Unit 4 - Molecular Genetics

• DNA Replication• Protein Synthesis– Transcription– Translation

• Cell Cycle

• 1928 – Griffith

• 1944 – Avery, McCarty, & McCleod

Bacteriophage

• Viruses that attack bacterial cells (“bacteria eaters”)

• 1952 – Harvey & Chase

• 1953 Watson & Crick (and Franklin)

• 1958 – Meselson & Stahl

DNA Structure

• Nucleotides – the building blocks of DNA (nucleic acids)

Base Pairing

• A-T (double) • G-C (triple)

3’ vs. 5’ ends of DNA

• Gene – a portion of DNA that codes for a specific protein

DNA Function• Transmission of genetic material to offspring• Basis of all cell processes (DNA RNA Protein)• Structure relates to function!– DNA structure provides a perfect template for semi-

conservative replication

Leading vs. Lagging Strands• Nucleotides can only be added to the 3’ end of a

DNA molecule– THUS - DNA elongates only in the 5’ 3’ direction

DNA Replication1. Starts at an origin of replication (ori)– Bacterial DNA is circular (only 1 ori)– Eukaryotic DNA is linear (multiple ori’s)

2. RNA primase (enzyme) recognizes an ori site on DNA, “sits” down on DNA, and synthesizes an RNA strand (5-10 nucleotides long) called a primer

3. DNA polymerase (enzyme) finds this primer and begins replication – Adds nucleotides in a 5’ 3’ direction only!

Replication fork – spots where DNA double strand “forks”• DNA helicase (enzyme) – unzips the DNA

double helix

3a – Leading Strand• As the helix is unzipped, the 3’ end of leading

strand is synthesized continuously• Only 1 RNA primer and 1 DNA polymerase is

needed!

3b – Lagging Strand• Lagging strand is synthesized BACKWARDS in

short chunks called Okazaki fragments with each getting: – RNA primer (thus RNA primase)– DNA polymerase (to synthesize DNA)

• DNA ligase – enzyme that ligates (“sticks together”) Okazaki fragments together to form a single DNA strand

DNA Replication Video

• http://www.youtube.com/watch?v=teV62zrm2

A few essential ideas to remember:

1. DNA copies itself almost always without error. (Approximately 1 true error per 10 billion bp; actually 1 error in 10,000 bp but DNA polymerases proof-read and fix)

2. Eukaryotic DNA replicates @ 50 nucleotides/second; bacterial DNA replicates much faster (10x)

Proof-reading & Repair• DNA polymerases proof-read each strand

against the parent template.• If error goes unrepaired by DNA polymerases,

then other repair mechanisms kick in.• There are 130 different DNA repair enzymes in

humans!

An example…

Telomeres• Telomere- a 100 to 1000 base-pair section at

the tips of Eukaryote’s chromosomes (DNA)• Repeating sequences of TTAGGG (humans)• Function to protect the end of genes from being

shortened by successive rounds of replication but telomeres themselves are shortened after each round of replication

• Telomeres can only be shortened so much (thus a cell can divide only so many times) and then the cell dies via apoptosis (programmed cell death)

• Get this:– Certain cells (e.g. germ cells) have telomerase, an

enzyme that catalyzes the repair of telomere DNA damaged after replication

– Certain types of cancer cells overexpress telomerase which allows those cells to continuously divide w/o consequence