Mv management of genetic information

Management of Genetic Information

Learning objectives

Understand the mechanism of DNA replication, RNA synthesis and protein synthesis

Flow of genetic information

Two possible models of the DNA replication

Expt by Meselson-Stahl proved the semiconservative model of replication

Which direction does replication go? Major enzyme: DNA polymerase III

DNA double helix unwinds at a specific point called an origin of replicationorigin of replication

Polynucleotide chains are synthesized in both directions from the origin of replication; DNA replication is bidirectionalbidirectional in most organisms

At each origin of replication, there are two replication replication forksforks, points at which new polynucleotide chains are formed

There is one origin of replication and two replication forks in the circular DNA of prokaryotes

In replication of a eukaryotic chromosome, there are several origins of replication and two replication forks at each origin

Replication in prokaryotes

Replication in eukaryotes

DNA synthesis based on two template strands: leading strand and lagging strand templates; mechanism in prokaryotes is presented

DNA is synthesized from its 5’ -> 3’ end (from the 3’ -> 5’ direction of the template) the leading strandleading strand is synthesized continuously in

the 5’ -> 3’ direction toward the replication fork the lagging strandlagging strand is synthesized

semidiscontinuously (Okazaki fragments)Okazaki fragments) also in the 5’ -> 3’ direction, but away from the replication fork

lagging strand fragments are joined by the enzyme DNA ligaseDNA ligase

Replication fork

Enzymes and proteins in DNA replication

The action of DNA polymerase

Why 53’ direction?

Start of DNA replication

Unwinding DNA gyrase introduces a swivel point in

advance of the replication fork a helicase binds at the replication fork and

promotes unwinding single-stranded binding (SSB) protein protects

exposed regions of single-stranded DNA

Primase catalyzes the synthesis of RNA primer Synthesis

catalyzed by Pol III primer removed by Pol I DNA ligase seals remaining nicks

Summary of DNA replication in prokaryotes DNA synthesis is bidirectional DNA synthesis is in the 5’ -> 3’ direction

the leading strand is formed continuously the lagging strand is formed as a series of

Okazaki fragments which are later joined

DNA polymerases Five DNA polymerases have been found to exist in

E. coli Pol I is involved in synthesis and repair Pol II, IV, and V are for repair under unique conditions Pol III is primarily responsible for new synthesis

Eukaryotic DNA replication

Not as understood as prokaryotic. Due in no small part to higher level of complexity.

Cell growth and division divided into phases: M, G1, S, and G2

DNA replication occurs during the S phase

RNA synthesis

Transcription Template is DNA Major enzyme: DNA directed RNA polymerase No need for primers 5’ to 3’ direction

RNA synthesis

Requires a promoter region in the template DNA to which the RNA polymerse will bind

Promoter 40 base pairs upstream (-40) away from the start site (+1)

Three stages:initiation, elongation, termination Termination may be

rho factor dependent – rho factor terminates synthesis

or rho factor independent – formation of a stable hairpin loop

Promoter 40 base pairs upstream (-40) away from the start site (+1)

INITIATION STEP

ELONGATION STEP

TERMINATION STEP

ρ-FACTOR INDEPENDENT- FORMATION OF HAIRPIN LOOP

Eukarotic transcription have 3 classes of RNA polymerases RNA pol I transcribes large ribosomal RNA

genes RNA pol II transcribes protein encoding gene RNA pol III transcribes small RNAs

(including tRNA and 5SRNA)

Post transcriptional modification of the eukaryotic mRNA Capping – methyl guanosine attachment at the

5’ end to protect the cleavage of the RNA by exonucleases as RNA moves out of the nucleus

Addition of poly A at the 3’ end (200-250 long) helps to stabilize the mRNA structure; increases resistance to cellular nucleases

Splicing – removal of non coding sequences (introns)

Protein synthesis

Translation Based on the m-RNA sequence, genetic

code Starts from 5’ end of the transcript Occurs in the ribosomes Activation of amino acids – attachment to the

tRNA Initiation, elongation, termination

Genetic code

Triplet nucleotide – one amino acid Nonoverlapping No punctuation Degenerate Almost universal

Initiation

Initiation factors Shine-Dalgarno sequence in mRNA 30S ribosome N-formylmet

Inhibitors of protein synthesis

Postranslational modification

Protein folding –chaperones Proteolytic cleavage (zymogens) – hydrolytic

enzymes in the gut Amino acid modifications Attachment of carbohydrates Addition of prosthetic groups

Regulation of protein synthesis and gene expression 20K to 25K genes in the human genome Only a fraction of the genes are expressed at

any given time Two types of gene expression: constitutive

and inducible Inducible genes are highly regulated –

regulatory proteins, hormones and metabolites