DNA, AND IN SOME CASES RNA, IS THE PRIMARY SOURCE OF HERITABLE INFORMATIONGenetic information is transmitted from one generation to the next through DNA or RNA

Structure of DNA Building Blocks

Nucleotide Deoxyribose sugar Phosphate group Nitrogenous bases

Uprights / Backbone Alternating DNA and

Phosphate Rungs / Core

Nitrogenous bases A-T and G-C A and G – purines (double

ring) T, C and U – pyrimidines

(single ring)

Structure of DNA Double helix

2 polynucleotides bonded together by hydrogen bonding between nitrogen bases

Antiparallel one strand runs 5’

to 3’ one strand runs 3’

to 5’

DNA replication Replication is a

semiconservative process Each strand serves

as the template for a new strand

Ends with two identical strands of DNA; each having one old and one new strand

http://youtu.be/teV62zrm2P0

DNA replicationHelicase unwinds DNA forming a Y-shaped region where new DNA strands are produced - replication fork/bubbles. There can be 100's or even 1,000's of replications forks/bubbles in a eukaryotic chromosome.

Single-stranded binding proteins bind to and stabilize the single-stranded DNA so that it can be used as a template (see above).

Topoisomerases correct "overwinding" ahead of the replication fork by breaking, swiveling, and rejoining DNA strands.

Primase adds a short RNA primer (5 - 10 nucleotides long) to add an initial starting point for the new DNA strand. DNA polymerase III can then add complementary DNA nucleotides to the 3' end of the RNA primer (at a rate of about 50/sec in humans.)

DNA ReplicationEach new nucleotide that is added to the growing DNA strand is a nucleoside triphosphate (see right). As each nucleoside triophosphate monomer is added it loses two phosphate groups - hydrolysis of 2 phosphates is exergonic & powers the polymerizeration of monomers.

One strand is continuously replicated. This strand is referred to as the leading strand and it is the strand that is replicated in a 5' to 3' direction.

The lagging strand on the other hand has discontinuous replication. The lagging strand is synthesized as a series of segments called Okazaki fragments, which are joined by DNA ligase (see right). When the replication fork opens a RNA primer is needed to change the change the direction of replication so that replication can run 5' to 3'.

When the entire replication is complete DNA polymerase I removes RNA primers and replaces them with DNA. DNA polymerases also proofread newly made DNA replacing any incorrect nucleotides.

Replication of Telomeres Tips of linear chromosomes

are called telomeres DNA polymerase works in a

5’ to 3’ direction Tips of lagging strand stops

short of the end during replication

Telomerase is an enzyme that extends the lagging strand

Telomerase prevents the premature shortening of telomeres / chromosomes

Length of telomeres has been linked to cell death (apoptosis)

http://youtu.be/AJNoTmWsE0s

Enzymes of Replication that you should know… Helicases

enzymes that untwist the double helix at the replication forks Topoisomerases

corrects "overwinding" ahead of replication forks by breaking, swiveling, and rejoining DNA strands

RNA Primase synthesizes an RNA primer at 5' end of leading strand and of each

Okazaki fragment of lagging strand DNA Polymerase

using parental DNA as a template synthesizes new DNA strand by covalently adding nucleotides to the 3' end of a pre-existing DNA strand or RNA primer; removes RNA nucleotides of primer from 5' end and replaces them with DNA nucleotides

DNA Ligase joins Okazaki Fragments