DNA Transmission

21 Jan, 2005

Overview• DNA replication is prerequisite to cell division.

• DNA is replicated by using each strand as template for synthesis of the complementary strand.– one strand is synthesized continuously, the other discontinuously

– all nucleotides are added at the 3’ end of the growing strand

• Replicated chromosomes are partitioned to nuclei of daughter cells in mitosis.

• Meiosis distributes replicated chromosomes to haploid daughter cells in two nuclear and cell divisions.

Overview

DNA Replication

Okazaki Fragments

Polymerization of DNA

• DNA polymerization requires the following:– dATP, dGTP, dCTP, dTTP (collectively dNTPs)– single-stranded template– primer with free 3’ hydroxyl to accept incoming

nucleotide– DNA polymerase, Mg+2

• Accessory enzymes– primosome containing RNA primase– helicase to unwind DNA– gyrase to relax supercoils formed by unwinding

Priming and Ligation

Replication Enzymes

Meselson and Stahl

Special cases• Some circular DNA molecules (e.g., plasmids) replicate by rolling circle

model– continuous replication from circular template

– discontinuous replication (Okazaki) from displaced single strand

• Telomeres– specialized ends of linear eukaryotic chromosomes

– replicated by telomerase• carries small template RNA

• a type of reverse transcriptase

– without telomerase, lagging strands continuously shorten with remover of primer, eventually deleting essential genes

Plasmid Replication

Telomeres and Telomerase

Cell division in eukaryotes• Mitosis

– an asexual division– equal division of replicated chromosomes– occurs with any number of chromosomes– part of cell cycle

• Cell cycle– G1: gap (growth)– S: DNA replication, forms sister chromatids– G2: gap (growth)– M: mitosis (nuclear division) and cytokinesis

Meiosis and Fertilization

Assignment: Concept map, solved problems 1 and 2, basic problems 7, 10, 13, 15, challenging problems 23, 24, 27

Web-based NCBI tutorial sections from Introduction to Using BLAST to compare sequences. Should now be finished with BLAST tutorial.