Download Slide 1

THE STRUCTURE OF THE
GENETIC MATERIAL
© 2012 Pearson Education, Inc.
SCIENTIFIC DISCOVERY: DNA is a doublestranded helix
 In 1953, James D. Watson and Francis
Crick deduced the secondary structure
of DNA, using X-ray crystallography
data of DNA from the work of Rosalind
Franklin and Maurice Wilkins and
Chargaff’s observation that in DNA the
amount of adenine was equal to the
amount of thymine and the amount of
guanine was equal to that of cytosine.
© 2012 Pearson Education, Inc.
SCIENTIFIC DISCOVERY: DNA is a doublestranded helix
 Watson and Crick reported that DNA consisted of
two polynucleotide strands wrapped into a double
helix.
– The sugar-phosphate backbone is on the outside.
– The nitrogenous bases are perpendicular to the
backbone in the interior.
– Specific pairs of bases give the helix a uniform shape.
– A pairs with T, with two hydrogen bonds
– G pairs with C, with three hydrogen bonds
© 2012 Pearson Education, Inc.
DNA and RNA are polymers of nucleotides
 DNA and RNA = nucleic acids
composed of long chains of
nucleotides
 A nucleotide = nitrogenous base,
five-carbon sugar, and phosphate
group
 Nucleotides are joined to one
another by a sugar-phosphate
backbone.
 DNA is antiparallel
© 2012 Pearson Education, Inc.
Figure 10.3D_2
Hydrogen bond
G
T
C
A
A
C
T
G
Partial chemical
structure
 Each DNA nucleotide has a different nitrogencontaining base: adenine (A), cytosine (C),
thymine (T), guanine (G).
Nitrogenous base
(can be A, G, C, or T)
Thymine (T)
Phosphate
group
Sugar
(deoxyribose)
DNA nucleotide
Thymine (T)
Cytosine (C)
Pyrimidines
Guanine (G)
Adenine (A)
Purines
RNA contains
the nitrogenous
base uracil (U)
instead of
thymine
Uracil (U)
Sugar
(ribose)
DNA REPLICATION
© 2012 Pearson Education, Inc.
DNA replication depends on specific base pairing
 In their description of the structure of DNA,
Watson and Crick noted that the structure of DNA
suggests a possible copying mechanism.
 DNA replication follows a semiconservative
model where,
– The two DNA strands separate.
– Each strand is used as a template to produce a
complementary strand, using specific base pairing.
– Semiconservative replication means that each new
DNA helix has one old strand with one new strand.
© 2012 Pearson Education, Inc.
A
T
G
A
A
T
C
T
T
A
Parental DNA
molecule
Daughter
strand
Parental
strand
Daughter DNA
molecules
DNA replication proceeds in two directions at
many sites simultaneously
 DNA replication begins at the origins of
replication where
– DNA unwinds at the origin to produce a replication
bubble
– Replication proceeds in both directions from the origin
– Replication ends when products from the bubbles
merge with each other
© 2012 Pearson Education, Inc.
Figure 10.5A
Parental
DNA
molecule
Origin of
replication
“Bubble”
Two
daughter
DNA
molecules
Parental strand
Daughter strand
DNA replication proceeds in two directions at
many sites simultaneously
 Both daughter strands are synthesized in the 5’ to
3’ direction = new nucleotides are only added to
the 3’ end of the growing strand
– One daughter strand is synthesized in one continuous
piece, toward replication fork=leading strand
– The other daughter strand is synthesized in (Okazaki)
fragments =lagging strand
– Consequence of the antiparallel nature of DNA
© 2012 Pearson Education, Inc.
DNA replication proceeds in two directions at
many sites simultaneously
 Two key proteins are involved in DNA replication.
1. DNA polymerase adds nucleotides to a growing chain
and proofreads and corrects improper base pairings.
2. DNA ligase joins Okazaki fragments into a continuous
chain.
3. Helicase-is a helix breaker; it unwinds the DNA double
helix by breaking the H-bonds.
© 2012 Pearson Education, Inc.
DNA polymerase
molecule
5
3
3
5
Leading Strand
Parental DNA
Replication fork
Lagging Strand
3
5
5
3
DNA ligase
Overall direction of replication
DNA replication proceeds in two directions at
many sites simultaneously
 DNA polymerases and DNA ligase also repair
DNA damaged by harmful radiation and toxic
chemicals.
 DNA replication ensures that all somatic cells in a
multicellular organism carry the same genetic
information.
© 2012 Pearson Education, Inc.
THE FLOW OF GENETIC
INFORMATION FROM DNA TO
RNA TO PROTEIN
© 2012 Pearson Education, Inc.
The DNA genotype is expressed as proteins,
which provide the molecular basis for
phenotypic traits
 DNA specifies traits by dictating protein synthesis.
 The molecular chain of command is from DNA to
RNA to protein
 Transcription is the synthesis of RNA under the
direction of DNA.
 Translation is the synthesis of proteins under the
direction of RNA.
© 2012 Pearson Education, Inc.
Figure 10.6A_s3
DNA
Transcription
RNA
NUCLEUS
Translation
Protein
CYTOPLASM
The DNA genotype is expressed as proteins,
which provide the molecular basis for
phenotypic traits
 The connections between genes and proteins
– In the 1940’s Beadle and Tatum suggested a one
gene–one enzyme hypothesis based on studies of
inherited metabolic diseases
– Their hypothesis is still accepted but with important
changes:
– The hypothesis includes all proteins not just
enzymes, and that one gene specifies a
polypeptide
© 2012 Pearson Education, Inc.
Genetic information written in codons is
translated into amino acid sequences
 The genetic code is written in DNA as a series of three
base sequences called triplets
 During transcription the genetic code in DNA is copied into
a complementary three base sequences in RNA called
codons
 Translation involves switching from the nucleotide
“language” to the amino acid “language.”
– Each amino acid is specified by a codon.
– 64 codons are possible.
© 2012 Pearson Education, Inc.
Figure 10.7_1
DNA
A A
A C
U U
U
C G G
C
A
A
A A
C G U
U
U
Transcription
RNA
Translation
Codon
Polypeptide
Amino
acid
G
G C
U
The genetic code dictates how codons are
translated into amino acids
 Characteristics of the genetic code
 Three nucleotides (a codon) specify
one amino acid.
 AUG = start codon that codes for
methionine; signals the start of
translation
 3 “stop” codons signal the end of
translation.
© 2012 Pearson Education, Inc.
The genetic code dictates how codons are
translated into amino acids
 The genetic code is
– redundant, more than one codon for the same
amino acid
– Unambiguous, in that any codon for one
amino acid does not code for any other amino
acid,
– nearly universal, the genetic code is shared
by organisms from the simplest bacteria to the
most complex plants and animals
© 2012 Pearson Education, Inc.
Third base
First base
Second base
Figure 10.8B_s3
Strand to be transcribed
T A C T
T
C A A A A T
C
DNA
A T G A A G T
T T
T A G
Transcription
RNA
A U G A A G U U U U A G
Translation
Start
codon
Polypeptide
Met
Stop
codon
Lys
Phe
Transcription produces genetic messages in the
form of RNA
 Overview of transcription
– RNA polymerase oversees transcription by unwinding
DNA, and linking RNA nucleotides together to
synthesize an RNA molecule
– The promoter is a nucleotide sequence in DNA that
signals the start of transcription
© 2012 Pearson Education, Inc.
Transcription produces genetic messages in the
form of RNA
 Steps of Transcription
– Initiation
– RNA polymerase attaches to promoter.
– Elongation
– RNA grows longer.
– As RNA peels away, DNA strands rejoin.
– Termination
– RNA polymerase reaches a sequence of bases in the
DNA template called a terminator, signaling the end
of the gene.
– RNA pol detaches from RNA and the gene being
transcribed
© 2012 Pearson Education, Inc.
RNA polymerase
DNA of gene
Promoter
DNA
1
Initiation
Terminator
DNA
2
Elongation
Area shown
in Figure 10.9A
Growing
RNA
3
Completed
RNA
Termination
Growing
RNA
RNA
polymerase
Eukaryotic RNA is processed before leaving the
nucleus as mRNA
 Messenger RNA (mRNA)
– The RNA formed from transcription, carrying the
genetic code is called mRNA
– mRNA carries the message from DNA (nucleus) to
ribosomes (cytoplasm)
– In prokaryotes transcription and translation occur in the same
place
– In eukaryotes, mRNA must exit nucleus via nuclear pores to
enter cytoplasm.
– Eukaryotic mRNA has introns or interrupting sequences that
separate exons, the coding regions
© 2012 Pearson Education, Inc.
Eukaryotic RNA is processed before leaving the
nucleus as mRNA
 Eukaryotic mRNA undergoes processing before
leaving the nucleus.
– RNA splicing removes introns and joins exons to
produce a continuous coding sequence.
– A 5’ cap and a 3’ tail of extra nucleotides are added to
the ends of mRNA to
– Facilitate export of mRNA from nucleus,
– Protect the mRNA from attack by cellular enzymes
– To help ribosomes bind to the mRNA.
© 2012 Pearson Education, Inc.
Figure 10.10
Exon Intron
Exon
Intron
Exon
DNA
Cap
RNA
transcript
with cap
and tail
Transcription
Addition of cap and tail
Introns removed
Tail
Exons spliced together
mRNA
Coding sequence
NUCLEUS
CYTOPLASM
Figure 10.13A
Start of genetic message
Cap
End
Tail
Transfer RNA molecules serve as interpreters
during translation
 Transfer RNA (tRNA)
molecules convert the
codons of mRNA into the
amino acid sequence of
proteins.
 tRNA’s recognize the
codons in mRNA with a
three base sequence alled
an anticodon
© 2012 Pearson Education, Inc.
Amino acid
attachment site
Anticodon
Ribosomes build polypeptides
 Translation occurs on the surface of the ribosome.
– Ribosomes coordinate the functioning of mRNA , tRNA &
therefore synthesis of polypeptides.
– Ribosomes have two subunits: small and large.
– Each subunit is composed of ribosomal RNAs and
proteins.
– Ribosomal subunits come together during translation
– Ribosomes have binding sites for mRNA and tRNAs.
© 2012 Pearson Education, Inc.
tRNA binding sites
Large
subunit
P
site
A
site
Small
subunit
mRNA binding site
Figure 10.12C
The next amino
acid to be added
to the polypeptide
Growing
polypeptide
mRNA
tRNA
Codons
An initiation codon marks the start of an mRNA
message
 Initiation occurs in two steps.
1. An mRNA molecule binds to a small ribosomal subunit and
the first tRNA binds to mRNA at the start codon.
– Start codon = AUG and codes for methionine.
– The first tRNA has the anticodon UAC.
2. A large ribosomal subunit joins the small subunit, allowing
the ribosome to function.
– The first tRNA occupies the P site, which will hold the growing
peptide chain.
– The A site is available to receive the next tRNA.
© 2012 Pearson Education, Inc.
Figure 10.13B
Large
ribosomal
subunit
Initiator
tRNA
P
site
mRNA
U A C
A U G
Start codon
1
Small
ribosomal
subunit
2
A
site
U A C
A U G
Elongation adds amino acids to the polypeptide
chain until a stop codon terminates
translation
 Elongation is the addition of amino acids to the
polypeptide chain and each cycle of elongation
has three steps.
© 2012 Pearson Education, Inc.
Elongation adds amino acids to the polypeptide
chain until a stop codon terminates
translation
1. Codon recognition: The anticodon of an incoming
tRNA molecule, carrying its amino acid, pairs with the
mRNA codon in the A site of the ribosome.
2. Peptide bond formation: The new amino acid is
joined to the chain.
3. Translocation: tRNA is released from the P site and
the ribosome moves tRNA from the A site into the P
site.
© 2012 Pearson Education, Inc.
Elongation adds amino acids to the polypeptide
chain until a stop codon terminates
translation
 Termination
1. Ribosome reaches a stop codon
2. Completed polypeptide is freed from the last tRNA,
3. The ribosome splits into separate subunits.
© 2012 Pearson Education, Inc.
Figure 10.14_s4
Polypeptide
P
site
mRNA
Amino
acid
A
site
Anticodon
Codons
1
Codon recognition
mRNA
movement
Stop
codon
2
New
peptide
bond
3
Translocation
Peptide bond
formation
Mutations can change the meaning of genes
 A mutation is any change in the nucleotide
sequence of DNA.
© 2012 Pearson Education, Inc.
Mutations can change the meaning of genes
 Mutations within a gene can be divided into two
general categories.
1. Base substitutions involve the replacement of one
nucleotide with another. Base substitutions may
– Have no effect at all, producing a silent mutation,
– A missense mutation still produces an amino acid, just not the
correct amino acid
– A nonsense mutation changes an amino acid codon into a
stop codon, usually producing a nonfunctional protein
© 2012 Pearson Education, Inc.
Missense Mutation
Nonsense Mutation
Mutations can change the meaning of genes
2. Mutations can result in deletions or insertions that may
– alter the reading frame (triplet grouping) of the mRNA, so that
nucleotides are grouped into different codons
– This leads to significant changes in amino acid sequence
downstream of the mutation, and produce a nonfunctional
polypeptide.
© 2012 Pearson Education, Inc.
Mutations can change the meaning of genes
 Mutations can be caused by
– spontaneous errors that occur during DNA replication
or recombination
– mutagens, which include high-energy radiation (Xrays) and UV light and chemicals
© 2012 Pearson Education, Inc.