DNA: HEREDITARY MOLECULES OF

LIFE

Chapter 6

DNA Consists of

Deoxyribose sugarPhosphate groupA, T, C, G

Double stranded molecule (Double Helix)Two strands of DNA run antiparallel to each

other (opposite direction)5’ to 3’ 5’ is the end with the phosphate group3’ is where deoxyribose sugar is located

Nitrogenous basesHeld together by hydrogen bondsA pairs with T ( forms double bond)C pairs with G (forms a triple bond)

Four Requirements for DNA to be Genetic Material

Must carry informationCracking the genetic code

Must replicateDNA replication

Must allow for information to changeMutation

Must govern the expression of the phenotypeGene function

DNA ReplicationProcess of duplication of

the entire genome prior to cell division

Biological significance extreme accuracy of DNA

replication is necessary in order to preserve the integrity of the genome in successive generations

In eukaryotes , replication only occurs during the S phase of the cell cycle.

Mitosis-prophase-metaphase-anaphase-telophase

G1 G2

Sphase

interphase

Basic rules of replication

A. Semi-conservativeB. Starts at the ‘origin’C. Synthesis always in the 5-3’ directionD. Semi-discontinuous E. RNA primers required

Mechanism of DNA Replication

Step 1: Strand SeparationProteins bind to DNA and open up double helixPrepare DNA for complementary base pairing

Step 2: Building Complementary StrandsProteins connect the correct sequences of

nucleotides into a continuous new strand of DNA Step 3: Dealing With Errors during DNA

ReplicationProteins release the replication complex

DNA Replication is Semi-Conservative

Separating the two parent strands and building new complementary strand for each

New DNA has one new strand and one old strand

Strand Separation Double Helix

Unwound at replication origins (many origins on DNA)Enzyme called helicase binds to origins and unwinds

the two strands creating replication bubblesTwo strands separating creates a replication fork

Strand Separation Unwinding DNA creates tension

Enzymes called topoisomerases relieves tension by cutting strands near the replication fork (supercoil)

Single strands want to join back togetherPrevented by single-strand binding proteins (SSBs)

by attaching to the DNA strands stabilizing them

Topoisomerase

Enzyme

DNA

Enzyme

Strand SeparationMultiple replication origins decrease the overall time

of DNA replication to about 1 hour

Building Complementary Strands

DNA polymerase III Adds nucleotides to

the 3’ end of a strandNew strands are

always assembled 5’ to 3’

Builds new strand using nucleoside triphosphates

Building Complementary Strands RNA primase begins the replication process

Builds small complementary RNA segments on strand at beginning of replication fork

RNA primersDNA polymerase III can start to add nucleotides

Building Complementary Strands Leading Strand

DNA that is copied in the direction toward the replication fork

Lagging StrandDNA that is copied

in the direction away from the replication fork

Leading and Lagging Strands

DNA polymerase III

5

3 5

3

leading strand

lagging strand

leading strand

lagging strandleading strand

5

3

3

5

5

3

5

3

5

3 5

3

growing replication fork

growing replication fork

5

5

5

5

53

3

5

5lagging strand

5 3

Building Complementary StrandsAnti parallel strands replicated

simultaneously Leading strand synthesis continuously in

5’– 3’ Lagging strand synthesis in fragments in

5’-3’

Leading Strand Single primer is used to start strand DNA polymerase III moves towards

replication fork 5’ to 3’ direction Continuous

Lagging Strand DNA polymerase III moves away from replication

fork Discontinuous Okazaki fragments are used to solve problem

1000 – 2000 base pairs long Multiple primers are used

Lagging Strand DNA polymerase I removes RNA primers and

replaces with DNA nucleotide Fills the gaps

Building Complementary Strands DNA ligase

Links last nucleotide to Okazaki fragmentFormation of phosphodiester bond

Dealing With Errors DNA polymerase

Proofread and correct errors

Errors are usually base pair mismatches

After replication Average of 1 error per

million base pairs DNA polymerase II

Repairs damage after strands have been synthesized

Loss of bases at 5 ends in every replicationDNA polymerase I cannot replace final RNA primer

DNA polymerase III

DNA polymerases can only add to 3 end of an existing DNA strand

Chromosome Erosion

5

5

5

5

3

3

3

3

growing replication fork

DNA polymerase I

Does it Create a Problem?

Repeating, non-coding sequences at the end of chromosomes = protective cap

limit to ~50 cell divisions

enzyme extends telomeres can add DNA bases at 5 end different level of activity in different cells

high in stem cells & cancers -- Why?

telomerase

Telomeres

5

5

5

5

3

3

3

3

growing replication fork

TTAAGGGTTAAGGGTTAAGGG

Cells Aging Process Cell senescence

Cells loses ability to function properly as a person ages

Decrease in telomeres with ageNo longer provide protection

for the chromosome Known as the Hayflick limit Possibly links to age-

related diseasesDementia, atherosclerosis,

macular degeneration

Packing of Eukaryotic DNA OrganizationNegative DNA wraps around positive histonesNucleosome – cluster of 8 histonesSolenoids – coiled strings of nucleosomes (chromatin

fibres)

Prokaryotic DNA Organization

Eubacteria/Archaea DNAOne chromosome – circular

in shapeUnbound by a nuclear

membrane

Genetic Variation Among Bacteria Plasmids

Smaller circular pieces of DNA that float throughout cell Conjugation

Plasmids are able to exit one cell and enter another (when two bacteria are close)

Useful in genetic engineering