1. DNA, RNA structure 2. DNA replication 3. Transcription, translation

1. DNA, RNA structure

2. DNA replication

3. Transcription, translation

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

• DNA is a nucleic acid, made of long chains of nucleotides

DNA and RNA are polymers of nucleotides

Figure 10.2A

Nucleotide

Phosphate group

Nitrogenous base

Sugar

Polynucleotide Sugar-phosphate backbone

DNA nucleotide

Phosphategroup

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

Thymine (T)

Sugar(deoxyribose)


• DNA has four kinds of bases, A, T, C, and G

Figure 10.2B

Pyrimidines

Thymine (T) Cytosine (C)

Purines

Adenine (A) Guanine (G)


• RNA is also a nucleic acid

– different sugar

– U instead of T

Figure 10.2C, D

Phosphategroup

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

Uracil (U)

Sugar(ribose)


• James Watson and Francis Crick worked out the three-dimensional structure of DNA, based on work by Rosalind Franklin

DNA is a double-stranded helix

Figure 10.3A, B


• Hydrogen bonds between bases hold the strands together: A and T, C and G

Figure 10.3D

Ribbon model Partial chemical structure Computer model

Hydrogen bond


Untwisting and replication of DNA

• each strand is a template for a new strand

Figure 10.4B

helicase

DNA polymerase


• DNA replication begins at many specific sites

How can entire chromosomes be replicated during S phase?

Figure 10.5A

Parental strandOrigin of replication

Bubble

Two daughter DNA molecules

Daughter strand


• Each strand of the double helix is oriented in the opposite direction

Figure 10.5B

5 end 3 end

3 end 5 end

P

P

P

PP

P

P

P


• DNA polymerase works in only one direction

5 end

P

P

Parental DNA

Figure 10.5C

DNA polymerasemolecule

53

35

35

Daughter strandsynthesizedcontinuously

Daughter strandsynthesizedin pieces

DNA ligase

Overall direction of replication

53

• Telomere sequences are lost with each replication.

• Cancer, aging

telomeres


– The DNA is transcribed into RNA, which is translated into the polypeptide

Figure 10.6A

DNA

RNA

Protein

TRANSCRIPTION

TRANSLATION

• The information constituting an organism’s genotype is carried in its sequence of bases


Transcription produces genetic messages in the form of mRNA

Figure 10.9A

RNApolymerase

RNA nucleotide

Direction oftranscription

Newly made RNA

Templatestrand of DNA


• In transcription, DNA helix unzips

– RNA nucleotides line up along one strand of DNA, following the base-pairing rules

– single-stranded messenger RNA peels away and DNA strands rejoin

RNA polymerase

DNA of gene

PromoterDNA Terminator

DNAInitiation

Elongation

Termination

Area shownin Figure 10.9A

GrowingRNA

RNApolymerase

Completed RNA

Figure 10.9B


• Noncoding segments, introns, are spliced out

• A cap and a tail are added to the ends

Eukaryotic RNA is processed before leaving the nucleus

Figure 10.10

DNA

RNAtranscriptwith capand tail

mRNA

Exon Intron IntronExon Exon

TranscriptionAddition of cap and tail

Introns removed

Exons spliced together

Coding sequence

NUCLEUS

CYTOPLASM

Tail

Cap


• 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

Translation of nucleic acids into amino acids


Figure 10.7

DNA molecule

Gene 1

Gene 2

Gene 3

DNA strand

TRANSCRIPTION

RNA

Polypeptide

TRANSLATIONCodon

Amino acid

U C A G

U

C

A

G

GACU

GACU

GACU

GACU

UUUUUCUUAUUG

CUUCUCCUACUG

AUUAUCAUAAUG

GUUGUCGUAGUG

phe

leu

leu

ile

met (start)

val

UCUUCCUCAUCG

CCUCCCCCACCG

ACUACCACAACG

GCUGCCGCAGCG

ser

pro

thr

ala

UAUUACUAAUAG

CAUCACCAACAG

AAUAAC

AAGAAA

GAUGACGAAGAG

tyr

stopstop

his

gln

asn

lys

asp

glu

UGUUGCUGAUGG

CGUCGCCGACGG

AGUAGCAGAAGG

GGUGGCGGAGGG

cys

stoptrp

arg

ser

arg

gly

Firs

t B

ase

Third

Base

Second Base

Virtually all organisms share the same genetic code “unity of life”


• An exercise in translating the genetic code

Figure 10.8B

Startcodon

RNA

Transcribed strand

StopcodonTranslation

Transcription

DNA

Polypeptide


• In the cytoplasm, a ribosome attaches to the mRNA and translates its message into a polypeptide

• The process is aided by transfer RNAs

Transfer RNA molecules serve as interpreters during translation

Figure 10.11A

Hydrogen bond

Amino acid attachment site

RNA polynucleotide chain

Anticodon


• Each tRNA molecule has a triplet anticodon on one end and an amino acid attachment site on the other

Figure 10.11B, C

Anticodon

Amino acidattachment site


Ribosomes build polypeptides

Figure 10.12A-C

Codons

tRNAmolecules

mRNA

Growingpolypeptide

Largesubunit

Smallsubunit

mRNA

mRNAbindingsite

P site A site

P A

Growingpolypeptide

tRNA

Next amino acidto be added topolypeptide


An initiation codon marks the start of an mRNA message

Figure 10.13A

End

Start of genetic message


• mRNA, a specific tRNA, and the ribosome subunits assemble during initiation

Figure 10.13B

1

Initiator tRNA

mRNA

Startcodon Small ribosomal

subunit

2

P site

Largeribosomalsubunit

A site


• The mRNA moves a codon at a time relative to the ribosome

– A tRNA pairs with each codon, adding an amino acid to the growing polypeptide

– A STOP codon causes the mRNA-ribosome complex to fall apart

Elongation


Figure 10.14

1 Codon recognition

Amino acid

Anticodon

AsiteP site

Polypeptide

2 Peptide bond formation

3 Translocation

Newpeptidebond

mRNAmovement

mRNA

Stopcodon

ba

Red object = ribosomeWhat molecules are present in this photo?

Table 14.2Types of RNA

Type of RNA Functions in Function

Messenger RNA(mRNA)

Nucleus, migratesto ribosomesin cytoplasm

Carries DNA sequenceinformation to ribosomes

Transfer RNA(tRNA)

Cytoplasm Provides linkage between mRNAand amino acids;transfers aminoacids to ribosomes

Ribosomal RNA(rRNA)

Cytoplasm Structural component of ribosomes


• The sequence of codons in DNA spells out the primary structure of a polypeptide

– Polypeptides form proteins that cells and organisms use

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


• Mutations are changes in the DNA base sequence

– caused by errors in DNA replication or by mutagens

– change of a single DNA nucleotide causes sickle-cell disease

Mutations can change the meaning of genes


Figure 10.16A

Normal hemoglobin DNA

mRNA

Normal hemoglobin

Glu

Mutant hemoglobin DNA

mRNA

Sickle-cell hemoglobin

Val


• Types of mutations

Figure 10.16B

mRNA

NORMAL GENE

BASE SUBSTITUTION

BASE DELETION

Protein Met Lys Phe Gly Ala

Met Lys Phe Ser Ala

Met Lys Leu Ala His

Missing


• Summary of transcription and translation

Figure 10.15

1Stage mRNA istranscribed from aDNA template.

Anticodon

DNA

mRNARNApolymerase

TRANSLATION

Enzyme

Amino acid

tRNA

InitiatortRNA

Largeribosomalsubunit

Smallribosomalsubunit

mRNA

Start Codon

2Stage Each amino acid attaches to its proper tRNA with the help of a specific enzyme and ATP.

3Stage Initiation of polypeptide synthesis

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

TRANSCRIPTION


Figure 10.15 (continued)

4Stage ElongationGrowingpolypeptide

Codons

5Stage Termination

mRNA

Newpeptidebondforming

Stop Codon

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

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

Polypeptide

Documents

1. DNA, RNA structure 2. DNA replication 3. Transcription, translation