chapter 10

© 2010 Pearson Education, Inc.

Lectures by Chris C. Romero, updated by Edward J. Zalisko

PowerPoint® Lectures forCampbell Essential Biology, Fourth Edition – Eric Simon, Jane Reece, and Jean DickeyCampbell Essential Biology with Physiology, Third Edition – Eric Simon, Jane Reece, and Jean Dickey

Chapter 10Chapter 10

The Structure and Function of DNA


DNA: STRUCTURE AND REPLICATION• DNA:

– Was known to be a chemical in cells by the end of the nineteenth century

– Has the capacity to store genetic information

– Can be copied and passed from generation to generation

• DNA and RNA are nucleic acids.

– They consist of chemical units called nucleotides.

– The nucleotides are joined by a sugar-phosphate backbone.

Sugar-phosphatebackbone

Phosphate group

Nitrogenous base

DNA nucleotide

Nucleotide Thymine (T)

Sugar

Polynucleotide

DNAdouble helix

Sugar(deoxyribose)

Phosphategroup

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

Figure 10.1


• The four nucleotides found in DNA differ in their nitrogenous bases. These bases are:

– Thymine (T)

– Cytosine (C)

– Adenine (A)

– Guanine (G)

• RNA has uracil (U) in place of thymine.

• James Watson and Francis Crick determined that DNA is a double helix.

James Watson (left) and Francis CrickFigure 10.3a

X-ray image of DNA

Rosalind Franklin

Figure 10.3b


• The model of DNA is like a rope ladder twisted into a spiral.

– The ropes at the sides represent the sugar-phosphate backbones.

– Each wooden rung represents a pair of bases connected by hydrogen bonds.


• DNA bases pair in a complementary fashion:

– Adenine (A) pairs with thymine (T)

– Cytosine (C) pairs with guanine (G)

(a) Ribbon modelFigure 10.5a


DNA Replication

• When a cell reproduces, a complete copy of the DNA must pass from one generation to the next.

• Watson and Crick’s model for DNA suggested that DNA replicates by a template mechanism.

Parental (old)DNA molecule

Daughter (new) strand

DaughterDNA molecules (double helices)

Figure 10.6


• DNA can be damaged by ultraviolet light.

• DNA polymerases:

– Are enzymes

– Make the covalent bonds between the nucleotides of a new DNA strand

– Are involved in repairing damaged DNA


• DNA specifies the synthesis of proteins in two stages:

– Transcription, the transfer of genetic information from DNA into an RNA molecule

– Translation, the transfer of information from RNA into a protein

TRANSLATION

Protein

RNA

TRANSCRIPTION

DNA

Cytoplasm

Nucleus

Figure 10.8-3


• When DNA is transcribed, the result is an RNA molecule.

• RNA is then translated into a sequence of amino acids in a polypeptide (protein).


• What are the rules for translating the RNA message into a polypeptide?

• A codon is a triplet of bases, which codes for one amino acid.

Second base of RNA codonF

irst

ba

se o

f R

NA

co

do

n

Phenylalanine(Phe)

Leucine(Leu)

Cysteine(Cys)

Leucine(Leu)

Isoleucine(Ile)

Valine(Val)

Met or start

Serine(Ser)

Proline(Pro)

Threonine(Thr)

Tyrosine(Tyr)

Histidine(His)

Glutamine(Gln)

Asparagine(Asn)

Alanine(Ala)

Stop Stop

Stop

Glutamicacid (Glu)

Asparticacid (Asp)

Lysine(Lys)

Arginine(Arg)

Tryptophan (Trp)

Arginine(Arg)

Serine(Ser)

Glycine(Gly)

Th

ird

ba

se o

f R

NA

co

do

n

Figure 10.11


Transcription: From DNA to RNA

• Transcription:

– Makes RNA from a DNA template

– Uses a process that resembles DNA replication

– Substitutes uracil (U) for thymine (T)

• RNA nucleotides are linked by RNA polymerase.

• The “start transcribing” signal is a nucleotide sequence called a promoter.


Initiation of Transcription

• The first phase of transcription is initiation, in which:

– RNA polymerase attaches to the promoter

– RNA synthesis begins

• During the second phase of transcription, called elongation:

– The RNA grows longer

– The RNA strand peels away from the DNA template

• During the third phase of transcription, called termination:

– RNA polymerase reaches a sequence of DNA bases called a terminator

– Polymerase detaches from the RNA

– The DNA strands rejoin

Newly made RNA

RNA nucleotides

RNA polymerase

Templatestrand of DNA

Direction oftranscription

(a) A close-up view of transcription

Figure 10.13a


Translation: The Players

• Translation is the conversion from the nucleic acid language to the protein language.

• Transfer RNA (tRNA):

– Acts as a molecular interpreter

– Carries amino acids

– Matches amino acids with codons in mRNA using anticodons

• Ribosomes are organelles that:

– Coordinate the functions of mRNA and tRNA

– Are made of two protein subunits

– Contain ribosomal RNA (rRNA)

tRNA polynucleotide(ribbon model)

RNA polynucleotidechain

Anticodon

Hydrogen bond

Amino acid attachment site

tRNA (simplifiedrepresentation)

Figure 10.15

Next amino acidto be added to polypeptide

Growingpolypeptide

tRNA

mRNA

Codons

(b) The “players” of translationFigure 10.16b


Translation: The Process

• Translation is divided into three phases:

– Initiation

– Elongation

– Termination


Initiation

• Initiation brings together:

– mRNA

– The first amino acid, Met, with its attached tRNA

– Two subunits of the ribosome

• The mRNA molecule has a cap and tail that help it bind to the ribosome.

• Initiation occurs in two steps:

– First, an mRNA molecule binds to a small ribosomal subunit, then an initiator tRNA binds to the start codon.

– Second, a large ribosomal subunit binds, creating a functional ribosome.


Elongation

• Elongation occurs in three steps.

– Step 1, codon recognition:

– the anticodon of an incoming tRNA pairs with the mRNA codon at the A site of the ribosome.

– Step 2, peptide bond formation:

– The polypeptide leaves the tRNA in the P site and attaches to the amino acid on the tRNA in the A site

– The ribosome catalyzes the bond formation between the two amino acids


– Step 3, translocation:

– The P site tRNA leaves the ribosome

– The tRNA carrying the polypeptide moves from the A to the P site

• Elongation continues until:

– The ribosome reaches a stop codon

– The completed polypeptide is freed

– The ribosome splits into its subunits

Newpeptidebond

Stopcodon

mRNAmovement

mRNA

P site

Translocation

Peptide bond formation

Polypeptide

ELONGATION

Codon recognition

Asite

Codons

Anticodon

Amino acid

Figure 10.19-4


Review: DNA RNA Protein• In a cell, genetic information flows from DNA to RNA in the

nucleus and RNA to protein in the cytoplasm.

Transcription RNA polymerase

mRNA DNA

Intron

Nucleus

mRNAIntron

TailCap

RNA processing

tRNA

Amino acid

Amino acidattachment

EnzymeATP Initiation

of translation

Ribosomalsubunits

Elongation

AnticodonCodon

Termination

Polypeptide

Stopcodon

Figure 10.20-6


Mutations

• A mutation is any change in the nucleotide sequence of DNA.

• Mutations can change the amino acids in a protein.

• Mutations can involve:

– Large regions of a chromosome

– Just a single nucleotide pair, as occurs in sickle cell anemia

• Mutations may result from:

– Errors in DNA replication

– Physical or chemical agents called mutagens


Types of Mutations

• Mutations within a gene can occur as a result of:

– Base substitution, the replacement of one base by another

– Nucleotide deletion, the loss of a nucleotide

– Nucleotide insertion, the addition of a nucleotide

Normal hemoglobin DNA

mRNA

Normal hemoglobin

Mutant hemoglobin DNA

mRNA

Sickle-cell hemoglobin

Figure 10.21

chapter 10

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