DNAT H E C

E L L U L A R BA S I S

FO R L

I FE

DNA STRUCTURE• DNA is a nucleic acid made up of nucleotides joined into two long strands by covalent bonds

• Nucleotides are made up of1. A 5-carbon sugar (deoxyribose)2. A phosphate group3. A nitrogenous base

• DNA has four nitrogenous bases• Adenine, guanine, thymine, cytosine

THE HEROES OF DNA DISCOVERY!

Hmm… the angle of the x produced by the x-ray must mean that there are two strands!

WATSON AND CRICK

ROSALIND FRANKLIN

DNA STRUCTURE• Base pairing• A always pairs with T• G always pairs with C• A-T• G-C

Strands of DNA are twisted into a double helix (like a twisted ladder)

Watson and Crick’s model- 1953

THE DOUBLE HELIX MODEL• The double helix model explains the

rule of base pairing and how the two strands are held together.

• Two strands are “antiparallel”- run in opposite directions

• Held together by hydrogen bonds • Relatively weak• The ability of strands to separate is

critical to DNA’s functions

DNA REPLICATION• Each strand has all the info needed to

reconstruct the other half (base pairing) = complementary

• Before dividing, a cell must duplicate its DNA in a process called replication• Takes place during S phase of interphase• DNA molecule separates into two strands• Then, produces two complementary strands by

base pairing

ROLE OF ENZYMES• Enzymes “unzip” DNA by breaking H

bonds and unwinding strands• Each strand serves as a template• DNA POLYMERASE- joins individual

nucleotides to produce a new strand of DNA• Also “proofreads” each new strand

TELOMERES• DNA at the tips of chromosomes;

protect our genetic data

• Difficult to replicate so require telomerase, a special enzyme that adds short, repeated DNA sequences to the telomeres (TTAGGG)• Helps prevent genes from being

damaged or lost during replication

• Telomerase is often switched off in adult somatic cells and activated in cancer cells, allowing to grow and proliferate rapidly

PROKARYOTIC DNA REPLICATION• Prokaryotes have circular DNA- in

cytoplasm

• Starts from a single point and proceeds in two directions until the entire chromosome is copied

EUKARYOTIC DNA REPLICATION• May begin at hundreds of places on the

DNA molecule, proceeding in both directions until chromosome is completely copied

• Proteins check for chemical damage or base pair mismatches but damaged regions are sometimes replicated• Results in changes to base sequences and

therefore may alter genes

EUKARYOTIC DNA REPLICATION

• Two copies of DNA produced by replication in each chromosome remain closely associated until prophase of mitosis

• After they condense, sister chromatids are clearly visible