Chapter 10

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings

PowerPoint Lectures forBiology: Concepts and Connections, Fifth Edition – Campbell, Reece, Taylor, and Simon

Lectures by Chris Romero

Chapter 10Chapter 10

Molecular Biology of the Gene


Original DNA sequence:

TAC ACC TTG GCG ACG ACT

mRNA:

tRNA:

A.A.:


DNA polynucleotide

A

C

T

G

T

Sugar-phosphate backbone

Phosphate group

Nitrogenous base

SugarA

C

T

G

T

Phosphategroup

O

O–

OO P CH2

H3C C

C

C

CN

C

N

H

H

O

O

C

O

O

H

C H H

H

C

H

Nitrogenous base(A, G, C, or T)

Thymine (T)

Sugar(deoxyribose)

DNA nucleotide

DNA nucleotide

10.2 DNA and RNA are polymers of nucleotides

• DNA is a nucleic acid

– Made of long chains of nucleotide monomers

Figure 10.2A


• DNA has four kinds of nitrogenous bases

– A, T, C, and G

CC

C

CC

C

O

N

C

H

H

ONH

H3C

H H

H

H

N

N

N

H

OC

H HN

H C

N

N N

N

C

CC

C

H

H

N

N

H

C

CN

C HN

CN

H C

O

H

H

Thymine (T) Cytosine (C) Adenine (A) Guanine (G)

PurinesPyrimidines

Figure 10.2B


• RNA is also a nucleic acid

– But has a slightly different sugar

– And has U instead of TNitrogenous base

(A, G, C, or U)

Phosphategroup

O

O–

OO P CH2

HC

C

C

CN

C

N

H

H

O

O

C

O

O

H

C H H

OH

C

H

Uracil (U)

Sugar(ribose)

KeyHydrogen atomCarbon atom

Nitrogen atom

Oxygen atom

Phosphorus atom

Figure 10.2C, D


10.3 DNA is a double-stranded helix

• James Watson and Francis Crick

– Worked out the three-dimensional structure of DNA, based on work by Rosalind Franklin

Figure 10.3A, B


• The structure of DNA

– Consists of two polynucleotide strands wrapped around each other in a double helix

Figure 10.3C Twist


• Hydrogen bonds between bases

– Hold the strands together

• Each base pairs with a complementary partner

– A with T, and G with C

Figure 10.3D

G C

T A

A T

G

G

C

C

A T

GC

T A

T A

A T

A T

G C

A T

O

O

OH–O

P

OO

–OPO

OO

P– O

– O OP

OO

O

OH

H2C

H2C

H2C

H2C

O

O

O

O

O

O

O

O

PO–

O–

O–

O–

OH

HO

O

O

O

P

P

P

O

O

O

O

O

O

O

O

T A

G C

C G

A T

CH2

CH2

CH2

CH2

Hydrogen bond

Basepair

Ribbon model Partial chemical structure Computer model


• DNA and RNA are identical except for two things:

– Nitrogenous bases

• DNA: A, C, G, T

• RNA: A, G, C, U

– Sugars

• DNA: deoxyribose

• RNA: ribose

Animation: DNA and RNA Structure


DNA REPLICATION

10.4 DNA replication depends on specific base pairing

• DNA replication

– Starts with the separation of DNA strands

• Then enzymes use each strand as a template

– To assemble new nucleotides into complementary strands

Figure 10.4A

A T

C G

G C

A T

T A

A T

C G

G C

A T

T A

A T

C G

G C

A T

T A

A T

C G

G C

A

T

A T

C G

AC

T

A

Parental moleculeof DNA

Both parental strands serve as templates

Two identical daughtermolecules of DNA

Nucleotides


• DNA replication is a complex process

– Due in part to the fact that some of the helical DNA molecule must untwist

Figure 10.4B

G C

A T

G C

A T

C G

AGA

CG

C

GC

G

TA

G

C

TAT

AA

TT

A

CG

CG

CG

T

AG

C

T

A

T

A

AT

T

A

TC

T


10.5 DNA replication: A closer look

• DNA replication begins at specific sites (origins of replication) on the double helix

– Proteins (such as enzyme helicase) attach and separate the strands

– Replication proceeds in both directions, creating replication bubbles

• Parent strands open, daughter strands elongate

– Replication occurs simultaneously at many sites


Figure 10.5A

Origin of replication

Two daughter DNA molecules

Parental strand

Daughter strand

Bubble


• Each strand of the double helix

– Is oriented in the opposite direction

Figure 10.5B

P

P

P

P

P

P

P

P

HO

OH

A

C

G

T

T

C

G

A

2 134

5

15 4

32

5 end 3 end

3 end 5 end


• Using the enzyme DNA polymerase

– The cell synthesizes one daughter strand as a continuous piece

• The other strand is synthesized as a series of short pieces

– Which are then connected by the enzyme DNA ligase

Figure 10.5C

3

53

53

5

53

Daughter strandsynthesizedcontinuously

Daughter strandsynthesizedin pieces

Parental DNA

DNA ligase

DNA polymerasemolecule

Overall direction of replication


• DNA's sugar-phosphate backbones are oriented in opposite directions

– The enzyme DNA polymerase adds nucleotides at only the 3’ end so the new strand is built in the 5’ 3’direction

• One daughter strand is synthesized as a continuous piece = leading strand

• The other strand is synthesized as a series of short pieces = lagging strands (AKA Okazaki fragments)

• The lagging strands are connected by the enzyme DNA ligase



DNA REPLICATON SUMMARY

ENZYME FUNCTION

Helicases

Primase

DNA Polymerases

Ligase


THE FLOW OF GENETIC INFORMATION FROM DNA TO RNA TO PROTEIN

• 10.6 The DNA genotype is expressed as proteins, which provide the molecular basis for phenotypic traits

• The information constituting an organism’s genotype

– Is carried in its sequence of its DNA bases

• A particular gene, a linear sequence of many nucleotides

– Specifies a polypeptide


• The DNA of the gene is transcribed into RNA

– Which is translated into the polypeptide

Figure 10.6A

DNA

Transcription

RNA

Protein

Translation


• The flow of genetic information (2 stages)

1. Transcription of the genetic information in DNA into RNA (DNA RNA) – occurs in the nucleus

2. Translation of RNA into the polypeptide (RNA proteins) – occurs on the ribosome either in the cytoplasm or attached to the rough e.r.


10.7 Genetic information written in codons is translated into amino acid sequences

• The “words” of the DNA “language”

– Are triplets of bases called codons

• The codons in a gene

– Specify the amino acid sequence of a polypeptide


DNA strand

Transcription

Translation

Polypeptide

RNA

Amino acid

Codon

A A A C C G G C A A A A

U U U G G C C G U U U U

Gene 1

Gene 2

Gene 3

DNA molecule

Figure 10.7


10.8 The genetic code is the Rosetta stone of life

• Nearly all organisms use exactly the same genetic code

Figure 10.8A

UUC

UGUUGC

UGA Stop

Met or start

Phe

Leu

Leu

Ile

Val Ala

Thr

Pro

Ser

Asn

Lys

His

Gln

Asp

Glu

Ser

Arg

Arg

Gly

CysTyr

G

A

C

U

U C A G

Th

ird

bas

e

Second base

Fir

st b

ase

UUA

UUU

CUC

CUU

CUG

CUA

AUC

AUU

AUG

AUA

GUC

GUU

GUG

GUA

UCC

UCU

UCG

UCA

CCC

CCU

CCG

CCA

ACC

ACU

ACC

ACA

GCC

GCU

GCG

GCA

UAC

UAU

UAG Stop

UAA Stop

CAC

CAU

CAG

CAA

AAC

AAU

AAG

AAA

GAC

GAU

GAG

GAA

UGG Trp

CGC

CGU

CGG

CGA

AGC

AGU

AGG

AGA

GGC

GGU

GGG

GGA

U

C

A

G

U

C

A

G

U

C

A

G

U

C

A

G

UUG


• An exercise in translating the genetic code

Figure 10.8B

T A C T T C A A A A T C

A T G A A G T T T T A G

A U G A A G U U U U A G

Transcription

Translation

RNA

DNA

Met Lys PhePolypeptide

Startcondon

Stopcondon

Strand to be transcribed


10.9 Transcription produces genetic messages in the form of RNA

• One DNA strand serves as a template for the new RNA strand

• RNA polymerase constructs the RNA strand in a multistep process

– Initiation

• RNA polymerase attaches to the promotor

• Synthesis starts


• Elongation:

– RNA synthesis continues as RNA polymerase brings in complementary RNA nucleotides

– RNA strand peels away from DNA template

– DNA strands come back together in transcribed region

• Termination

– RNA polymerase reaches a terminator sequence at the end of the gene

– Polymerase detaches




LE 10-9a

RNApolymerase

RNA nucleotides

Templatestrand of DNA

Direction oftranscription

Newly made RNA

T A AC CA

CT

C C AA

U

T

T

GG

U

G T T AA

C

CUAC

GG

T A




LE 10-9bRNA polymerase

Initiation

PromoterDNA

TerminatorDNA

Area shownIn Figure 10.9A

Initiation

Elongation

TerminationGrowingRNA

Completed RNA RNApolymerase

DNA of gene


THREE TYPES OF RNA

•Messenger RNA = mRNA holds the genetic code (contains the codons) and is “read” by the ribosome to make proteins

•Transfer RNA = tRNA carries (transfers) the correct amino acid to the ribosome

•Ribosomal RNA = rRNA combines with other proteins to make ribosomes


http://www.youtube.com/watch?v=SMtWvDbfHLo&safety_mode=true&persist_safety

_mode=1&safe=active

See transcription animation!

http://www.youtube.com/watch?feature=endscreen&v=ztPkv7wc3yU&NR=1&safety_mode=true&persist_safety_mode=1&safe=a

ctive

http://www.youtube.com/watch?v=NJxobgkPEAo&feature=related&safety_mode

=true&persist_safety_mode=1&safe=active

http://www.youtube.com/watch?v=983lhh20rGY&safety_mode=true&persist_saf

ety_mode=1&safe=active


10.10 Eukaryotic RNA is processed before leaving the nucleus

RNA PROCESSING:

• Doesn’t occur in prokaryotes because prokaryotes use all of their DNA

• In eukaryotes, RNA transcribed in the nucleus is processed before moving to the cytoplasm for translation


Steps to RNA Processing:

– Noncoding segments called introns are cut out

– Remaining exons are joined to form a continuous coding sequence

– A 5’cap and a 3’ polyA-tail are added to the ends of the mRNA


10.10 Eukaryotic RNA is processed before leaving the nucleus

• Noncoding segments called introns are spliced out

– And a cap and a tail are added to the ends

Exon Intron Exon Intron Exon

DNA

Cap TranscriptionAddition of cap and tail

RNAtranscript with capand tail

Introns removedTail

Exons spliced together

mRNA

Coding sequence Nucleus

Cytoplasm

Figure 10.10


10.11 Transfer RNA molecules serve as interpreters during translation

• Translation

– Takes place in the cytoplasm


• A ribosome attaches to the mRNA

– And translates its message into a specific polypeptide aided by transfer RNAs (tRNAs)

Amino acid attachment site

Hydrogen bond

RNA polynucleotide chain

AnticodonFigure 10.11A


• Each tRNA molecule

– Is a folded molecule bearing a base triplet called an anticodon on one end

• A specific amino acid

– Is attached to the other endAmino acidattachment site

AnticodonFigure 10.11B, C


10.12 Ribosomes build polypeptides

• A ribosome consists of two subunits

– Each made up of proteins and a kind of RNA called ribosomal RNA

tRNAmolecules

mRNA Small subunit

Growingpolypeptide

Largesubunit

Figure 10.12A


• The subunits of a ribosome

– Hold the tRNA and mRNA close together during translation

Largesubunit

mRNA-binding site

Smallsubunit

tRNA-binding sites

Growing polypeptide

Next amino acid to be added to polypeptide

mRNA

tRNA

Codons

Figure 10.12B, C


10.13 An initiation codon marks the start of an mRNA message

Start of genetic message

End

Figure 10.13A


• mRNA, a specific tRNA, and the ribosome subunits

– Assemble during initiation

Met Met

Initiator tRNA

1 2mRNA Small ribosomal

subunit

Startcodon

Large ribosomalsubunit

A siteU A CAU C

A U G A U G

P site

Figure 10.13B


10.14 Elongation adds amino acids to the polypeptide chain until a stop codon terminates translation

• Once initiation is complete

– Amino acids are added one by one to the first amino acid


• Each addition of an amino acid

– Occurs in a three-step elongation process

Polypeptide

P site

mRNA Codons

mRNAmovement

Stopcodon

NewPeptidebond

Anticodon

Aminoacid

A site

Figure 10.14

1 Codon recognition

2 Peptide bondformation

3 Translocation


• The mRNA moves a codon at a time

– And a tRNA with a complementary anticodon pairs with each codon, adding its amino acid to the peptide chain


• Elongation continues

– Until a stop codon reaches the ribosome’s A site, terminating translation


10.15 Review: The flow of genetic information in the cell is DNARNAprotein

• The sequence of codons in DNA, via the sequence of codons

– Spells out the primary structure of a polypeptide


Polypeptide

TranscriptionDNA

mRNA

RNApolymerase

Amino acid Translation

tRNA

Enzyme

Anticodon

ATP

InitiatortRNA

Largeribosomalsubunit

Start Codon

Codons

mRNA

Stop codon

Smallribosomalsubunit

Growingpolypeptide

New peptidebond forming

mRNA

Figure 10.15

• Summary of transcription and translation

mRNA is transcribed from a DNA template.1

Each amino acidattaches to its propertRNA with the help of aspecific enzyme and ATP.

2

Initiation ofpolypeptide synthesis

The mRNA, the first tRNA,and the ribosomal subunits come together.

3

Elongation4A succession of tRNAsadd their amino acids to the polypeptide chain as the mRNA is moved through the ribosome, one codon at a time.

5

The ribosome recognizes a stop codon. The poly-peptide is terminated and released.

Termination


10.16 Mutations can change the meaning of genes

• Mutations are changes in the DNA base sequence

– Caused by errors in DNA replication or recombination, or by mutagens

C T T C A T

Normal hemoglobin

Mutant hemoglobin DNA

G A A G U A

Sickle-cell hemoglobin

Normal hemoglobin DNA

Glu Val

mRNA mRNA

Figure 10.16A


• Substituting, inserting, or deleting nucleotides alters a gene

– With varying effects on the organismNormal gene

mRNA

Base substitution

Base deletion Missing

Met Lys Phe Gly Ala

Met Lys Phe Ser Ala

Met Lys Leu Ala His

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

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

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

U

Protein

Figure 10.16B


10.21 The AIDS virus makes DNA on an RNA template

• HIV, the AIDS virus

– Is a retrovirus Envelope

Glycoprotein

Protein coat

RNA (two identical strands)

Reverse transcriptase

Figure 10.21A


• Inside a cell, HIV uses its RNA as a template for making DNA

– To insert into a host chromosome

Viral RNA

RNAstrand

Double-strandedDNA

Viral RNAand proteins

CYTOPLASM

NUCLEUSChromosomal DNA

Provirus DNA

RNA

Figure 10.21B

1

2

3

45

6


10.22 Bacteria can transfer DNA in three ways

• Bacteria can transfer genes from cell to cell by one of three processes

– Transformation, transduction, or conjugation

DNA enterscell

Fragment of DNAfrom anotherbacterial cell

Bacterial chromosome

(DNA)

Phage

Fragment of DNA fromanotherbacterial cell(former phagehost)

Phage

Sex pili

Mating bridge

Donor cell(“male”)

Recipient cell(“female”)

Figure 10.22A–C


• Once new DNA gets into a bacterial cell

– Part of it may then integrate into the recipient’s chromosome

Recipient cell’schromosome

Recombinantchromosome

Donated DNACrossovers Degraded DNA

Figure 10.22D


10.23 Bacterial plasmids can serve as carriers for gene transfer

• Plasmids

– Are small circular DNA molecules separate from the bacterial chromosome


• Plasmids can serve as carriers

– For the transfer of genes

Plasmids

Co

loriz

ed

TE

M 2

,00

0

Cell now male

Plasmid completes transferand circularizes

F factor starts replication and transfer

Male (donor) cell

Bacterial chromosome

F factor (plasmid)

Recombination can occur

Only part of the chromosome transfers

F factor starts replication and transfer of chromosome

Origin of F replicationBacterial chromosome

Male (donor) cellF factor (integrated)

Recipient cell

Figure 10.23A–C

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