CH. 14.1 -14.5
Dr. Marchettehttp://learning.mgccc.cc.ms.us/jd/science/carter/chapter14/animations/csRicin.html
Section 14.1How is RNA transcribed from DNA?
Key Concepts Life depends on enzymes and other
proteins. All proteins consist of polypeptide
chains. The chains are sequences of amino
acids that corresponds to genes—sequences of nucleotide bases in DNA.
The path leading from genes to proteins has two steps. Transcription and translation.
Cont. During transcription, the two stands
of DNA double helix are unwound in a gene region.
Exposed chains of one strand become the template for assembling a single strand of RNA.
Only one type of RNA transcript encodes the message that gets translated into protein.
It is called messenger RNA (mRNA).
cont. In transcription, the first step in
protein synthesis, a sequence of nucleotide bases is exposed in a unwound region of a DNA strand.
That sequence is the template upon which a single stand of RNA is assembled from adenine, cytosine, quanine and uracil subunits.
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Nitrogen Bases DNAA-TC-G
RNAA-UC-G
http://learning.mgccc.cc.ms.us/jd/science/carter/chapter14/animations/dna_rna_compared.html
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Three Classes of RNAs Messenger RNA (mRNA)
Carries protein-building instruction Ribosomal RNA (rRNA)
Major component of ribosomes Transfer RNA (tRNA)
Delivers amino acids to ribosomes
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A Nucleotide Subunit of RNA
phosphate group
sugar (ribose)
uracil (base)
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Same two steps produce all proteins:1) Transcription- DNA is transcribed to
form mRNA Occurs in the nucleus mRNA moves into cytoplasm
Steps from DNA to Proteins
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Transcription & DNA Replication
Like DNA replicationNucleotides added in 5’ to 3’
direction Unlike DNA replication
Only small stretch is templateRNA polymerase catalyzes
nucleotide additionProduct is a single strand of RNA
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Promoter
A base sequence in the DNA that signals the start of a gene
For transcription to occur, RNA polymerase must first bind to a promoter
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Gene Transcription
transcribed DNA winds up again
DNA to be transcribed unwinds
mRNAtranscript
RNA polymerase
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Adding Nucleotides
growing RNA transcript5’
3’ 5’
3’
direction of transcription
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Transcript Modificationunit of transcription in a DNA strand
exon intron
mature mRNA transcript
poly-A tail
5’
5’ 3’
3’
snipped out
snipped out
exon exonintron
cap
transcription into pre-mRNA
3’ 5’
http://learning.mgccc.cc.ms.us/jd/science/carter/chapter14/animations/rna_modifications.html
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Base Pairing during Transcription
DNA
DNA DNA
RNAG C A T
C G T A
G C A U
C G T A
base pairing in DNA replication base pairing in transcription
http://learning.mgccc.cc.ms.us/jd/science/carter/chapter14/animations/transcription_details_v2.html
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tRNA Structure
codon in mRNA
anticodon
amino acid OH
amino-acidattachment site
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Ribosomestunnel
small ribosomal subunit large ribosomal subunit intact ribosome
Summary In gene, transcription, a sequence of
exposed bases on one of the two strands of the DNA molecule serves as a template for synthesizing a complementary strand of mRNA.
RNA polymerase assemble the RNA from four kinds of ribonucelotides that differ I their bases: A,U,C,G.
Before leaving the nucleus, each new mRNA transcript, or pre-mRNA, undergoes modification into final form.
Section 14.2The Genetic Code
Key Concepts The nucleotide sequence in DNA is read
three bases at a time. Sixty four base triplets correspond to specific
amino acids and represent the genetic code. The code words have been highly conserved
through time. Only in a few eukaryotes, prokaryotes, and
prokaryote derived organelles have slight variation on the code.
Cont. The correspondence between genes and
proteins is encoded in protein-building “words” in mRNA transcripts.
Three nucleotide bases make up each three letter code.
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Genetic Code Set of 64 base
triplets Codons 61 specify amino
acids 3 stop translation
http://learning.mgccc.cc.ms.us/jd/science/carter/chapter14/animations/genetic_code.html
Summary The gentic code is a set of 64 different
codons, which are nucleotide bases in mRNA that are “read” in sets of three.
Different codons (base triplets) specify different amino acids.
Section 14.3The Other RNA’s
Key Concepts During translation, amino acids are
bonded together in a polypeptide chain in a sequence specified by the base triplets in mRNA.
Transfer RNA delivers amino acids one at a time to the ribosome.
An RNA component of ribosome catalyzes the chain reaction.
Cont. The codons in mRNA transcript are the
words in protein building messages. Without translators, words that originated
from DNA mean nothing: it take the other two classes of RNA to synthesize the proteins.
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Steps from DNA to Proteins cont.
2) Translation- mRNA with tRNA and rRNA at a ribosome is translated to form polypeptide chains of amino acids, which fold to form proteins, occurs in cytoplasm
Summary Only mRNA carries DNA’s protein building
instructions from the nucleus to the cytoplasm.
Transfer RNA (tRNA) deliver amino acids to ribosome.
Their anticodons base pair with codons in the order specified by mRNA.
Polypeptide chains are built on ribosomes, each consisting of a large and a small subunit made of tRNA and proteins.
Section 14.4The three Stages of Translation Key Concepts An mRNA transcript encodes DNA’s
information about a protein enters an intact ribosome.
There, its codons are translated into polypeptide chain – a protein primary structure.
Translation of the protein building message proceeds through three continuous stages called: initiation, elongation and termination.
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Three Stages of Translation
Initiation
Elongation
Termination
http://learning.mgccc.cc.ms.us/jd/science/carter/chapter14/animations/translation_v2.html
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Initiation
Initiator tRNA binds to small ribosomal subunit
Small subunit/tRNA complex attaches to mRNA and moves along it to an AUG “start” codon
Large ribosomal subunit joins complex
http://learning.mgccc.cc.ms.us/jd/science/carter/chapter14/animations/ribosome.html
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Binding Sites
binding site for mRNA
P (first binding site for tRNA)
A (second binding site for tRNA)
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Elongation mRNA passes through ribosomal subunits tRNAs deliver amino acids to the
ribosomal binding site in the order specified by the mRNA
Peptide bonds form between the amino acids and the polypeptide chain grows
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Elongation
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Termination Stop codon into place No tRNA with
anticodon Release factors bind to
the ribosome mRNA and polypeptide
are releasednew polypeptide chain
mRNA
http://learning.mgccc.cc.ms.us/jd/science/carter/chapter14/animations/protein_synthesis.html
Summary Translation is initiated when a small ribosomal
subunit and an initiator tRNA arrive at a mRNA transcript's start codon, and a large ribosomal subunit binds to them.
tRNA delivers amino acids to a ribosome in the order dictated by the linear sequence of mRNA codons.
A polypeptide chain lengthens as peptide bonds from between amino acids.
Translation ends when a stop codon triggers events that cause the polypeptide chain and the mRNA to detach from the ribosome.
Section 14.5Mutated Genes & Their protein Products
Key Concepts Gene mutations introduce changes in
protein structure, protein function, or both.
The changes may lead to small variation in the shared traits that characterize individuals of a population.
When a cell taps its genetic code, it is making proteins with precise structural and functional roles that keep it alive.
Cont. If a gene changes, the mRNA transcribed
from it may change a specify an altered protein.
If the protein has a critical role, the out come will be a dead or abnormal cell.
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Gene Mutations
Base-Pair Substitutions
Insertions
Deletions
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Base-Pair Substitution
original base triplet in a DNA strand
During replication, proofreading enzymes make a substitution
a base substitution within the triplet (red)
original, unmutated sequence
a gene mutation
possible outcomes:
or
http://learning.mgccc.cc.ms.us/jd/science/carter/chapter14/animations/base_pair_sub.html
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Frameshift Mutations
Insertion Extra base added into gene region
Deletion Base removed from gene region
Both shift the reading frame Result in many wrong amino acids
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Frameshift Mutation
mRNA parental DNA amino acids
altered mRNADNA withbase insertionaltered amino-acid sequence
arginine glycine tyrosine tryptophan asparagine
arginine glycine leucine glutamateleucine
http://learning.mgccc.cc.ms.us/jd/science/carter/chapter14/animations/frameshift.html
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Transposing
DNA segments that move spontaneously about the genome
When they insert into a gene region, they usually inactivate that gene
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Mutation Rates Each gene has a characteristic mutation
rate Average rate for eukaryotes is between
10-4 and 10-6 per gene per generation Only mutations that arise in germ cells
can be passed on to next generation
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Mutagens
Ionizing radiation (X rays)
Nonionizing radiation (UV)
Natural and synthetic chemicals
Summary A gene mutation is permanent in one or more
bases in the nucleotide sequence of DNA. The most common types are base pair
substitution, deletion, and insertion . Exposure to harmful radiation and chemicals
in the environment can cause mutations in DNA.
A protein specified by a mutated gene may be harmful, neutral, or beneficial effect on the individual’s capacity to function in the environment.
Ionizing Radiation
Fig. 14-12, p.227
Fig. 14-14, p.229
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