Lecture 14 - DNA & Chromosomes

Bio

Watson and Crick

identified the double-helix model of DNA

DNA

Made up of nitrogenous base, deoxyribose, and a phosphate group

Nitrogenous bases

Adenine, thymine, guanine, cytosine

Chargaff Rules:

A=T and G=C

A=T

2 hydrogen bonds

C=G

3 hydrogen bondsP

Purines

A and G, containing two carbon rings

Pyrimidines

T and C, contain one carbon ring

Sugar phosphate backbone

Made up of deoxyribose and phosphate molecules, run antiparallel to each other: One strand is 5’ - 3’ and the other runs 3’ - 5’

Semiconservative model of replication

Predicts that when DNA replicates, one strand is a parent strand and the other is the newly synthesized strand.

Origin Sites

Where DNA strands separate, forming a replication “bubble”, 100s-1000s per chromosome

DNA replication is remarkably accurate

1 error per 10 billion nucleotides

Replication Forks

Y shape structure that forms as helicase unwinds a DNA strand into to single parent strands.

Helicase

Untwists double helix, separating template DNA at the replication fork

Primase

Initial RNA primer (5-10 nucleotides long), short stretch. Necessary for initiating the synthesis of a new polynucleotide.

DNA polymerase

Synthesizes new DNA strand by adding nucleotides to preexisting chain (initial primer)

Can only place nucleotides on 3’ end… elongates from 5’—> 3’

Anitparallel

Two strands of DNA in a double helix are _____… meaning one strans runs 5’—> 3’ and other runs 3’—> 5’

3’ end has a free hydroxyl (-OH) group attached to deoxyribose and a 5’ end with a free phosphate group attached to deoxyribose

What is attached to the 3’ end? What is attached to the 5’ end?

DNA Polymerase III

Synthesizes a complementary strand continuously by adding nucleotides to the new strand in the 5’ to 3’ direction

Leading Strand

Requires a single primer, DNA polymerase III moves towards junction of replication fork along the template strand, continuously adding nucleotides to the new complementary strand as the fork progresses

Lagging Strand

Synthesized through a series of short segments (okazaki fragments), copied away from the fork and requires multiple primers

Okazaki Fragments

How lagging strand is synthesized, each ____ must be primed separately on the lagging strand

DNA ligase

Joins sugar-phosphate backbones of Okazaki fragments to form a single DNA strand — joining 3’ end of fragment 1 to 5’ end of fragment 2

Initial Mistakes occur 1 per 100,000 base pairs

DNA polymerase proofreads each new nucleotides against template, if wrong, enzyme removes wrong base and resumes synthesis

Mismatch Repair

Enzymes fix incorrectly paired nucleotides that are missed by DNA polymerase or mutations that occur after DNA synthesis completed

DNA molecules subjected to harmful chemicals or physical agents (ex: X-rays, cigarette smoke)

DNA molecules incorrectly paired/altered nucleotides arise after replication by… OR spontaneous chemical changes under normal cellular conditions

Mutations

Mistakes in DNA replications. Can change phenotype of organism. If in germ cells, can be passed on from generation to generation.

Mutations

Source of variation in which natural selection operates during evolution… responsible for all traits and features for all species in our biosphere. Majority of changes are harmful… very small percentage beneficial.

Bacteria Chromosome

Single, circular, double-stranded DNA molecule w/ small amount of proteins

Eukaryote chromosome

Linear, double helix DNA molecule associated with large amount of protein

Histones

DNA packaged with ____ proteins form chromatin

Nucleosome

Basic structure of chromosome, made up of DNA would 2 ½ times around a bundle of 8 histone proteins. 10nm in diameter, looks like beads on a string

Euchromatin & Heterochromatin

Two forms of highly decondensed chromatin found during interphase

Euchromatin

10-nm fiber of chromatin found during interphase

Heterochromatin

30-nm fiber of chromatin found during interphase, can be further folded into more dense arrangements by a host of other proteins

Chromatin coils and condenses, formes short, thick metaphase chromosomes

Chromatin during Mitosis/Meiosis