DNA Structure & Replication

Chapter 5

Discovery of DNA as genetic material

1869 a compound found in the nucleus1920’s the chemical components of nuclein

determined1930’s experiments on single celled algae

Experiment 1 stalk removed, and regrew, therefore the DNA can not be in the stalk

Experiment 2 foot remove, and did NOT regrow, therefore DNA must be in foot.

Foot contained nucleus, so nucleus must contain genetic material1

Rosalind Franklin (1920-1958) :-X-ray diffraction photograph of the DNA double helix in 1951-determined DNA must be a corkstrew

Watson and Crick• James Watson & Francis Crick

•Described DNA as a double helix in 1953

•Nobel Prize for Physiology in 1962

DNA StructureDNA consists of two molecules that are arranged

into a ladder-like structure called a Double Helix.

A molecule of DNA is made up of millions of tiny subunits called Nucleotides.

Each nucleotide consists of:1. Phosphate group2. Pentose sugar3. Nitrogenous base

Nucleotide

Phosphate

Pentose

Sugar

Nitrogenous

Base

NucleotidesThe phosphate and sugar form the backbone

of the DNA molecule, whereas the bases form the “rungs”.

There are four types of nitrogenous bases.

Nucleotides

A

Adenine

T

Thymine

G

Guanine

C

Cytosine

NucleotidesEach base will only bond with one other

specific base.

Adenine (A)Thymine (T)

Cytosine (C)Guanine (G)

Form a base pair.

Form a base pair.

“The Code of Life”

•Nitrogenous Bases

•Purines: Adenine & guanine Double-ringed

•Pyrimidines: Thymine & cytosine Single-ringed

Triple bonds

double bonds

DNA StructureBecause of this complementary base pairing,

the order of the bases in one strand determines the order of the bases in the other strand.

Anti-Parallel Strands

DNA STRUCTURE https://www.youtube.com/watch?v=qy8dk5iS1f0

DNA SEMICONSERVATIVE REPLICATION• The process of copying a

double-stranded DNA molecule

• DNA strands are antiparallel and complementary

• Each strand serves as a template for the reproduction of the opposite strand

• one strand of the newly formed DNA molecule is “conserved” from the original parent strand.

The process of DNA Semiconservative Replication Continued:

• The 2 nucleotide chains unwind and unzip from each other. • Each old chain base pairs with new

nucleotides forming 2 identical DNA molecules that are identical to the original DNA molecule.• The 2 old chains determine the sequence of

the new chains since we know A pairs with T and C pairs with G.

DNA structure and Replication

https://www.youtube.com/watch?v=8kK2zwjRV0M

http://www.sinauer.com/cooper/4e/animations0601.html

http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120076/micro04.swf::DNA Replication Fork

BIOFACT

If extended a single human DNA molecule would be ~ 6 cm long, that

is 1000s times longer than the diameter of the nucleus. This chromosome and another 45 chromosomes can fit into the nucleus because of efficient

chromatin packing.

ANOTHER BIOFACT !!

If you arrange all of the DNA strands in your body end-to-end, their total length would stretch 2 x 1010 ......That is well over 100 times the

distance between the earth and the sun !!!!