NOTES: 12.2 – 12.3DNA Structure & Replication

12.2: DNA Structure

● it was known that DNA was made up of

nucleotides joined into long strands by

covalent bonds…but HOW were they

connected…what was the structure?

DNA Structure

● Made up of nucleotides (monomer)

● Each nucleotide composed of 3 parts:

– 5-carbon sugar (DEOXYRIBOSE)

– Phosphate group

– Nitrogen-containing base

● Adenine (A)

● Thymine (T)

● Guanine (G)

● Cytosine (C)

Chargaff’s Rules

What do these data suggest to you?

Percentage of Bases in Four Organisms

Source of DNA A T G C

Streptococcus 29.8 31.6 20.5 18.0

Yeast 31.3 32.9 18.7 17.1

Herring 27.8 27.5 22.2 22.6

Human 30.9 29.4 19.9 19.8

● A = T (A pairs with T)

● C = G (C pairs with G)

What DNA Looks Like:

● Rosalind Franklin: used X-ray

diffraction patterns to discover that DNA

strands twisted around each other like a HELIX

● Watson & Crick: while

trying to build a 3-D

model of DNA, Watson

saw Franklin’s

photograph and

measurements and

within weeks, he and

Crick figured out the

structure of DNA

● Backbone

– deoxyribose &

phosphate group of

each nucleotide

●“Steps of Ladder”

– bases (A, G, C, T)

● Any base sequence is

possible!

DNA Structure = DOUBLE HELIX

(“twisted ladder”)

What’s holding the strands together?

● HYDROGEN BONDS!!!

● Hydrogen Bonds form between A and T base pairs as well as betweenC and G base pairs

12.3 - DNA Replication

Vocabulary:

• Chromatin

• Replication

• DNA polymerase

Key Concept:

•What happens during DNA Replication?

The Review

● Prokaryotic cells do not have a nucleus

– DNA molecules are located in the cytoplasm

– Usually a circular DNA molecule and it is referred

to as the cell’s chromosome

● Eukaryotic cells have a nucleus

– Can have 1000x more DNA than prokaryotic cells

– DNA is located in the form of a number of

chromosomes

– # of chromosomes varies widely from species to species

DNA molecules are long…how does DNA fit in the nucleus?

● It forms chromosomes!

● DNA coils around proteinscalled histones and then this chromatin supercoils around itself until chromosomes form

DNA Replication

● before a cell divides, it must copy its DNA so that all cells have a copy of the genetic instructions

● this process involves A LOT of ENZYMES!

● each half of the DNA double helix can serve as a "template" for the replication of another DNA double helix molecule

– The strands are said to be

complementary

DNA REPLICATION:

THE PROCESS

1) An enzyme unwinds

& "unzips" DNA

(separates the 2

DNA strands) by

breaking the

hydrogen bonds

between base pairs

Hydrogen Bonds Breaking!

2) a complementary base is inserted to each

side of the DNA strand with the help of DNA

POLYMERASE

Why does the

“blue” always pair

with the “green?”

3) The sugar-phosphate

groups are covalently

bonded to the

growing DNA chain (new "backbone")

1 nucleotide

COVALENT BOND

4) an enzyme (DNA

polymerase) will

"proof-read" the

order of bases &

make corrections

A SIMPLIFIED VERSION…

A Little More Complicated…