DNA Structure and Function

Chapter 8

Mystery of the
Hereditary Material

• Originally believed to be an unknown class of proteins

• Thinking was

– Heritable traits are diverse

– Molecules encoding traits must be diverse

– Proteins are made of 20 amino acids and are structurally diverse

Structure of the
Hereditary Material

• Experiments in the 1950s showed that DNA is the hereditary material

• Scientists raced to determine the structure of DNA

• 1953 - Watson and Crick proposed that DNA is a double helix

Structure of Nucleotides
in DNA

• Each nucleotide consists of

– Deoxyribose (5-carbon sugar)

– Phosphate group

– A nitrogen-containing base

• Four bases

– Adenine, Guanine, Thymine, Cytosine

Nucleotide Bases

How does DNA encode information?

• The secret is in the order of A’s, T’s G’s and C’s –the sequence.

• These are read as three letter “words”, for example TAG that code for specific amino acids.

• Every human is 99.9% identical, but some variable regions are more alike in close relatives – DNA fingerprinting

Watson-Crick Model

• DNA consists of two nucleotide strands

• Strands run in opposite directions

• Strands are held together by hydrogen bonds between bases

• A binds with T and C with G

• Molecule is a double helix

Watson-Crick Model

Why Replicate?

• Every cell needs a complete instruction book – the genome.

• Before dividing, the genome must be copied exactly and completely.

DNA Structure Helps
Explain How it Duplicates

• DNA is two nucleotide strands held together by hydrogen bonds

• Hydrogen bonds between two strands are easily broken

• Each single strand then serves as template for new strand

DNA Replication

Base Pairing During Replication

Each old strand serves as the template for complementary new strand

Enzymes in Replication

• Helicase unwinds the two strands

• DNA polymerase attaches complementary nucleotides

• DNA ligase fills in gaps

• Enzymes wind two strands together

Continuous and Discontinuous Assembly

DNA Repair

• Mistakes can occur during replication

• DNA polymerase can read correct sequence from complementary strand and, together with DNA ligase, can repair mistakes in incorrect strand

• But some mistakes lead to mutations

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