Chapter 8

DNA Structure and Replication8.2 and 8.3

What is DNA?• Deoxyribonucleic acid (DNA) is a long

molecule made up of units called nucleotides.– Nucleotides are made up of:• 5-carbon sugar (deoxyribose)• Phosphate group• Nitrogenous base (adenine, guanine,

cytosine, or thymine)

• All the genetic information for an organism is carried on its DNA.

Nucleotides Form DNA

Nucleotide Structure

History of DNA

• DNA winds around itself, forming a twisted ladder (double helix)– Chargaff discovered the rules of base

pairs in 1950– Rosalind Franklin took a picture of DNA

in 1952 using X-ray diffraction–Watson and Crick coined the term

“Double Helix” in 1953

Groups of Bases• Purines (two ring)– Adenine– Guanine

• Pyrimidines (one ring)– Cytosine– Thymine

Structure of DNA Cont’d

• Chargaff’s Rules:– The four bases (adenine, guanine,

thymine, cytosine) pair up in a certain way • A = T• C = G

– The bases are held together by hydrogen bonds (hydrogen bonds are weak bonds)

G

G

A

T

T

A

A

C

T

G

C

A

T

C

Chargaff’s

Rules

Storage of DNA

• Prokaryotes – DNA is found in the cytoplasm–Most have single circular DNA

molecule

• Eukaryotes – Much more DNA than in prokaryotes (up to 1000x as much!)– Condensed into chromosomes• Chromosome number varies wildly from one

species to the next. (humans have 46)

DNA Replication• Before a cell divides, it must copy its

DNA in a process called replication.• During DNA replication:– DNA unwinds using an enzyme called

topoisomerase– DNA then unzips into two strands using

helicase– DNA polymerase makes complimentary

strands using base pairing.• Each strand of the original DNA

molecule is used as a template for the new DNA.

•The area where DNA unwinds is called a replication fork.