DNA STRUCTURE AND REPLICATION

• pentose sugar• nitrogen base• phosphate group

Sugar

Base

PO4

DNA IS A POLYMER OF DNA IS A POLYMER OF NUCLEOTIDESNUCLEOTIDES

NUCLEOSIDESNUCLEOSIDES

SUGAR IN DNA

NITROGENOUS BASES

PURINES

Found in DNA and RNA

PYRIMIDINESPYRIMIDINES

C in DNA and RNA T in DNA, U in RNA

NUCLEOSIDE/NUCLEOTIDE NUCLEOSIDE/NUCLEOTIDE LINKAGESLINKAGES

- N- glycosidic linkage between the ribose sugar and base

NOMENCLATURENOMENCLATURE

A DNA POLY-NUCLEOTIDE

(5’ - TACG- 3’)’

BASE PAIRING

CHARGAFF’S RULE

PURINES = PYRIMIDINES

THE DNA DOUBLE HELIXTHE DNA DOUBLE HELIX

The DNA strands are anti parallel

STRUCTURAL FORMS OF THE DNA DOUBLE HELIX

A-DNA B-DNA Z-DNA

CHROMATIN CHROMATIN STRUCTURESTRUCTURE

SEPARATION OF THE DNA DOUBLE HELIX

DNA ReplicationDNA Replication

““It has not escaped our notice that the It has not escaped our notice that the specific pairing we have postulated specific pairing we have postulated

immediately suggests a possible immediately suggests a possible copying mechanism for the genetic copying mechanism for the genetic

material”material”

James Watson and Francis CrickJames Watson and Francis Crick

NatureNature, 1953, 1953

DNA Replication: Very Replication: Very Simply….Simply….

DNA Replication: DNA Replication: The ComplexitiesThe Complexities

[1]The site of initiation of replication is not chosen randomly

How does the cell know where to start replication???

Origins of replicationOrigins of replication

[2][2]

• For replication to occur, the parental DNA double helix has to be separated into single strands

What protein is required to make the DNA single stranded during replication????

DNA helicaseDNA helicase

[3][3]• Single stranded DNA is vulnerable to damage• Normally complementary single DNA strands

reform the double helix

What proteins are required keep the DNA single stranded during replication and to protect it from damage????

SSB ProteinsSSB Proteins

• Each eukaryotic chromosome is one linear DNA double helix

• Average ~108 base pairs long• With a replication rate of 2 kb/minute,

replicating one human chromosome would require ~35 days (S phase of eukaryotic cell cycle = 6-8 hrs)

How are eukaryotic DNA molecules replicated in time????

[4][4]

Multiple Origins of replicationMultiple Origins of replication

[5][5]• The two DNA strands twist around each

other to form a helix. As the two strands are pulled apart it leads to the tightening of the twists in regions upstream of replication initiation points and the introduction of ‘supercoils’.

How are supercoils removed from the DNA helix????

SUPERCOILINGSUPERCOILING

TOPOISOMERASE ITOPOISOMERASE I

TOPOISOMERASE IITOPOISOMERASE II

[6][6]• The primary DNA replication enzyme,

DNA polymerase, can only add nucleotides to an existing strand of nucleotides i.e., it cannot start the replication process by itself.

How does replication get started????

RNA PrimerRNA Primer

[7][7]• DNA polymerase can only add

nucleotides in one direction, 5’ 3’ i.e., creation of both the new strands is in the 5’ 3’ direction

• However, the two DNA strands are antiparallel to each other --- one runs 5’ 3’ and the other 3’ 5’. Both strands have to be replicated. How is the problem of directionality solved????

[8][8]

• The solution to problems 6 requires the excision of RNA primers and their replacement by DNA.

Which enzyme is involved in this step and how does it work ????

DNA polymerase IDNA polymerase I

•5’→3’ exonuclease activity•5’→3’ polymerase activity

[9][9]• The solutions to problems 7 and 8 leave

the newly synthesized DNA with single strand breaks or ‘nicks’.

Which enzyme is involved this sealing these breaks ????

[10][10]

• The mis-incorporation of a single nucleotide during replication could lead to deleterious or even lethal mutations.

How is replication fidelity maintained ????

[11]• In eukaryotes DNA polymerase cannot fill the

gaps at ends of chromosomes after RNA primer is removed.

• If this gap is not filled, chromosomes would become shorter each round of replication!

How are the telomeric regions replicated without shortening of chromosomes???

TELOMERESTELOMERES