Chapter 12 ppt

(1)

Biology

Sylvia S. Mader Michael Windelspecht

Chapter 12

Molecular Biology of the Gene

Lecture Outline

See separate FlexArt PowerPoint slides for all figures and tables pre-inserted into

PowerPoint without notes.

(2)

Outline

• 12.1 The Genetic Material

• 12.2 Replication of DNA

• 12.3 The Genetic Code of Life

• 12.4 First Step: Transcription

• 12.5 Second Step: Translocation

• 12.6 Structure of the Eukaryotic Chromosome

(3)

12.1 The Genetic Material

• Frederick Griffith investigated virulence of

Streptococcus pneumoniae

 Concluded that virulence could be passed

from a dead strain to a nonvirulent living strain

 Transformation

• Further research by Avery et al.

 Discovered that DNA is the transforming

substance

 DNA from dead cells was being incorporated

into the genome of living cells

(4)

The Genetic Material

• Griffith’s Transformation Experiment

 Mice were injected with two strains of

pneumococcus: an encapsulated (S) strain and a non-encapsulated (R) strain.

• The S strain is virulent (the mice died); it has a mucous capsule and forms “shiny” colonies.

• The R strain is not virulent (the mice lived); it has no capsule and forms “dull” colonies.

(5)

Griffith’s Transformation

Experiment

5

capsule

Injected live S strain has capsule and causes mice

to die.

a. b.

Injected live R strain has no capsule and mice do not die.

c.

Injected heat- killed S strain does not cause mice to die.

d.

Injected heat-killed S strain plus live

R strain causes

(6)

The Genetic Material

• Transformation of organisms today:

 Result is the so-called genetically modified

organisms (GMOs)

• Invaluable tool in modern biotechnology today

• Commercial products that are currently much used • Green fluorescent protein (GFP) can be used as a

marker

– A jellyfish gene codes for GFP

– The jellyfish gene is isolated and then transferred to a bacterium, or the embryo of a plant, pig, or mouse. – When this gene is transferred to another organism, the

organism glows in the dark

(7)

Transformation of Organisms

(8)

The Genetic Material

• DNA contains:

 Two Nucleotides with purine bases

• Adenine (A)

• Guanine (G)

 Two Nucleotides with pyrimidine bases

• Thymine (T)

• Cytosine (C)

(9)

The Genetic Material

• Chargaff’s Rules:

 The amounts of A, T, G, and C in DNA:

• Are constant among members of the same species • Vary from species to species

 In each species, there are equal amounts of:

• A and T • G and C

 All this suggests that DNA uses

complementary base pairing to store genetic information

 Each human chromosome contains, on

average, about 140 million base pairs

 The number of possible nucleotide sequences

is 4140,000,000

(10)

Nucleotide Composition of DNA

10

O N N CH CH C C NH2 cytosine (C)

3 C C2

C 1 O HO P O

O H H H H H OH CH3 O HN N C CH C C O HO P O

O H H H H H OH HN N N

C CH

O

C C C

N H₂N

C 2 C 2 C 1 C 1 O HO P O

O guanine (G) phosphate H H H H H OH N N N HC CH NH₂ C C C N 4

3 C 2 C 1 5 O O O O O O H H H H H OH

c. Chargaff’s data

DNA Composition in Various Species (%) Species

Homo sapiens (human)

Drosophila melanogaster (fruit fly) Zea mays (corn)

Neurospora crassa (fungus) Escherichia coli (bacterium) Bacillus subtilis (bacterium)

31.0 27.3 25.6 23.0 24.6 28.4 31.5 27.6 25.3 23.3 24.3 29.0 19.1 22.5 24.5 27.1 25.5 21.0 18.4 22.5 24.6 26.6 25.6 21.6 A T G C

a. Purine nucleotides b. Pyrimidine nucleotides nitrogen-containing

base

sugar = deoxyribose thymine

(T) adenine

(A)

HO P O CH₂

5 CH2 5 CH₂ 5 CH2 C

4 C

4 C C

3 C

(11)

The Genetic Material

• X-Ray diffraction:

 Rosalind Franklin studied the structure of DNA using X-rays.

 She found that if a concentrated, viscous solution of DNA is made, it can be separated into fibers.

 Under the right conditions, the fibers can produce an X-ray diffraction pattern

• She produced X-ray diffraction photographs.

• This provided evidence that DNA had the following features: – DNA is a helix.

– Some portion of the helix is repeated.

(12)

X-Ray Diffraction of DNA

X-ray beam

b. c.

Rosalind Franklin

diffraction pattern

Crystalline DNA diffracted

X-rays

(13)

The Genetic Material

• The Watson and Crick Model (1953)

 Double helix model is similar to a twisted ladder

• Sugar-phosphate backbones make up the sides

• Hydrogen-bonded bases make up the rungs

 Complementary base pairing ensures that a purine is always bonded to a pyrimidine (A with T, G with C)

 Received a Nobel Prize in 1962

(14)

Watson and Crick Model of DNA

P P P P c . P

S G C _S

P P S S A T G C C G T T A A C G P 0.34 nm 3.4 nm sugar-phosphate backbone a. b. d.

5′ end 3′ end

3′ end 5′ end

complementary base pairing hydrogen bonds sugar 2 nm

(15)

12.2 Replication of DNA

• DNA replication

is the process of copying

a DNA molecule.

• Semiconservative replication

-

each

strand of the original double helix (

parental

molecule) serves as a

template

(mold or

model) for a new strand in a daughter

molecule.

(16)

Semiconservative Replication

old strand new

strand

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

region of parental DNA double helix

region of replication: new nucleotides are pairing with those of parental strands region of completed replication old strand new strand

daughter DNA double helix

daughter DNA double helix DNA

polymerase enzyme

5′ 3′

3′

(17)

17

Replication of DNA

• Replication requires the following steps:

 Unwinding, or separation of the two strands of the parental DNA molecule

 Complementary base pairing between a new

nucleotide and a nucleotide on the template strand

 Joining of nucleotides to form the new strand

(18)

Replication of DNA

• Prokaryotic Replication

 Bacteria have a single circular loop of DNA

 Replication moves around the circular DNA molecule in both directions

 Produces two identical circles

 The process begins at the origin of replication

 Replication takes about 40 minutes, but the cell divides every 20 minutes

• A new round of replication can begin before the previous round is completed

(19)

Replication of DNA

• Eukaryotic Replication

 DNA replication begins at numerous points

along each linear chromosome

 DNA unwinds and unzips into two strands

 Each old strand of DNA serves as a template

for a new strand

 Complementary base-pairing forms a new

strand paired with each old strand

• Requires enzyme DNA polymerase

(20)

Replication of DNA

• Eukaryotic Replication

 Replication bubbles spread bidirectionally until they meet

 The complementary nucleotides are joined to form

new strands. Each daughter DNA molecule contains an old strand and a new strand.

 Replication is semiconservative:

• One original strand is conserved in each daughter molecule, i.e., each daughter double helix has one parental strand and one new strand.

(21)

Prokaryotic versus Eukaryotic Replication

replication fork replication bubble

parental strand

daughter strand

new DNA duplexes a. Replication in prokaryotes

b. Replication in eukaryotes

replication is occurring in two directions

(22)

Aspects of DNA Replication

22

G C A T G C G C P P P P P P P P P P P P 5′ 4′_C

3′C C_2′ C 1′ H H H HH O

5′ end

3′ end Deoxyribosemolecule

3′ 3′ 3′ 5′ 5′ 3′ 5′ 3 5 2 4 6 7 1

OH Is attached here base is attached here

helicase at replication fork DNA polymerase template strand Okazaki fragment template strand lagging strand Direction of replication

template strand

parental DNA helix DNA polymerase

Replication fork introduces complications DNA ligase RNA primer 3 ′end OH CH2 OH

5′ end new strand

leading new strand DNA polymerase

(23)

Replication of DNA

• Accuracy of Replication

 DNA polymerase is very accurate, yet makes

a mistake about once per 100,000 base pairs.

• Capable of identifying and correcting errors

(24)

12.3 The Genetic Code of Life

• Genes Specify Enzymes



Beadle and Tatum:

• Experiments on the fungus Neurospora

crassa

• Proposed that each gene specifies the synthesis of one enzyme

• One-gene-one-enzyme hypothesis

(25)

The Genetic Code of Life

• The mechanism of gene expression

 DNA in genes specify information, but

information is not structure and function

 Genetic information is expressed into

structure and function through protein synthesis

• The expression of genetic information into

structure and function:

 DNA in a gene determines the sequence of

nucleotides in an RNA molecule

 RNA controls the primary structure of a

(26)

The Central Dogma of Molecular Biology

C G

G C A

G C C

G

C

G T

T

A G

C G

C

DNA

3' 5'

(27)

The Central Dogma of Molecular Biology

nontemplate strand

template strand

C G

G C A

G C C

G

C

G T

T

A G

C G

C

G C

A G A C C C C

DNA

transcription in nucleus

mRNA

3 5

(28)

The Central Dogma of Molecular Biology

codon 1 codon 2

template strand

codon 3

C G

G C A

G C C

G

C

G T

T

A G

C G

C

G C

A G A C C C C

DNA

transcription at ribosome

mRNA

3 5

3 3

5

(29)

The Central Dogma of Molecular Biology

29

codon 1 codon 2

template strand

codon 3

polypeptide N C C N C N C C R1 R2 R3

Serine Aspartate Proline O O O

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

A G A C C C C

transcription in nucleus translation at ribosome mRN A 3 5 3 5 5 3

(30)

The Genetic Code of Life

• RNA is a polymer of RNA nucleotides

 RNA nucleotides contain the sugar ribose instead of deoxyribose

 RNA nucleotides are of four types: uracil (U),

adenine (A), cytosine (C), and guanine(G)

 Uracil (U) replaces thymine (T) of DNA

• Types of RNA

• Messenger (mRNA) - Takes a message from DNA in the nucleus to ribosomes in the cytoplasm

• Ribosomal (rRNA) - Makes up ribosomes, which read the message in mRNA

• Transfer (tRNA) - Transfers the appropriate amino acid to the ribosomes for protein synthesis

(31)

Structure of RNA

G

U

A

C

S

P

P P

base is

uracil instead of thymine

one nucleotide

ribose 3′ end

(32)

RNA Structure Compared to

DNA structure

(33)

The Genetic Code of Life

• The unit of the genetic code consists of codons, each of which is a unique arrangement of symbols • Each of the 20 amino acids found in proteins is

uniquely specified by one or more codons

 The symbols used by the genetic code are the mRNA bases

• Function as “letters” of the genetic alphabet

• Genetic alphabet has only four “letters” (U, A, C, G)

 Codons in the genetic code are all three bases (symbols) long

• Function as “words” of genetic information • Permutations:

– There are 64 possible arrangements of four symbols taken three at a time

– Often referred to as triplets

• Genetic language only has 64 “words”

(34)

Messenger RNA Codons

34

U C A G

U C A G U C A G U C A G U C A G U C A G Second Base _Third

Base First Base CGA arginine CGG arginine AGU serine AGC serine AGA arginine AGG arginine GGU glycine GGC glycine GGA glycine GGG glycine UGG tryptophan CGU arginine CGC arginine UGA stop AUG (start) methionine CAC histidine CAA glutamine CAG glutamine AAU asparagine AAC asparagine AAA lysine AAG lysine GAU aspartate GAC aspartate GAA glutamate GAG glutamate UUU phenylalanine CUU leucine CUC leucine CUA leucine CUG leucine AUU isoleucine AUC isoleucine AUA isoleucine GUU valine GUC valine GUA valine GUG valine UCA serine CCU proline CCC proline CCA proline CCG proline ACU threonine ACC threonine ACA threonine ACG threonine GCU alanine GCC alanine GCA alanine GCG alanine CAU histidine UAA stop UCU serine UAU tyrosine UGU cysteine UUC phenylalanine UCC serine UAC tyrosine UGU cysteine UUA leucine UCG serine UAG stop UUG leucine

(35)

The Genetic Code of Life

• Properties of the genetic code:

 Universal

• With few exceptions, all organisms use the code the same way

• Encode the same 20 amino acids with the same 64 triplets

 Degenerate (redundant)

• There are 64 codons available for 20 amino acids • Most amino acids encoded by two or more codons

 Unambiguous (codons are exclusive)

• None of the codons code for two or more amino acids • Each codon specifies only one of the 20 amino acids

 Contains start and stop signals

• Punctuation codons

• Like the capital letter we use to signify the beginning of a sentence, and the period to signify the end

(36)

12.4 First Step: Transcription

• Transcription

 A segment of DNA serves as a template for

the production of an RNA molecule

 The gene unzips and exposes unpaired bases

 Serves as template for mRNA formation

 Loose RNA nucleotides bind to exposed DNA

bases using the C=G and A=U rule

 When entire gene is transcribed into mRNA,

the result is a pre-mRNA transcript of the

gene

 The base sequence in the pre-mRNA is

complementary to the base sequence in DNA

(37)

First Step: Transcription

• A single chromosome consists of one very long molecule encoding hundreds or thousands of genes

• The genetic information in a gene describes the amino acid sequence of a protein

 The information is in the base sequence of one side (the “sense” strand) of the DNA molecule

 The gene is the functional equivalent of a “sentence”

• The segment of DNA corresponding to a gene is unzipped to expose the bases of the sense strand

 The genetic information in the gene is transcribed (rewritten) into an mRNA molecule

 The exposed bases in the DNA determine the sequence in which the RNA bases will be connected together

 RNA polymerase connects the loose RNA nucleotides together

• The completed transcript contains the information from the gene, but in a mirror image, or complementary form

(38)

Transcription

T C

terminator

gene strand

promoter

template strand

direction of polymerase movement

RNA polymerase

DNA template strand

mRNA transcript

to RNA processing 3′

3′ 5′

5′

(39)

RNA Polymerase

39 © Oscar L. Miller/Photo Researchers, Inc.

a. 200m

b.

spliceosome

DNA

RNA polymerase

(40)

First Step: Transcription

• Pre-mRNA is modified before leaving the

eukaryotic nucleus.

 Modifications to the ends of the primary

transcript:

• Cap on the 5′ end

– The cap is a modified guanine (G) nucleotide

– Helps a ribosome determine where to attach when translation begins

• Poly-A tail of 150-200 adenines on the 3′ end

– Facilitates the transport of mRNA out of the nucleus – Inhibits degradation of mRNA by hydrolytic enzymes.

(41)

First Step: Transcription

• Pre-mRNA, is composed of exons and introns.

 The exons will be expressed,

 The introns, occur in between the exons.

• Allows a cell to pick and choose which exons will go into a particular mRNA

 RNA splicing:

• Primary transcript consists of:

– Some segments that will not be expressed (introns) – Segments that will be expressed (exons)

• Performed by spliceosome complexes in nucleoplasm

– Introns are excised

– Remaining exons are spliced back together

• Result is a mature mRNA transcript

(42)

First Step: Transcription

• In prokaryotes, introns are removed by

“self-splicing”—that is, the intron itself has

the capability of enzymatically splicing

itself out of a pre-mRNA

(43)

Messenger RNA Processing in Eukaryotes

(44)

Messenger RNA Processing in Eukaryotes

exon

intron intron

exon exon

(45)

Messenger RNA Processing in Eukaryotes

exon

exon exon

DNA

transcription

exon

exon exon

pre-mRNA

(46)

Messenger RNA Processing in Eukaryotes

46

exon

exon exon

transcription

exon

exon exon pre-mRNA

5 3

exon

exon exon

cap

3 5

DNA

poly-A tail

(47)

Messenger RNA Processing in Eukaryotes

exon

exon exon transcription

exon

exon exon pre-mRNA

5 3

exon

exon exon

exon exon exon

cap poly-A tail spliceosome

cap

3 3

5 5

DNA

(48)

Messenger RNA Processing in Eukaryotes

48

exon

intron intron exon exon DNA

transcription

exon

intron intron exon exon

pre-mRNA

5 3

exon

intron intron exon exon

exon

intron RNA exon

pre-mRNA splicing

exon

cap poly-A tail spliceosome

mRNA

cap poly-A tail

cap poly-A tail 3 3

3 5

5

(49)

Messenger RNA Processing in Eukaryotes

49

exon

exon exon

DNA

transcription

exon

exon exon

pre-mRNA

5 3

exon

exon exon

exon

spliceosome

mRNA

cytoplasm

3 3

3 5

5

(50)

Messenger RNA Processing in Eukaryotes

50

exon

exon exon

DNA

transcription

exon

exon exon

5 3

exon

exon exon

exon

spliceosome

nucleus

mRNA

cytoplasm

nuclear pore in nuclear envelope

cap _poly-A_tail

3 3

5 5 pre-mRNA

3 5

(51)

First Step: Transcription

• Functions of introns:

 As organismal complexity increases;

• The number of protein-coding genes does not keep pace • But the proportion of the genome that is introns increases

 Possible functions of introns:

• Exons might combine in various combinations

– Would allow different mRNAs to result from one segment of DNA

• Introns might regulate gene expression

• Introns may encourage crossing-over during meiosis

 Exciting new picture of the genome is

emerging

(52)

12.5 Second Step: Translation

• Translation

 The sequence of codons in the mRNA at a ribosome directs the sequence of amino acids into a polypeptide

 A nucleic acid sequence is translated into a protein sequence

• tRNA molecules have two binding sites:

 One associates with the mRNA transcript

 The other associates with a specific amino acid

 Each of the 20 amino acids in proteins associates with one or more of 64 types of tRNA

• Translation

 An mRNA transcript associates with the rRNA of a ribosome in the

cytoplasm or a ribosome associated with the rough endoplasmic reticulum

 The ribosome “reads” the information in the transcript

 Ribosome directs various types of tRNA to bring in their specific amino acid “fares”

 The tRNA specified is determined by the code being translated in the mRNA transcript

(53)

Second Step: Translation

• tRNA molecules come in 64 different kinds

• All are very similar except that

 One end bears a specific triplet (of the 64 possible) called the anticodon

 The other end binds with a specific amino acid

type

 tRNA synthetases attach the correct amino acid

to the correct tRNA molecule

• All tRNA molecules with a specific anticodon

will always bind with the same amino acid

(54)

3’

5’

hydrogen bonding amino

acid leucine

G A A

Structure of a tRNA Molecule

(55)

Structure of a tRNA Molecule

3’

5’

hydrogen bonding

amino acid end amino

(56)

Structure of a tRNA Molecule

G A A 3’

5’

amino acid end

anticodon end amino

(57)

Structure of a tRNA Molecule

G G

A

A A C

C

C U U U C C U C

3’

5’

amino acid end

anticodon end

mRNA

5’ 3’

amino acid leucine

(58)

Structure of a tRNA Molecule

G G

A

A A C

C

C U U U C C U C

3’

5’

amino acid end

anticodon end

mRNA

5’ 3’

amino acid leucine

anticodon

a. b.

(59)

Second Step: Translation

• Ribosomes

 Ribosomal RNA (rRNA):

• Produced from a DNA template in the nucleolus of a nucleus • Combined with proteins into large and small ribosomal

subunits

 A completed ribosome has three binding sites to facilitate pairing between tRNA and mRNA

• The E (for exit) site

• The P (for peptide) site, and • The A (for amino acid) site

(60)

Ribosome Structure and Function

60

tRNA binding sites

outgoing tRNA

3

mRNA incoming tRNA polypeptide

5

small subunit

mRNA large subunit

a. Structure of a ribosome b. Binding sites of ribosome

c. Function of ribosomes d. Polyribosome

(61)

Second Step: Translation

• Initiation:

 Components necessary for initiation are:

• Small ribosomal subunit • mRNA transcript

• Initiator tRNA, and

• Large ribosomal subunit

• Initiation factors (special proteins that bring the above together)

 Initiator tRNA:

• Always has the UAC anticodon

• Always carries the amino acid methionine • Capable of binding to the P site

(62)

Second Step: Translation

• Small ribosomal subunit attaches to

mRNA transcript

 Beginning of transcript always has the START

codon (AUG)

• Initiator tRNA (UAC) attaches to P site

• Large ribosomal subunit joins the small

subunit

(63)

Initiation

63 U A C

A U G

A small ribosomal subunit binds to mRNA; an initiator tRNA pairs with the mRNA

start codon AUG. The large ribosomal subunit completes the ribosome. Initiator tRNA occupies the P site. The A site is ready for the next tRNA.

Met

amino acid methionine

initiator tRNA

mRNA

3 5

small ribosomal subunit

large ribosomal subunit

P site A site E site

3 5

Met

Initiation

(64)

Second Step: Elongation

• “

Elongation

” refers to the growth in length

of the polypeptide

• RNA molecules bring their amino acid

fares to the ribosome

 Ribosome reads a codon in the mRNA

• Allows only one type of tRNA to bring its amino acid

• Must have the anticodon complementary to the mRNA codon being read

• The incoming tRNA joins the ribosome at its A site

 Methionine of the initiator tRNA is connected

to the amino acid of the 2nd_{tRNA by a peptide}

(65)

Second Step: Elongation

• The second tRNA moves to P site (translocation)

• The spent initiator tRNA moves to the E site and exits

• The ribosome reads the next codon in the mRNA

 Allows only one type of tRNA to bring its amino acid

• Must have the anticodon complementary to the mRNA codon being read

• Joins the ribosome at its A site

 The dipeptide on the 2nd_{amino acid is connected to}

(66)

Elongation

U A

C A

U

G G A C

3 5

peptide bond Met

Ala Trp Ser

(67)

Elongation

67

U A C A

U G

C U G

G A C

A tRNA–amino acid approaches the ribosome and binds at the A site.

3 5

anticodon

tRNA peptide

bond Met

asp

Ala Trp Ser

Val

1

(68)

Elongation

U A C A U G

C U G

G A C

U A C A

U

G G A C C U G

A tRNA–amino acid approaches the

ribosome and binds at the A site.

Two tRNAs can be at a ribosome at one time; the anticodons are paired to the codons.

anticodon tRNA peptide bond Met asp Ala Trp Ser Val 3

5 5 3

Met

Ala Trp Ser

Val Asp

1 2

(69)

Elongation

69 U A C A U G

C U G

G A C

U A C A

U

G GA C C U G

Two tRNAs can be at a ribosome at one time;

the anticodons are paired to the codons.

anticodon tRNA peptide bond Met asp Ala Trp Ser Val Met Ala Trp Ser

Val Asp

1 2

U A C A

U

G GA C C U G

Peptide bond formation attaches the peptide chain to the newly arrived amino acid.

3

5 5 3 5 3

Met Val Asp Ala Trp Ser 3 peptide bond

(70)

Elongation

70 U A C A U G

CUG

G A C

U A C A

U

G G A C C U G

Two tRNAs can be at a ribosome at one time; the anticodons are paired to the codons. anticodon tRNA peptide bond asp 3

5 5

Asp

1 2 4

U A C A

U

G G A C C U G

G A C C U G A U

G A C C

The ribosome moves forward; the “empty” tRNA exits from the E site; the next amino acid–tRNA complex is approaching the ribosome. 3 5 Met Val Asp Ala Trp Ser

3 ₅ 3

Met Val Asp Ala Trp Ser 3 peptide bond Met Ala Trp Ser Val Met Ala Trp Ser Val

(71)

Elongation

71 U A C A U G

CUG

G A C

U A C A

U

G G A C C U G

5 5

Asp

1 2 4

U A C A

U

G G A C C U G

G A C C U G A U

G A C C

3 ₅ 3

Met

Val Asp Ala

Trp

Ser Thr

3

peptide bond

(72)

Elongation

72

Elongation

U A C A

U G

CUG

G A C

U A C A

U

G G A C C U G

5 5

Asp

1 2 4

U A C A

U

G G A C C U G

G A C C U G A U

G A C C

3 ₅ 3

Met

Val Asp Ala

Trp

Ser Thr

(73)

Second Step: Translation

• Termination:

 Previous tRNA moves to the P site

 Spent tRNA moves to the E site and exits

 Ribosome reads the STOP codon at the end of the mRNA

• UAA, UAG, or UGA

• Does not code for an amino acid

 A protein called a release factor binds to the stop

codon and cleaves the polypeptide from the last tRNA

 The ribosome releases the mRNA and dissociates into subunits

 The same mRNA may be read by another ribosome

(74)

Termination

74

The ribosome comes to a stop codon on the mRNA. A release factor binds to the site.

The release factor hydrolyzes the bond between the last tRNA at the P site and the polypeptide, releasing them. The ribosomal subunits dissociate.

release factor

U A U

A U G A

Termination

stop codon

Trp Trp

Val Val

3′

5′

(75)

Summary of Protein Synthesis in Eukaryotes

75 5 5 3 mRN A pre-mRN A DN A

introns

6. During elongation, polypeptide synthesis takes place one amino acid at a time.

7. Ribosome attaches to rough ER. Polypeptide enters lumen, where it folds and is

modified.

3. mRNA moves into cytoplasm and becomes associated with ribosomes.

4. tRNAs with anticodons carry amino acids to mRNA. TRANSLATION TRANSCRIPTION nuclear pore mRNA

large and small ribosomal subunits amino acids anticodon tRNA peptide codon ribosome 5 3 3 CC GG G C G C G C G U A U A U A U A U A A U C G

1. DNAin nucleus serves as a template for mRNA.

2. mRNA is processed before leaving the nucleus.

5. During initiation, anticodon-codon complementary base pairing begins as the ribosomal subunits come together at a start codon.

8. During termination, a ribosome reaches a stop codon; mRNA and ribosomal subunits disband.

(76)

12.6 Structure of the

Eukaryotic Chromosome

• Each chromosome contains a single linear DNA molecule, but is composed of more than 50% protein.

• Some of these proteins are concerned with DNA and RNA synthesis

• Histones play primarily a structural role

 The most abundant proteins in chromosomes

 Five primary types of histone molecules

 Responsible for packaging the DNA

• The DNA double helix is wound at intervals around a core of eight histone molecules (called a nucleosome)

• Nucleosomes are joined by “linker” DNA.

(77)

Structure of Eukaryotic Chromosomes

77

a: © Ada L. Olins and Donald E. Olins/Biological Photo Service; b: Courtesy Dr. Jerome Rattner, Cell Biology and Anatomy, University of Calgary; c: Courtesy of Ulrich Laemmli and J.R. Paulson, Dept. of Molecular Biology, University of Geneva, Switzerland; d: © Peter Engelhardt/Department of Pathology and Virology, Haartman Institute/Centre of Excellence in Computational Complex Systems Research,

Biomedical Engineering and Computational Science, Faculty of Information and Natural Sciences, Helsinki University of Technology, Helsinki, Finland Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

1. Wrapping of DNA around histone proteins.

2. Formation of a three-dimensional zigzag structure via histone H1 and other DNA-binding proteins.

3. Loose coiling into radial loops .

4. Tight compaction of radial loops to form heterochromatin.

5. Metaphase chromosome forms with the help of a protein scaffold.

DNA double helix

nucleosome

euchromatin c. Radial loop domains

d. Heterochromatin

e. Metaphase chromosome

1,400 nm 700 nm 300 nm 30 nm

histone H1 histones 11 nm

2nm

a. Nucleosomes (“beads on a string”)