the structure of the genetic material © 2012 pearson education, inc

THE STRUCTURE OF THE GENETIC MATERIAL

© 2012 Pearson Education, Inc.

SCIENTIFIC DISCOVERY: DNA is a double-stranded 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.


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

SCIENTIFIC DISCOVERY: DNA is a double-stranded helix


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


Figure 10.3D_2

Hydrogen bond

Partial chemicalstructure

G C

T A

A T

C G

Phosphategroup

Sugar(deoxyribose)

DNA nucleotide

Thymine (T)

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

Each DNA nucleotide has a different nitrogen-containing base: adenine (A), cytosine (C), thymine (T), guanine (G).

Thymine (T) Cytosine (C)

Pyrimidines Purines

Adenine (A) Guanine (G)

Sugar(ribose)

Uracil (U)RNA contains the nitrogenous base uracil (U) instead of thymine

DNA REPLICATION


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.


Parental DNAmolecule

Daughterstrand

Parentalstrand

Daughter DNAmolecules

A T

G C

A T

A T

T A

T T

A

C

G

C

GC

G

A

T

A

T

G

CT

C G

T

C G

C G

AC

G C

A TA TG C

A T

G

A

A

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

DNA replication proceeds in two directions at many sites simultaneously


Figure 10.5A

ParentalDNAmolecule Origin of

replication

“Bubble”

Parental strand

Daughter strand

TwodaughterDNAmolecules

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




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.


http://www.youtube.com/watch?v=zdDkiRw1PdU

Overall direction of replication

DNA ligase

Replication fork

Parental DNA

DNA polymerasemolecule Leading Strand

Lagging Strand35

35

3

5

35

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.



THE FLOW OF GENETIC INFORMATION FROM DNA TO

RNA TO PROTEIN


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.


Figure 10.6A_s3

DNA

NUCLEUS

CYTOPLASM

RNA

Transcription

Translation

Protein

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


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.


Figure 10.7_1

A

Transcription

RNA

Translation Codon

Polypeptide

Aminoacid

A A C C G G C A A A A

U U G G C C G U UU U

DNA

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.


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


Second base

Th

ird

bas

e

Fir

st b

ase

Figure 10.8B_s3

T

Strand to be transcribed

A C T T C AA

A A A T

DNAAA T C

T T T T G A G G

RNA

Transcription

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

Translation

Polypeptide Met Lys Phe

Stopcodon

Startcodon

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


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


RNA polymerase

DNA of gene

PromoterDNA

Initiation1

TerminatorDNA

2 Elongation Area shownin Figure 10.9A

GrowingRNA

Termination

RNApolymerase

CompletedRNA

3GrowingRNA

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


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.


Figure 10.10

DNA

Cap

Exon Intron Exon

RNAtranscriptwith capand tail

ExonIntron

TranscriptionAddition of cap and tail

Introns removed Tail

Exons spliced together

Coding sequenceNUCLEUS

CYTOPLASM

mRNA

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


Amino acidattachment 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.


tRNA binding sites

mRNA binding site

Large subunit

Small subunit

Psite A

site

Figure 10.12C

mRNA

Codons

tRNA

Growingpolypeptide

The next aminoacid to be addedto the polypeptide

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.


Figure 10.13B

InitiatortRNA

mRNA

Start codon

Smallribosomalsubunit

Largeribosomalsubunit

Psite

Asite

Met

A U G

U A C

2

A U G

U A C

1

Met

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.


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.



Termination

1. Ribosome reaches a stop codon

2. Completed polypeptide is freed from the last tRNA,

3. The ribosome splits into separate subunits.



Figure 10.14_s4

Polypeptide

mRNA

Codon recognition

Anticodon

Aminoacid

Codons

Psite

Asite

1

Peptide bond2

formation

Translocation3

Newpeptidebond

Stopcodon

mRNAmovement

Mutations can change the meaning of genes

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



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,

– change the amino acid coding, producing a missense mutation, which produces a different amino acid,

– change an amino acid into a stop codon, producing a nonsense mutation.



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.



Mutations can be caused by

– spontaneous errors that occur during DNA replication or recombination

– mutagens, which include high-energy radiation (X-rays) and UV light and chemicals


the structure of the genetic material © 2012 pearson education, inc

Documents

pearson education

new dna helix

secondary structure

dna strands separate

semiconservative replication

replication fork

thyminedna replication

otherdna replication