Download dna

Chapter 10
The Structure and Function
of DNA
PowerPoint® Lectures for
Campbell Essential Biology, Fourth Edition
– Eric Simon, Jane Reece, and Jean Dickey
Campbell Essential Biology with Physiology, Third Edition
– Eric Simon, Jane Reece, and Jean Dickey
Lectures by Chris C. Romero, updated by Edward J. Zalisko
© 2010 Pearson Education, Inc.
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.
© 2010 Pearson Education, Inc.
Phosphate group
Nitrogenous base
Sugar
Nucleotide
DNA
double helix
Nitrogenous base
(can be A, G, C, or T)
Thymine (T)
Phosphate
group
Sugar
(deoxyribose)
DNA nucleotide
Polynucleotide
Sugar-phosphate
backbone
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.
© 2010 Pearson Education, Inc.
James Watson (left) and Francis Crick
Figure 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.
© 2010 Pearson Education, Inc.
• DNA bases pair in a complementary fashion:
– Adenine (A) pairs with thymine (T)
– Cytosine (C) pairs with guanine (G)
© 2010 Pearson Education, Inc.
(a) Ribbon model
Figure 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.
© 2010 Pearson Education, Inc.
Parental (old)
DNA molecule
Daughter
(new) strand
Daughter
DNA 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
© 2010 Pearson Education, Inc.
• 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
© 2010 Pearson Education, Inc.
Nucleus
DNA
TRANSCRIPTION
RNA
TRANSLATION
Protein
Cytoplasm
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).
© 2010 Pearson Education, Inc.
• 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.
© 2010 Pearson Education, Inc.
Second base of RNA codon
First base of RNA codon
Leucine
(Leu)
Leucine
(Leu)
Isoleucine
(Ile)
Serine
(Ser)
Stop
Stop
Proline
(Pro)
Threonine
(Thr)
Met or start
Valine
(Val)
Tyrosine
(Tyr)
Alanine
(Ala)
Histidine
(His)
Glutamine
(Gln)
Cysteine
(Cys)
Stop
Tryptophan (Trp)
Arginine
(Arg)
Asparagine
(Asn)
Serine
(Ser)
Lysine
(Lys)
Arginine
(Arg)
Aspartic
acid (Asp)
Glutamic
acid (Glu)
Third base of RNA codon
Phenylalanine
(Phe)
Glycine
(Gly)
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.
© 2010 Pearson Education, Inc.
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
© 2010 Pearson Education, Inc.
RNA nucleotides
RNA polymerase
Newly made RNA
Direction of
transcription
Template
strand of DNA
(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)
© 2010 Pearson Education, Inc.
Amino acid attachment site
Hydrogen bond
RNA polynucleotide
chain
Anticodon
tRNA polynucleotide
(ribbon model)
tRNA
(simplified
representation)
Figure 10.15
Next amino acid
to be added to
polypeptide
Growing
polypeptide
tRNA
mRNA
Codons
(b) The “players” of translation
Figure 10.16b
Translation: The Process
• Translation is divided into three phases:
– Initiation
– Elongation
– Termination
© 2010 Pearson Education, Inc.
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.
© 2010 Pearson Education, Inc.
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
© 2010 Pearson Education, Inc.
– 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
© 2010 Pearson Education, Inc.
Amino acid
Polypeptide
P site
mRNA
Anticodon
A
site
Codons
Codon recognition
ELONGATION
Stop
codon
New
peptide
bond
Peptide bond formation
mRNA
movement
Translocation
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.
© 2010 Pearson Education, Inc.
Transcription
RNA polymerase
Polypeptide
Nucleus
DNA
mRNA
Stop
codon
Intron
RNA processing
Cap
Tail
Termination
mRNA
Intron
Anticodon
Ribosomal Codon
subunits
Amino acid
tRNA
ATP
Enzyme
Amino acid
attachment
Initiation
of translation
Elongation
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
© 2010 Pearson Education, Inc.
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
© 2010 Pearson Education, Inc.
Normal hemoglobin DNA
Mutant hemoglobin DNA
mRNA
mRNA
Normal hemoglobin
Sickle-cell hemoglobin
Figure 10.21