DNA Structure and Replication

DNA (deoxyribonucleic acid) is a two stranded molecule called double helix

Each strand are made of smaller parts called nucleotides

The two strands are bonded with weak hydrogen bonds

Structure of DNA

Structure of DNA

Nucleotides have 3 parts:

• deoxyribose (sugar)• phosphate group• Nitrogenous base

(A,C,G,T)

Nitrogenous Bases Double Rings called PURINES

• Adenine (A)• Guanine (G)

Single Ring called PYRIMIDINES• Thymine (T)• Cytosine (C)

A or G

T or C

Base-Pairing Rules Adenine bonds with Thymine using

two hydrogen bonds

T A

Base Pairing Rules Guanine bonds with Cytosine using

three hydrogen bonds

CG

3 H-bonds

How did we discover this? Base-Pairing Rules (1950)

• Erwin Chargaff discovered DNA from any species had equal amounts C’s and G’s, and equal amounts A’s and T’s

• In human DNA A = 30% T = ________________ C = 20% G = ________________

How did we discover this? Structure of DNA

(1953)• Rosalind Franklin

takes diffraction x-ray photographs of DNA

• Watson and Crick build the first model of DNA using Franklin’s pictures

Discovery of DNA

DNA Replication When a cell divides (mitosis and

meiosis) chromosomes (DNA) needs to divide to create

DNA division happens during Interphase

Carried by a series of enzymes (proteins)

DNA Replication (in eukaryotic cells)

First, DNA is “unzipped” along the hydrogen bonds by DNA helicase

Each strand is a template to add new nucleotides

DNA Replication DNA polymerase adds new

nucleotides Nucleotides are added using Base

Pairing Rules• A – T• C - G

DNA Replication New DNA consists of one parent

(original) strand and one new strand of DNA

DNA Replication

Question What would be the complementary

DNA strand for the following DNA sequence?

Use base pairing rules• A - T• C - G

CGTATG

Answer

Original Strand: CGTATGNew Strand:GCATAC