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

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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

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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