M OLECULAR B IOLOGY OF THE G ENE Chapter 10. W HAT IS MOLECULAR B IOLOGY ? study of heredity and biology at the molecular level interactions between the.

MOLECULAR BIOLOGY OF THE GENEChapter 10

WHAT IS MOLECULAR BIOLOGY?

study of heredity and biology at the molecular level

interactions between the various systems of a cell Between different types of

DNA, RNA and protein biosynthesis

learning how these interactions are regulated

DNA VERSE RNA….REVIEWDNA VERSE RNA….REVIEW

DNA and RNA are nucleic acids DNA – genetic information RNA – used to build proteins

Built by nucleotides Can be single stranded or double stranded Bases Bonds

DNA VERSE GENE??DNA VERSE GENE??

DNA Nucleic acid Comprised of

chromosomes Every person

genetically unique

Gene Segment of DNA Carries instructions

for products to be made

Every person has same genes, but each varies

The genetic information in a chromosome is encoded in the nucleotide sequence of DNA

DNA REPLICATION

follows a semiconservative model two DNA strands separate Each strand is used as a pattern to

produce a complementary strand using specific base pairing

Each new DNA helix has one old strand with one new strand

A parentalmoleculeof DNA

A

C

G C

A T

T A

The parental strandsseparate and serve

as templates

Freenucleotides

T A T

T

A

A

T

AG

G GC

C

A T

C G

C

Two identicaldaughter moleculesof DNA are formed

A T A T

A TA T

T A T A

C G C G

G C G C

DNA REPLICATION

begins at the origins of replication: DNA unwinds at the origin to produce a

“bubble” replication proceeds in both directions

from the origin replication ends when products from the

bubbles merge with each other

ParentalDNAmolecule Origin of

replication

“Bubble”

Parental strand

Daughter strand

TwodaughterDNAmolecules

DNA REPLICATION

Occurs in the 5 to 3 direction. Replication is continuous on the 3 to 5

template Replication is discontinuous on the 5 to 3

template, forming short segments

Two key proteins are involved:o DNA ligase

o joins small fragments into a continuous chain

o DNA polymerase adds nucleotides to a growing chain proofreads and corrects improper base

pairings

FIGURE 10.5C

Overall direction of replication

DNA ligase

Replication fork

Parental DNA

DNA polymerasemolecule This daughter

strand is synthesizedcontinuously

This daughterstrand is synthesizedin pieces

35

35

3

5

35

FLOW OF GENETIC INFORMATIONFLOW OF GENETIC INFORMATION

Information contained in DNA is stored in blocks called genesgenes code for proteinsproteins determine what a cell will be like

DNA stores information in nucleus instructions are copied from the DNA into

messages comprised of RNAthese messages are sent out into the cell

direct the assembly of proteins

FLOW OF GENETIC INFORMATIONFLOW OF GENETIC INFORMATION

The path of information is often referred to as the central dogma

DNA RNA protein /trait

Gene expression : Transcription

messenger RNA (mRNA) made from a gene within the DNA Nucleus

Translation Using the mRNA to direct the production of a protein Cytoplasm

DNA

NUCLEUS

CYTOPLASM

RNA

Transcription

Translation

Protein

AMINO ACID SEQUENCES

flow of information from gene to protein is based on a triplet code

genetic instructions for the amino acid sequence of a polypeptide chain are written in DNA and RNA

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. Some amino acids have more than one possible codon.

Second base

Th

ird

base

Fir

st

base

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

TRANSCRIPTION OVERVIEW

RNA molecule is transcribed from a DNA template RNA nucleotides are linked by the transcription

enzyme RNA polymerase Specific sequences of nucleotides along the DNA

mark where transcription begins and ends The “start transcribing” signal is a nucleotide

sequence called a promoter

TRANSCRIPTION PROCESS

begins with initiation RNA polymerase attaches to the promoter.

second phase elongation RNA grows longer. As the RNA peels away, the DNA strands rejoin

third phase termination RNA polymerase reaches a sequence of bases

in the DNA template called a terminator signals the end of the gene

FIGURE 10.9B

RNA polymerase

DNA of gene

PromoterDNA

Initiation1

2

TerminatorDNA

3

Elongation

TerminationGrowingRNA

RNApolymerase

CompletedRNA

RNApolymerase

Free RNAnucleotides

Templatestrand of DNA

Newly made RNA

Direction oftranscription

TG

AG G

A

A

U C C AC

T TA

A

CC

GGU

T UTAACCT

A

TC

RNA PROCESSED BEFORE LEAVING THE NUCLEUS AS MRNA

Messenger RNA (mRNA) encodes amino acid sequences

conveys genetic messages from DNA to the translation machinery of the cell,

Eukaryotic mRNA has Introns

interrupting sequences

Exons coding regions

RNA 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 cap and tail of extra nucleotides are added to the ends of the mRNA to

facilitate the export of the mRNA from the nucleus

protect the mRNA from attack by cellular enzymes

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

TRANSLATION

TRANSFER RNA MOLECULES SERVE AS INTERPRETERS DURING TRANSLATION

Transfer RNA (tRNA) converting the genetic message of mRNA

into language of proteins perform this interpreter task by:

picking up the appropriate amino acid using a special triplet of bases to

recognize the appropriate codons in the mRNA

anticodon

RIBOSOMES BUILD POLYPEPTIDES

Translation occurs on the surface of the ribosome

coordinate the functioning of mRNA and tRNA and, ultimately, the synthesis of polypeptides

have two subunits: small and large. Each subunit is composed of ribosomal RNAs

and proteins subunits come together during translation

Ribosomes have binding sites for mRNA and tRNAs

FIGURE 10.12C

mRNA

Codons

tRNA

Growingpolypeptide

The next aminoacid to be addedto the polypeptide

INITIATION

Initiation codon marks start of mRNA message

Initiation brings together

mRNA

a tRNA bearing the first amino acid

the two subunits of a ribosome

FIGURE 10.13A

Start of genetic message

Cap

End

Tail

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

The start codon reads 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

InitiatortRNA

mRNA

Start codonSmallribosomalsubunit

Largeribosomalsubunit

Psite

Asite

Met

A U G

U A C

2

A U G

U A C

1

Met

amino acids are added one by one to the first amino acid

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

ELONGATION

FIGURE 10.14_S4

Polypeptide

mRNA

Codon recognition

Anticodon

Aminoacid

Codons

Psite

Asite

1

Peptide bond2

formation

Translocation3

Newpeptidebond

Stopcodon

mRNAmovement

Elongation continues until the termination stage of translation, when

the ribosome reaches a stop codon, the completed polypeptide is freed from the last

tRNA the ribosome splits back into its separate

subunits

TERMINATION

FIGURE 10.15DNA

Transcription

mRNARNApolymerase

Transcription

Translation

Amino acid

Enzyme

CYTOPLASM

Amino acidattachment2

1

3

4

tRNA

ATP

Anticodon

Initiation ofpolypeptide synthesis

Elongation

Largeribosomalsubunit

InitiatortRNA

Start Codon

mRNA

Growingpolypeptide

Smallribosomalsubunit

New peptidebond forming

Codons

mRNA

Polypeptide

Termination5

Stop codon

BACTERIA

CHROMOSOME single, circular, double-

stranded DNA molecule contains all the genetic

information required by a cell Binary fission – asexual

reproduction DNA is tightly coiled around a

protein dense area called the nucleoid central subcompartment in the

cytoplasm where DNA aggregates

DNA TRANSFER

Bacteria use three mechanisms to move genes from cell to cell:

1. Transformation

2. Transduction

3. Conjugation

Once new DNA gets into a bacterial cell, part of it may then integrate into the recipient’s chromosome

uptake of DNA from the surrounding environment

DNA enterscell

A fragment ofDNA from anotherbacterial cell

TRANSFORMATION

gene transfer by phages

Phage

A fragmentof DNA fromanotherbacterial cell(former phage host)

TRANSDUCTION

Mating bridge

Sex pili

CONJUGATION

transfer of DNA from a donor to a recipient bacterial cell

through a cytoplasmic (mating) bridge

BACTERIAL PLASMIDS CAN SERVE AS CARRIERS FOR GENE TRANSFER

Plasmid Nonessential small circular DNA molecule

separate from the bacterial chromosome Some plasmids can bring about conjugation and

move to another cell in linear form transferred plasmid re-forms a circle in the

recipient cell