step 1 = transcribe
nucleotide base sequence
from DNA to RNA
• Transcribed info contains
code for making proteins
• Proteins made by joining
amino acids together =
polypeptides
• 20 amino acids
• RNA contains 4 bases:
adenine, cytosine, guanine,
uracil
• 4 letters form a language =
genetic code
• The genetic code is read 3
letters at a time
• Codon – each 3-letter “word”
in mRNA – consists of 3
bases that specify a single
amino acid
A. How to Read Codons
• 64 possible 3-base
codons from 4 letters
• Genetic code table (start
at center of circle, move
outward through 2 more
rings, read amino acid
name)
B. Start and Stop Codons
• Punctuation marks for
messages
• Methionine = AUG = initiation
(“start”) codon for protein
synthesis
• mRNA is read (3 bases at a
time)
• “Stop” codon – ends
translation
• Polypeptide is complete
Do Now
• Transcribe the following DNA sequence:
– ATGCTAGCTAAT
• What DNA sequence indicates a start
codon? Stop Codon?
• What is the difference between an intron
and exon?
• How do you read the genetic table chart?
II. Translation
• Sequence of nucleotide bases in
mRNA = set of instructions that
gives order amino acids
• Once complete  polypeptide
folds into final shape to become a
functional protein
• Ribosomes use sequence of
codons in mRNA to assemble
amino acids
• Translation – process of decoding
an mRNA message into a protein
A. Steps in Translation
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Transcription in nucleus
Translation in ribosomes
1. Ribosome attaches to mRNA
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Each codon passes through ribosome
tRNAs bring proper amino acids into
ribosome
Ribosome attaches amino acids one at a
time to growing chain
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Each tRNA carries only 1 kind of amino
acid
Each tRNA has 3 unpaired bases
(anticodon – complementary to one
mRNA codon)
- Next tRNA brings next amino acid w/
anticodon
2. Ribosome helps form
peptide bond b/w first and
second amino acids
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At same time bond holding first
tRNA molecule to its amino
acid is broken
tRNA moves into 3rd binding
site  exits ribosome
Ribosome moves to 3rd codon
 tRNA brings amino acid for
3rd codon
3. Polypeptide chain continues to grow until
ribosome reaches “stop” codon on mRNA
- ribosome releases both newly formed
polypeptide & mRNA
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Translation process complete
READING DNA TO MAKING A
PROTEIN
• DNA : TTA GCG AGC GTT (base Triplets) holds
instructions
• mRNA: AAU
CGC UCG CAA (Codons) each
codon codes for an amino acid that builds a protein.
• tRNA: UUA GCG AGC GUU (anti-codons) these
are the tRNA letters that go out and find the correct
amino acid for the protein
Take DNA and Transcribe it into
mRNA and tRNA
• DNA Strand:
• mRNA:
• tRNA:
TAC GGC TAT ACT
B. The Roles of tRNA and rRNA in
Translation
• All 3 types come together in
ribosome during translation
– mRNA – carries coded message
that directs the process
– tRNA – deliver exactly the right
amino acid called for by each
codon on mRNA
• Enable ribosome to “read” mRNA
message
– rRNA – help hold ribosomal
proteins in place & help locate
beginning of mRNA message
• May carry out chemical reactions that
joins amino acids together
III. The Molecular Basis of
Heredity
• Most genes contain nothing more
than instructions for assembling
proteins
• Many proteins = enzymes that
catalyze and regulate chemical
reactions
• Gene codes for enzyme to produce
pigment that controls color of flower
• Another gene produces proteins that
regulate patterns of tissue growth in a leaf
• Proteins = microscopic tools 
each specifically designed to build
or operate a component of a living
cell
• Molecular biology – seeks to
explain living organisms by
studying them at the
molecular level (DNA and
RNA)
• “Central dogma” of molecular
biology is that information is
transferred from DNA to RNA
to protein
– Many exceptions: viruses
(transfer info in opposite
direction)
• Gene expression – way in
which DNA, RNA and
proteins are involved in
putting genetic information
into action in living cells
• Discovery – near-universal
nature of genetic code
– Some organisms w/ slight
variations in amino acids
assigned to particular codons
– Code is always read 3 bases at
a time
– Always “read” in same direction
• Despite enormous diversity in
form and function, living
organisms display remarkable
unity in molecular biology of
genes
Exit Ticket
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DNA  _______  _________
What is transcription?
What is Translation?
What is a codon?
What does a codon determine?
Do Now
• What process involves RNA getting the
instructions from DNA?
• What process involves the assembling of a
protein?
• What join together to form the protein?
• Where are proteins assembled?
Section 13.3
Mutations
• Objectives:
– What are mutations?
– How do mutations affect genes?
• Define:
– Mutation
– Point mutation
– Frameshift mutation
– Mutagen
– polyploidy
I. Types of Mutations
• Cells make mistakes in
copying DNA
• Mutations – heritable
changes in genetic
information
• Gene mutations: produce
changes in a single gene
• Chromosomal mutations:
produce changes in whole
chromosome
A. Gene Mutations
• Point mutation – involve
changes in one or a few
nucleotides; occur at a
single point in DNA
sequence
• Occur during replication
• Can be passed on to every
cell that develops from
original mutated cell
1. Substitutions
• One base changed to a
different base
• Affect no more than single
amino acid
• Sometimes have no effect at all
– Changed mRNA from CCC to
CCA = no effect
– Changed CCC to ACC = proline
replaced with threonine
2. Insertions and Deletions
• One base inserted or removed
from DNA sequence
• Effects can be dramatic
– Code read 3 bases at a time
– If nucleotide is added or deleted 
bases still read in groups of 3 
groupings shift in every codon that
follows mutation
• Frameshift mutation – shift
“reading frame” of genetic
message
– Can change every amino acid that
follows the point of mutation
– Can alter a protein so much that is
unable to perform its normal function
B. Chromosomal Mutations
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Involve changes in number or
structure of chromosomes
Can change location of
genes on chromosomes
Can change number of
copies of some genes
4 types:
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Deletion – loss of all or part of a
chromosome
Duplication – produces extra
copy of all or part of a
chromosome
Inversion – reverses direction of
parts of a chromosome
Translocation – part of one
chromosome breaks off and
attaches to another
II. Effects of Mutations
• Genes can be altered by natural
events or artificial means
• Mutations may or many not affect
organism
• Some mutations that affect an
individual can also affect a species or
ecosystem
• Many produced by errors in genetic
processes
– Errors during DNA replication: inserts
incorrect base 1 in 10million bases
– Small changes can accumulate over time
• Stressful environmental conditions
cause bacteria to increase mutation
rates
– Can be helpful: ability to consume new
food source or resist poison
A. Mutagens
• Mutagen – chemical or physical
agents in environment
– Chemical: Pesticides, natural plant
alkaloids, tobacco smoke,
environmental pollutants
– Physical: electromagnetic radiation;
interact w/ DNA & produce high
rates of mutation
• Cells can repair some damage
– When they cannot  DNA
sequence changes permanently
• Some=interfere w/ base-pairing
 increases error rate of DNA
replication
• Some=weaken DNA strand 
breaks & inversions create
chromosomal mutations
B. Harmful and Helpful Mutations
• Effects of mutations on
genes vary widely
– Some have little or no effect
and some produce beneficial
variations
– Some negatively disrupt gene
function
• Depends on how its DNA
changes relative to the
organism’s situation
• Without mutations 
organisms could not evolve 
mutations are source of
genetic variability in a species
1. Harmful Effects
• Most harmful – dramatically change
protein structure or gene activity
• Defective proteins can disrupt normal
biological activities  result in genetic
disorders (cancers) (sickle-cell disease)
2. Beneficial Effects
• Mutations often produce proteins
with new or altered functions that
can be useful to organisms in
different or changing environments
• Insects resist chemical pesticides
(mosquitoes)
• Humans increase bone strength/density;
increase resistance to HIV
• Plant/animal breeders take
advantage of “good” mutations
– Polyploidy – multiple sets of
chromosomes in gametes b/c
chromosomes failed to separate
during meiosis
• Polyploidy plants – often larger and
stronger than diploid plants
(bananas/limes)
Exit Ticket
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What is a mutation?
What is a point mutation?
What types of point mutations are there?
How can a mutation be a good thing?
How can a mutation be bad for the
organism?
• What is a mutagen?