Chapter 13 (Sections 13.1-13.3) Gene Expression

www.cengage.com/biology/solomon

Albia Dugger • Miami Dade College

Eldra SolomonLinda BergDiana W. Martin

Chapter 13(Sections 13.1-13.3)

Gene Expression

From DNA to Proteins

• Most genes specify the structure of proteins

• DNA affects the phenotype of the organism at the molecular level through the process of gene expression, in which DNA specifies the makeup of a cell’s proteins

• The first major step of gene expression is transcription, the synthesis of RNA molecules complementary to DNA

• The second major step is translation, in which RNA becomes a coded template to direct polypeptide synthesis

13.1 DISCOVERY OF THE GENE–PROTEIN RELATIONSHIP

LEARNING OBJECTIVES:

• Summarize the early evidence indicating that most genes specify the structure of proteins

• Describe Beadle and Tatum’s experiments with Neurospora

Early Evidence

• 1908: Archibald Garrod proposed that a rare genetic disease (alkaptonuria) resulted in the absence of an enzyme in the metabolic pathway that breaks down the amino acid tyrosine

• 1926: James Sumner was the first to prove that an enzyme, urease, was a protein

Beadle and Tatum

• 1940s: George Beadle and Edward Tatum • looked for mutations interfering with known metabolic

reactions that produce essential molecules (amino acids and vitamins)

• They worked with the bread mold Neurospora, (expresses recessive mutations)• Wild-type Neurospora manufactures essential molecules• Mutant strains require an additional nutrient

• Showed that each mutant strain had a mutation in only one gene and that each gene affected only one enzyme (the one-gene, one-enzyme hypothesis)

More Evidence

• 1949: Linus Pauling showed that a mutation of a single gene alters the structure of the protein hemoglobin

• Other scientists showed that many proteins are constructed from two or more polypeptide chains, each under the control of a different locus

• The definition of a gene was extended to include that one gene is responsible for one polypeptide chain

KEY CONCEPTS 13.1

• Beadle and Tatum demonstrated the relationship between genes and proteins in the 1940s

13.2 INFORMATION FLOWFROM DNA TO PROTEIN


• Outline the flow of genetic information in cells, from DNA to RNA to polypeptide

• Compare the structures of DNA and RNA

• Explain why the genetic code is said to be redundant and virtually universal, and discuss how these features may

reflect its evolutionary history

Ribonucleic Acid (RNA)

• When a gene that codes for a protein is expressed, the information in DNA is copied to a ribonucleic acid (RNA)

• Like DNA, RNA is a polymer of nucleotides, but RNA has some important differences:• RNA is usually single-stranded• The sugar in RNA is ribose• The base uracil substitutes for thymine

Nucleotide Structure of RNA

• RNA nucleotides form complementary base-pairs with DNA

• Uracil base-pairs with adenine

Fig. 13-3, p. 285

Ribose

Uracil

Ribose

Adenine

Ribose

Cytosine

Ribose

Guanine

Transcription

• The process of transcription copies the information in DNA to RNA

• The sequence of RNA bases is determined by complementary base pairing with the DNA template strand

• Three main kinds of RNA molecules are transcribed: messenger RNA, transfer RNA, and ribosomal RNA

Three Types of RNA

• Messenger RNA (mRNA)• Single RNA strand that carries information for making a

protein

• Transfer RNA (tRNA)• Single RNA strand that folds back on itself to form a

specific shape; each kind of tRNA bonds with one kind of amino acid and carries it to the ribosome

• Ribosomal RNA (rRNA)• Globular, structural part of ribosomes with catalytic

functions needed during protein synthesis

Translation

• The process of translation uses the information transcribed in mRNA to specify the amino acid sequence of a polypeptide

• A sequence of three consecutive bases in mRNA (codon) specifies one amino acid

• The series of codons that specifies the amino acid sequence is a triplet code

• Codons for amino acids and for start and stop signals are collectively called the genetic code

Genetic Code

Second letterU C A G

UUUUUCUUAUUG

LeuUCG UAG Stop UGG Trp GUCA

SerUAAStop UGAStop A

Phe UCC UAC UGC CUCU UAU Tyr UGU

CysU

U

CUU CCU CAUCUC

CUG CCG CAG CGG GCUA Gln

LeuCCA CAA CGA ACCC

ProCAC

ArgHis

CGC CCGU U

C

AUU ACU AAUAUC Ile ACC

ThrAAA

AUGor start

Met ACG AAG GLysAGGAGA Arg

A Th

ird

lett

er (

3’ e

nd

)

AAC CAsnAGCAGU

SerU

Fir

st le

tter

(5’

en

d)

AAUA ACA

GUU GCU GAUGUCGUA Glu

ValGCA GAA GGA

AGCCAla

GACGly

AspGGC

CGGUU

G

GUG GCG GAG GGGG

Fig. 13-5, p. 286

tRNA

• Transfer RNAs connect amino acids and nucleic acids • Each tRNA links with a specific amino acid • tRNA recognizes the mRNA codon for that amino acid

• Each tRNA has a sequence of three bases (anticodon) that hydrogen-bonds with the mRNA codon by complementary base pairing

• The amino acids carried by the tRNAs are linked in the order specified by the sequence of codons in the mRNA

A tRNA Molecule

Fig. 13-6a, p. 287

Loop 3

Loop 1

Loop 2

Anticodon

(a) The 3-D shape of a tRNA molecule is determined by hydrogen bonds formed between complementary bases.

Hydrogen bonds

Fig. 13-6b, p. 287

OH 3′ endAmino acid accepting end

P 5′ end

Hydrogen bonds

Loop 3 Loop 1

Modified nucleotides

Anticodon

(b) One loop contains the anticodon; these unpaired bases pair with a complementary mRNA codon. The amino acid attaches to the terminal nucleotide at the hydroxyl (OH) 3′ end.

Loop 2

Fig. 13-6c, p. 287

Amino acid (phenylalanine)

Anticodon

(c) This stylized diagram of an aminoacyl-tRNA shows that the amino acid attaches to tRNA by its carboxyl group, leaving its amino group exposed for peptide bond formation.

Ribosomes

• Ribosomes are the site of translation• Composed of two different subunits, each containing

protein and rRNA• Attach to the 5′ end of mRNA and travel along it, allowing

tRNAs to attach sequentially to the codons of mRNA

• Amino acids carried by tRNAs are positioned in the correct order and joined by peptide bonds to form a polypeptide

Overview: Transcription and Translation

How Genetic Code Works

• mRNA consists of a linear sequence of four different RNA nucleotides (A, U, G, and C)

• Three-letter combinations (triplets) of the four bases form a total of 64 codons representing the 20 amino acids, plus stop and start codons

• The code is read, one triplet at a time, from a fixed starting point that establishes the reading frame for the message

Genetic Code is Redundant

• More than one codon specifies most amino acids – only methionine and tryptophan are specified by single codons

KEY CONCEPTS 13.2

• The transmission of information in cells is typically from DNA to RNA to polypeptide

13.3 TRANSCRIPTION


• Compare the processes of transcription and DNA replication, identifying both similarities and differences

• Compare bacterial and eukaryotic mRNAs, and explain the functional significance of their structural differences

RNA Polymerases

• In eukaryotic transcription, most RNA synthesis requires one of three RNA polymerases• RNA polymerase I catalyzes synthesis of several kinds of

rRNA molecules that are components of ribosomes• RNA polymerase II catalyzes production of protein-coding

mRNA• RNA polymerase III catalyzes synthesis of tRNA and one

of the RNA molecules

RNA Synthesis

• RNA polymerases carry out synthesis in the 5′ → 3′ direction

• The template strand of DNA and the complementary RNA strand are antiparallel – the DNA template is read in the 3′ → 5′ direction

Upstream and Downstream

Molecular View of Transcription

Initiation

• The nucleotide sequence in DNA to which RNA polymerase and associated proteins initially bind is called the promoter

• Once RNA polymerase has recognized the correct promoter, it unwinds the DNA double helix and initiates transcription

Elongation

• During the elongation stage, as each nucleotide is added to the 3′ end of the RNA molecule, two phosphates are removed in an exergonic reaction

• The remaining phosphate becomes part of the sugar–phosphate backbone

• The last nucleotide to be incorporated has an exposed 3′ hydroxyl group

Termination

• Termination occurs when RNA polymerase recognizes a termination sequence of bases in the DNA template

• RNA polymerase separates from the template DNA and the newly synthesized RNA

• In eukaryotic cells, RNA polymerase adds about 10 to 35 nucleotides to the mRNA molecule after it passes the termination sequence

KEY POINT:Initiation, Elongation, and Termination

RNA transcript RNA polymerase

DNA 4

Rewinding of DNA Unwinding

of DNA

3

Direction of transcription DNA template strand

RNA transcript

2

Fig. 13-9, p. 290

RNA polymerase binds to promoter region in DNA

DNA

Promoter region

Termination sequence

1

Stepped Art

Template and Nontemplate Strands

• Only one DNA strand is transcribed for a given gene, but the opposite strand may be transcribed for another gene

Fig. 13-10, p. 291

mRNA transcript Promoter

regionPromoter

region

mRNA transcriptPromoter

regionRNA polymerase Gene 2

Gene 1 Gene 3

mRNA transcript

Only one of the two strands is transcribed for a given gene, but the opposite strand may be transcribed for a neighboring gene. Each transcript starts at its own promoter region (orange). The orange arrow associated with each promoter region indicates the direction of transcription.

Noncoding mRNA Sequences

• At the 5′ end, a noncoding leader sequence has recognition sites that bind and position ribosomes for translation

• The start codon follows the leader sequence and signals the beginning of the coding sequence for the polypeptide

• At the end of each coding sequence, a stop codon (UAA, UGA, UAG) signals the end of the protein

• Followed by noncoding 3′ trailing sequences

Bacterial mRNA

Fig. 13-11, p. 291

Promoter region

Transcribed region

mRNA termination sequence

DNA

Upstream leader

sequences

Downstream trailing

sequences

Protein-coding sequences

Translated region

Start codon Stop codonmRNA

Polypeptide

mRNA Modification

• In eukaryotes, the original transcript (precursor mRNA, or pre-mRNA) is modified in before it leaves the nucleus

• These posttranscriptional modification and processing activities produce mature mRNA for transport and translation

• A 5′ cap stabilizes the mRNA and allows ribosomes to bind

• Polyadenylation adds a poly-A tail at the 3′ end, which helps export mRNA from the nucleus, stabilizes mRNA, and facilitates initiation of translation

Noncoding and Coding Sequences

• Most eukaryotic genes have interrupted coding sequences: long sequences of bases within protein-coding sequences that do not code for amino acids in the final polypeptide

• introns• Intervening sequences

• exons • Expressed sequences which are parts of the protein-

coding sequence

Posttranscriptional Modifications

• When a gene is transcribed:• the entire gene is copied as a large pre-mRNA transcript

containing both introns and exons

• To become a functional message:• the pre-mRNA is capped, a poly-A tail added, introns are

removed, and exons spliced together to form a continuous protein-coding message

• Following pre-mRNA processing:• mature mRNA is transported through a nuclear pore into

the cytosol to be translated by a ribosome

Splicing Exons

• Splicing may involve the association of several small nuclear ribonucleoprotein complexes (snRNPs) to form a large ribonucleoprotein complex (spliceosome) which catalyzes reactions that remove introns.

• RNA within the intron may act as an RNA catalyst (ribozyme), splicing itself without the use of a spliceosome or protein enzymes

KEY POINT: Eukaryotic Posttranscriptional Modification

Fig. 13-12, p. 292

1st exon

1st intron

2nd intron

3rd exon

mRNA termination sequence

PromoterTemplate

DNA strand

7-methylguanosine cap Transcription, capping of 5′ end

Start codon Stop codon

Small nuclear ribonucleoprotein complex

1st intron 2nd intron

–AAA... Poly-A tail 3′ end

1st exon

2nd exon

3rd exon

–AAA... Poly-A tail 3 ′ endProtein-coding region

Nuclear envelopeNuclear pore

CytosolTransport through nuclear envelope to cytosol

–AAA... Poly-A tail 3 ′ end

Start codon

Protein-coding region

1

2

3

4 Stop codon

2nd exon

1. Formation of pre-mRNA

2. Processing of pre-mRNA

3. Mature mRNA in nucleus

4. Mature mRNA in cytosol

KEY CONCEPTS 13.3

• A sequence of DNA base triplets is transcribed into mRNA codons

KEY CONCEPTS 13.4

• A sequence of mRNA codons is translated into a sequence of amino acids in a polypeptide

13.5 VARIATIONS INGENE EXPRESSION


• Describe a polyribosome in bacterial cells

• Briefly discuss RNA interference

• Describe retroviruses and the enzyme reverse transcriptase

Gene Expression in Bacteria and Eukaryotes

• Significant differences in transcription and translation exist between bacteria and eukaryotes

• Bacteria lack a nucleus

• In eukaryotes, mRNA must move from the nucleus (transcription) to the cytosol (translation or polypeptide synthesis)

Flow of Genetic Information in Bacteria and Eukaryotes

Fig. 13-18a, p. 298

Bacterial cell

DNA

Transcription

mRNA

Translation

Polypeptide Ribosome

(a) In a bacterial cell, mRNA is immediately ready for translation by ribosomes.

Fig. 13-18b, p. 298

Eukaryotic cell

Nucleus

DNA

Transcription

Pre-mRNA

mRNA

RNA processing

Translation

Polypeptide Ribosome

(b) In a eukaryotic cell, RNA processing occurs in the nucleus before the mRNA exits the nucleus for translation.

Transcription and Translationin Bacteria

• Translation begins very soon after transcription, and several ribosomes are attached to the same mRNA

• Ribosomes bind to the 5′end and initiate translation long before the mRNA is fully synthesized

• An mRNA molecule bound to clusters of ribosomes is a polyribosome (polysome)

• The half-life of mRNA in bacteria is only about 2 minutes

Evolution of Eukaryotic Gene Structure

• As much as 75% of the original transcript of a eukaryotic nuclear gene must be removed to make a working message

• 1980s: Walter Gilbert proposed that exons are nucleotide sequences that code for important regions of protein tertiary structure (protein domains), but most exons are too small to code for an entire protein domain

• Regardless of how split genes originated, intron excision is one way present-day eukaryotes regulate gene expression

Eukaryotic RNA in Gene Expression

• Small nuclear RNAs (snRNA) bind to specific proteins to form snRNPs, which combine to form spliceosomes

• Small nucleolar RNAs (snoRNAs) process pre-rRNA in the nucleolus during ribosome subunit formation

• Small interfering RNAs (siRNAs) help control damage from viral infections; and regulate gene expression of protein-coding genes

Selected Kinds of RNA in Eukaryotic Cells

Table 13-1, p. 299

Definition of a Gene

• About 80% of our genome is expressed• About 2% of our genome codes for polypeptides • A single gene may produce more than one polypeptide

chain by modifications in the way mRNA is processed• Many genes code for various kinds of RNA molecules

• gene • A DNA nucleotide sequence that carries the information

needed to produce a specific RNA or polypeptide product

Reversing Information Flow

• 1970: Howard Temin and David Baltimore discovered an enzyme (reverse transcriptase) in RNA tumor viruses that synthesizes DNA using RNA as a template

• Because they reverse the usual direction of information flow, viruses that use reverse transcriptase are called retroviruses

• HIV-1 (the AIDS virus) is the most widely known retrovirus

An RNA Tumor Virus

(a) After an RNA tumor virus enters the host cell, the viral enzyme reverse transcriptase synthesizes a DNA strand that is complementary to the viral RNA. Next, the RNA strand is degraded and a complementary DNA strand is synthesized, thus completing the double-stranded DNA provirus, which is then integrated into the host cell’s DNA. Fig. 13-21a, p. 301

Chromosome DNA in nucleus of host cell Provirus inserted

into chromosome DNA

DNA provirus

DNA replication

Digestion of RNA strandRNA/DNA hybrid

Viral RNA Reverse transcription

RNA virus

Fig. 13-21b, p. 301

Provirus DNA transcribed

Viral mRNA

Viral RNAViral proteins

RNA virus(b) The provirus DNA is transcribed, and the resulting viral mRNA is translated to form viral proteins. Additional viral RNA molecules are produced and then incorporated into mature viral particles enclosed by protein coats.

KEY CONCEPTS 13.5

• Bacterial and eukaryotic cells differ in certain details of gene expression

13.6 MUTATIONS

LEARNING OBJECTIVE:

• Give examples of the different classes of mutations that affect the base sequence of DNA and explain the effects

that each has on the polypeptide produced

Mutations

• Genes undergo changes in the nucleotide sequence of the DNA (mutations)

• The frequency of damage to DNA is low because organisms have enzyme systems that repair DNA damage

• Most uncorrected mutations are either silent or harmful, a few are useful

• Mutations provide the variation among individuals that evolutionary forces act on

Base-Pair Substitution Mutations

• A base-pair substitution involves a change in one pair of nucleotides

• May result in a polypeptide chain with one amino acid different from the normal sequence (missense mutations)

• Silent mutations do not alter the amino acid sequence

• Nonsense mutations convert an amino acid–specifying codon to a stop codon

Base-Pair Silent Mutation

Fig. 13-22a, p. 302

(a) Normal DNA sequence

T A C T G A T C T T T A A C C C T A

A T G A C T A G A A A T T G G G A T

DNA (normal sequence)

A U G A C U A G A A A U U G G G A U

mRNA (normal sequence)

Met Thr Arg Asn Trp Asp

Polypeptide (normal sequence)

DNA template strand

Fig. 13-22b, p. 302

(b) Base-pair substitution (silent mutation)

T A C T G A T C C T T A A C C C T A

A T G A C T A G G A A T T G G G A T

A U G A C U A G G A A U U G G G A U


Base-Pair Missense Mutation

Fig. 13-22a, p. 302









DNA template strand

Fig. 13-22c, p. 302

(c) Base-pair substitution (missense)

T A C T G A T G T T T A A C C C T A

A T G A C T A C A A A T T G G G A T

A U G A C U A C A A A U U G G G A U

Met Thr Thr Asn Trp Asp

Base-Pair Nonsense Mutation

Fig. 13-22a, p. 302









DNA template strand

Fig. 13-22d, p. 302

(d) Base-pair substitution (nonsense)

T A C T G A T C T T T A A T C C T A

A T G A C T A G A A A T T A G G A T

A U G A C U A G A A A U U A G G A U

Met Thr Arg Asn (Stop)

Frameshift Mutations

• In frameshift mutations, one or two nucleotide pairs are inserted into or deleted from the molecule, altering the reading frame

• Codons downstream of the insertion or deletion site specify an entirely new sequence of amino acids

• May produce a stop codon (nonsense mutation) or an altered amino acid sequence

Frameshift Nonsense Mutation

Fig. 13-22a, p. 302









DNA template strand

Fig. 13-22e, p. 302

(e) Frameshift (nonsense)

T G missing

T A C A T C T T T A A C C C T A G G

A T G T A G A A A T T G G G A T C C

A C missing

A U G U A G A A A U U G G G A U C C

Met (Stop)

Frameshift Altered Sequence

Fig. 13-22a, p. 302









DNA template strand

Fig. 13-22f, p. 302

(f) Frameshift (altered amino acid sequence)

T missing

T A C T G A C T T T A A C C C T A G

A T G A C T G A A A T T G G G A T C

A missing

A U G A C U G A A A U U G G G A U C

Met Thr Glu Ile Gly Ile

Mobile Genetic Elements

• Mobile genetic elements or transposons are movable sequences of DNA that cause mutations by “jumping” into the middle of a gene – inactivating the gene

• A DNA transposon moves genetic material from one site to another using a “cut-and-paste” method

• Retrotransposons replicate by forming an RNA intermediate, converted back to DNA by reverse transcriptase

Causes of Mutations

• Most mutations occur spontaneously from mistakes in DNA replication or defects in mitosis or meiosis

• Some regions of DNA (mutational hot spots) are much more likely than others to undergo mutation

• Mutations in certain genes (e.g. for DNA polymerase) increase the overall mutation rate

Mutagens

• Mutagens (agents that cause mutation) include various types of radiation (X-rays, gamma rays, cosmic rays, UV rays) and certain chemicals

• Chemical mutagens may modify bases in DNA, leading to mistakes in complementary base pairing, or cause nucleotide pairs to be inserted into or deleted from DNA

• Many mutagens are also cancer-causing agents (carcinogens)

KEY CONCEPTS 13.6

• Mutations can cause changes in phenotype

88

The Genetic Code

89

DNA and Genes• DNA: contains genes• instructions for every protein in the body

• Gene: functional units of heredity• DNA instructions for a product: RNA or protein

• Humans have 30-75 thousand potential genes (only 1.5% of total DNA)• Remainder is involved with control of genes or appear to be

junk (25%)• Noncoding parts of DNA (non-genes) is highly variable from

one person to the next• Variability allows for identification of an individual by DNA

fingerprinting

90

Gene Activation

• In order for a gene to be expressed (used to make a product) it must be unwound from the histone proteins so it can be read

• Disassembly of the nucleosomes and unwinding of the DNA is called gene activation

91

Genetic Code

• The chemical language of DNA instructions:• Read off a gene in order to assemble a protein• sequence of bases (A, T, C, G)• triplet code:• 3 bases of DNA = 1 amino acid (codon)

• A gene = all the codons for all the amino acids in one protein in the correct order

92

Gene Structure and Expression

• Structure

• Expression (original) (copy) (product) DNA RNA Protein Transcription Translation

Open Reading FramePromoter Terminator

Start Codon

Stop Codon

93

KEY CONCEPT

• The nucleus contains chromosomes• Chromosomes contain DNA• DNA stores genetic instructions for proteins• Proteins determine cell structure and function

94

How DNA instructions become proteins

95

Protein Synthesis

• Transcription:• copies instructions from DNA to mRNA (in nucleus)

• Translation:• ribosome reads code from mRNA (in cytoplasm)• assembles amino acids into polypeptide chain

• Processing:• by RER and Golgi apparatus produces protein

96

mRNA Transcription

• A DNA gene is transcribed to mRNA in 3 steps:• gene activation• DNA to mRNA• RNA processing

97

mRNA Transcription

G

A

A

T

G

A

G

T

A

C

G

G

C

T

C

G

A

T

T A

A

T

C

G

A

G

C

C

G

T

A

C

G C

A

G

C

G

A

C

C

C

G

U

U

A

T

G

A

G

T

A

A

C

C

GC

G

G

C

C

T

C

G

A

T

T

T

C

G

A

A

T

G

G

T

A

A

C

G

G

C

T

G

C

A

T

T

T

T

A

C

C

T

STEP STEP STEP

•

DNA

Gene

Promoter

Triplet 2

Triplet 3

Triplet 4

Triplet 1

Codon1

Codingstrand

Templatestrand

Codon2

Codon3

Codon 4(stop codon)

mRNAstrandRNA

polymerase

Codon1

RNAnucleotide

Adenine

Thymine

Guanine

Cytosine

1

2 2

3

4KEY

Uracil (RNA)

98

Step 1: Gene Activation

• Uncoils DNA, removes histones• Start (promoter) and stop codes on DNA mark location of

gene:• coding strand is code for protein• template strand used by RNA polymerase molecule

99

Step 2: DNA to mRNA

• Enzyme RNA polymerase transcribes DNA:• binds to promoter (start) sequence• reads DNA code for gene• binds nucleotides to form messenger RNA (mRNA)• mRNA duplicates DNA coding strand, uracil replaces

thymine

100

Step 3: RNA Processing in Eukaryotes

• At stop signal, mRNA detaches from DNA molecule:• code is edited (RNA processing)• unnecessary codes (introns) removed• good codes (exons) spliced together• triplet of 3 nucleotides (codon) represents one amino acid

101

Key Concept

• The timing of gene activation (transcription) for any gene is controlled by signals from outside the nucleus, either from within the cell or in response to external cues• E.g. Hormones

102

Translation

• Making a protein using the mRNA blueprint

• Occurs in the cytoplasm on free ribosomes or on fixed ribosomes on the RER

• mRNA moves: • from the nucleus• through a nuclear pore

Figure 3–13

103

Translation

A

G

C

U

U A C

STEP STEP

NUCLEUS

mRNA

Adenine

Guanine

Cytosine

Uracil

Smallribosomal

subunit

Amino acid

tRNA

Anticodon

tRNA binding sites

mRNA strandStart codon

Largeribosomalsubunit

The mRNA strand binds to the smallribosomal subunit and is joined at thestart codon by the first tRNA, whichcarries the amino acid methionine.Binding occurs between complementarybase pairs of the codon and anticodon.

The small and large ribosomal subunitsinterlock around the mRNA strand.

11

2

KEY

•tRNA delivers amino

acids to mRNA

104

Translation

AA

A GGG G

GUU C

CCC

A UG G CCC

A

STEP STEP STEP

1 2 12

3

1

2

3

Peptide bond

A second tRNA arrives at the adjacentbinding site of the ribosome. Theanticodon of the second tRNA binds tothe next mRNA codon.

The first amino acid is detached from itstRNA and is joined to the second aminoacid by a peptide bond. The ribosomemoves one codon farther along themRNA strand; the first tRNA detachesas another tRNA arrives.

The chain elongates until the stopcodon is reached; the componentsthen separate.

Small ribosomalsubunit

CompletedpolypeptideStop

codon

Largeribosomalsubunit

105

Genetic Code

106

Examples using the Genetic Code:

Coding Strand DNA: ATgCAgTTTACgCAgAAgATCAgTTAgTemplate strand DNA: complement A-T, C-G TACgTCAAATgCgTCTTCTAgTCAATC

Transcription to form mRNA: complementary base pairing to template, U replaces T AUgCAgUUUACgCAgAAgAUCAgUUAg

Translation to form protein: read codons from genetic code e.g. AUg = Met/Start (start codon) Aug/CAg/UUU/ACg/CAg/AAg/AUC/AgU/UAg Met-Gln-Phe-Thr-Glu-Lys-Ile-SerUAg = stop codon (no tRNA, no amino acid)

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Mutations

• Point mutations = change in 1 base of DNA can be a silent mutation if the amino acids is not changed • common at the 3rd base in a codon

• Insertion mutation = addition of a base which changes the reading frame;whole protein after the mutation is wrong

• Deletion Mutation = removal of a base, alter reading frame, protein wrong.

YouTube Videos

• Transcription & Translation Video:http://www.youtube.com/watch?v=983lhh20rGY

Transcription Video:http://www-class.unl.edu/biochem/gp2/m_biology/animation/gene/gene_a2.html

Translation Video:• http://www.youtube.com/watch?v=D5vH4Q_tAkY

Chapter 13 (Sections 13.1-13.3) Gene Expression

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Transcript of Chapter 13 (Sections 13.1-13.3) Gene Expression