Chromosomes and DNA Replication

DNA and Chromosomes Bacteria Eukaryotes have multiple chromosomes. Single, circular DNA molecule referred to as a chromosome. Loose in the cytoplasm Eukaryotes have multiple chromosomes. Up to 1000 times as much DNA Located in the nucleus Numbers vary greatly from species to species

DNA and Chromosomes DNA Length Long In an E. coli bacterium, it is folded into a space 1/1000 of its length! (It would be like trying to fit a 300-meter rope into your backpack!)

DNA and Chromosomes Chromosome structure The smallest human chromosome has 30 million base pairs! To fold so much DNA into the nucleus, eukaryotic chromosomes contain both DNA and protein packed together as chromatin. DNA is coiled around proteins called histones. Together, the DNA and histone molecules form a beadlike nucleosome. Nucleosomes pack together to form a thick fiber which is shortened by loops and coils to make the chromosome.

Chromosome structure

DNA and Chromosomes Chromosome structure During most of the cell cycle, the chromatin is spread throughout the nucleus and is not visible. When the cell prepares to divide (in mitosis), the fibers pack tightly into chromosomes. Changes in chromatin structure and histone-DNA binding influences gene expression. It can expose DNA that needs to be “read” to make proteins. Nucleosomes help fold lots of DNA into the small space inside the nucleus.

DNA Replication Duplicating DNA Each half of a DNA strand contains the information needed to make a copy, or “replicate” it. Replication – copying DNA - happens before a cell divides. The 2 strands are “complimentary” – the rules of base pairing allow you to use each strand as a template to reconstruct the base sequence of the other strand.

DNA Replication How replication works: Enzymes unzip the DNA. Hydrogen bonds break and DNA unwinds Complementary bases are attached to each strand. DNA polymerase is the enzyme that adds nucleotides and proofreads the new strand. Each DNA molecule will now have one original strand and one new strand.

DNA Replication