DNA: Structure and Function

The DNA Revolution

1940s-1960s Griffith & Avery—DNA transformed

pneumococcus bacteria. Encouraged the study of prokaryotic

chromosomes. Hershey and Chase—Bacteriophage

experiment. DNA from viruses is injected to host bacteria cells.

Griffith and Avery

Hershey & Chase

Hershey & Chase

The Search for the Structure of the DNA Molecule

1951—Rosalind Franklin—X-ray crystallography

Chargaff—Chargaff’s rules. Ratio of nitrogenous bases in DNA. Complimentary bases.

Watson & Crick--1953

Franklin

Chargaff

Watson & Crick Model of DNA

Nucleotides Sugar—Deoxyribose Phosphate Group Nitrogenous Bases Adenine & Thymine

are complementary. Cytosine and

Guanine are complimentary.

Purines—Adenine & Guanine

Pyrimidines—Cytosine & Thymine

5’ End—Phosphate side

3’ End—Other side

Watson & Crick Model

DNA is composed of 2 chains of nucleotides that form a double helix shape.

The two strands are antiparallel.

The backbone of the DNA molecule is composed of alternating phosphate groups and sugars.

The complimentary nitrogenous bases form hydrogen bonds between the strands.

A is complimentary to T and G is complimentary to C.

DNA Model

DNA

Functions 1. Storage of genetic information 2. Self-duplication & inheritance. 3. Expression of the genetic message. DNA’s major function is to code for proteins. Information is encoded in the order of the

nitrogenous bases.

Chromosomes Prokaryotic Circular Very small 1 chromosome per

cell Some enzymes and

proteins are associated with the DNA.

Not housed in a nucleus.

Eukaryotic

Linear

Fairly long

Several chromosomes per cell.

Histone proteins---”spools”. Same in all eukaryotes

Housed in a nucleus.

Nucleosome—2 loops of DNA wrapped around 8 histone proteins.

Unity theme.

Summary

Describe the structure and function of DNA.

DNA Replication

DNA replicates in a semiconservative way 1 strand is the “leading strand” (3’ 5’) 1 strand is the “lagging strand” (5’ 3’)

DNA Replication Steps

1. Helicase (protein) unzips the DNA

2. DNA polymerase (protein) binds to the strands

3. DNA polymerase adds the floating nucleotides to the new DNA strand.

4. 2 new strands are made

Summary

Describe the process of DNA Replication

RNA

3 types—mRNA, tRNA, rRNA Ribose Uracil replaces Thymine Single stranded helix

Transcription

RNA polymerase Messenger RNA mRNA Unzipping of DNA helix. Sense strand or template strand Codons Genetic code

Translation Vocabulary

1. Amino Acid- The basic building block of proteins

2. tRNA- a RNA molecule that carries the amino acid to the ribosome

3. codon- a set of 3 nucleotides used to code for an amino acid

4. polypeptide- a string of many molecule (ex: protein is a polypeptide of amino acids)

5. P site- where the tRNA detaches leaving behind the amino acid

6. A site- where the tRNA arrives

Translation

Steps:

1. mRNA enters the cytoplasm from the nucleus

2. mRNA finds a ribosome

3. Ribosome starts translation by using the tRNA that carries Methionine (AUG)

4. Protein chain elongates

5. Stop codons terminates the translation of protein elongation