Chapter 8 DNA Structure and Replication 8.2 and 8.3.
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Transcript of Chapter 8 DNA Structure and Replication 8.2 and 8.3.
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.