Ch.9: DNA Structure & Replication Genetics

Structure of DNA DNA is a very long thin molecule that is made up of units called nucleotides. A nucleotide consists of 3 things: 1) Phosphate group 2) 5-carbon sugar called deoxyribose 3) a nitrogen carrying base

There are 4 nitrogen carrying bases which are put in 2 categories depending on their makeup. I. Purine (have a double ring of carbon)-adenine (A) and guanine (G) II. Pyrimidines (have a single ring of carbon)-thymine (T) and cytosine (C)

The DNA molecule is double stranded and is shaped like a double helix (spiral staircase, or twisted ladder). Using the ladder analogy: ----”sides” are composed of the alternating phosphate groups and sugar molecules. ----”rungs” consists of pairs of nitrogen-carrying bases. The two bases that make-up each rung are joined to each other by weak hydrogen bonds.

Base-pairing rules: a purine must always pair up with a pyrimidine Base-pairing rules: a purine must always pair up with a pyrimidine. Therefore, adenine pairs only with thymine; and guanine pairs only with cytosine. The strictness of base pairing always results in 2 strands that are complementary to each other. This means that the sequence of bases on 1 strand determines the sequence of bases on the other strand.

***A single DNA molecule maybe composed of many thousands of such nucleotides.*** The pairs of nucleotides forming the DNA ladder can appear in any order. ***The sequence of the nucleotides is the code that controls the production of all the proteins of an organism.*** **A gene is a sequence of nucleotides that controls the production of a protein or an RNA molecule.**

The activity of the protein is responsible for the trait associated w/ the gene. DNA is directional. The two chains of the double helix run in an anti-parallel arrangement (1 chain ends in the 3’ (3 prime) carbon & the opposite chain ends in the 5’ (5 prime) carbon. See pg.178. DNA wraps around proteins (histones) in a tight coil). Nucleosome-146 DNA nucleotides entwined in 2 loops around 8 histone proteins. Chromatin-formed by DNA and associated histone proteins and nucleosomes.

Replication The process by which DNA copies itself. DNA replicates before mitosis and before meiosis (in interphase). In replication, the 2 strands of the DNA molecule unwind and separate at the hydrogen bonds that link the complementary bases with the help of enzymes called helicases.

Replication, cont’d. The point where the molecule splits into separate strands is called the “replication fork” because of the Y shape. At the replication fork, an enzyme known as RNA polymerase (primase) directs the making of short RNA primer at the start of each segment of DNA to be replicated. Then enzymes known as DNA polymerases move along each of the 2 old strands adding nucleotides to the exposed bases according to the base-pairing rules forming 2 new complementary strands. Each completed new DNA molecule contains 1 old strand and 1 new strand (semiconservative replication). The DNA polymerase also has a proofreading function. In the event of a mismatched nucleotide, DNA polymerase is capable of replacing the nucleotide with the correct one. An enzyme known as a ligase joins the sugar-phosphate backbone.

Replication begins in several places on a linear strand of eukaryotic DNA, and proceeds in 1 direction on one strand and is discontinuous on the other strand. The 5’ to 3’ directionality on 1 strand is accomplished in short pieces from the inner part of the fork outward and ligase joins the pieces. These pieces are called Okazaki fragments. See pg.182