Chromosomes and DNA Replication 12-2

DNA & Chromosomes Prokaryotes - Lack nuclei and cellular organelles Have single circular DNA molecule Contains nearly all cell’s genetic information Referred to as cell’s chromosome

Eukaryotes - Have as much as 1000 times the amount of DNA as prokaryotes Not free in cytoplasm Contained in nucleus in the form of a number of chromosomes

DNA Length DNA molecule is very long Ex: E. coli DNA contains 4,639,221 base pairs, or ~1.6 mm long Like trying to pack 300 m of rope into a backpack

Chromosome Structure Human cells contain almost 1000 times as many base pairs as bacteria The nucleus of a human cell contains > 1m of DNA The composition of chromosomes allows them to hold a lot of DNA

Chromosomes contain both proteins and DNA Form chromatin Chromatin consists of DNA tightly coiled around histone proteins Forms beadlike structure - nucleosome Nucleosomes pack with one another to form a thick fiber, shortened by loops and coils

Nucleosomes seem to make it possible to fold enormous lengths of DNA Histones have changed very little during evolution During most of cell cycle fibers are dispersed - can’t see chromosomes During mitosis, fibers are drawn together, forming tightly packed chromosomes you can see with a microscope

DNA Replication When Watson and Crick discovered the double helix, the structure explained how DNA could be copied Each strand of DNA has all the information needed to reconstruct the other half because of base pairing

Since each strand is used to make the other half, the strands are said to be complimentary In eukaryotes, DNA replication begins at 100s of places along the molecule, and continues until replication is complete Sites known as replication forks

Duplicating DNA During replication - DNA molecule separates into 2 strands Produces 2 new complimentary strands following rules of base pairing Each strand is a template for a new strand

DNA replication is carried out by a series of enzymes Enzymes “unzip” the DNA molecule, creating replication forks Breaks hydrogen bonds between bases A pairs of with T, and G pairs up with C, until the new strand is complete

Each strand created is composed of an old strand and a new strand The principal enzyme involved in this reaction is DNA polymerase Joins individual nucleotides to produce DNA molecule Proofreads each new molecule