Chromosomes & DNA Replication

Memory Refresher!! What is DNA? –Deoxyribonucleic acid/molecule to store and transmit genetic information What THREE things make up a nucleotide? –Sugar, phosphate, nitrogenous base What is the “tool” to help remember the base- pairing rule? –A two T, G three C What is the “other side,” or complimentary strand, to this strand of DNA: –AATCGTAACGGGTTC TTAGCATTGCCCAAG

Where is DNA found? –Prokaryotes – no nucleus, so DNA is located in the cytoplasm and is one large chromosome –Eukaryotes – have a nucleus, where DNA is located in many chromosomes (number of chromosomes varies from one species to the next)

DNA Length DNA very long – millions of base pairs in length Human cells contain more than 1 meter of DNA inside the nucleus

Chromosome Structure (pages in book) Eukaryotic chromosomes contain both DNA and protein, packed together to form chromatin Chromatin is DNA coiled tightly around proteins called histones Having the DNA packed so tightly allows it to fit into the nucleus Chromosome Supercoils Coils Nucleosome Histones DNA double helix

Duplicated chromosomes (sister chromatids) Tight helical fiber Thick supercoil Centromere Nucleosome “Beads on a string” Histones DNA double helix

DNA Replication Double helix structure allows for easy replication Each strand of DNA double helix has all the information needed to construct the other half by the mechanism of base pairing Strands are complementary = each strand can be used to make the other

Replicating DNA Before a cell divides, it duplicates its DNA in a copying process called replication During DNA replication, the DNA molecule separates into two strands, then produces two new complementary strands following the rules of base-pairing. Each strand of the double helix of DNA serves as a template, or model for the new strand Eukaryotes – DNA replication occurs at hundreds of places at the same time until each chromosome is complete copied – 50 nucleotides copied per second! Sites where replication and separation occur is called the origin of replication (aka replication forks)

How Replication Occurs DNA replication is carried out by a series of enzymes which “unzip” the DNA at the hydrogen bonds between the base pairs Each strand serves as a template for the attachment of complementary bases –Example: DNA strand with bases TACGTT will produce a strand with complementary bases ATGCAA

How Replication Occurs (continued) Each DNA molecule resulting from replication has one original strand (parental strand) and one new strand (daughter strand) Principle enzyme involved in DNA replication is DNA polymerase. This protein helps build the complementary DNA strand and also “proofreads” each new DNA strand to make sure the new DNA strand is an exact copy of the original. DNA polymerase also repairs damaged DNA.

DNA Replication Growth Origin of Replication DNA polymerase New strand Original strand DNA polymerase Origin of Replication Original strand New strand

DNA Replication Notice how the white strands represent the original DNA molecule and the black strands represent the new strands being replicated.

DNA Replication