Chromosomes and DNA Replication

What We Will Be Covering

• DNA and Chromosomes– Length and Structure

• DNA Replication

DNA and Chromosomes

• DNA is stored as chromosomes

• Chromosomes are organized structures of DNA that contain large numbers of nucleotides

• Large numbers of nucleotides that are expressed are often called genes

DNA and Chromosomes

• DNA can be extremely long

• A chromosome for an E. coli bacterium living in your gut is 4,639,221 base pairs

DNA and Chromosomes

• The E. Coli DNA when stretched out would be 1.6 centimeters

• The bacteria is only 1.6 micrometers– That is one thousand

times smaller than the DNA

• It is like fitting a 300m rope into your backpack

DNA and Chromosomes

• Eukaryotic cells contain even more DNA

• Humans contain about a meter of DNA per cell

• In order to fit DNA into a cell, it must be coiled

DNA and Chromosomes

• Eukaryotic chromosomes pack DNA and Protein together– It is called chromatin

• Chromatin consists of DNA tightly coiled around proteins called histones

DNA and Chromosomes

• By coiling the DNA around histones, it decreases its overall length

• Histones are then coiled

Copying Information

• What happens when you make a hand drawn copy of a picture?

• Is it then really the same image?

Demo

• Try this…• Have one partner sit facing the blackboard,

one sit facing the opposite direction• The partner that faces the direction of the

board will describe and the other partner will draw

Now Switch

Copying Information

• What happens when you put a piece of paper in a copier?

• Can you then copy the copy?

• Does DNA need to be exactly copied in order to give the same information?

Copying Information

• DNA can be easily copied based on its structure

• If you split DNA in half, you can easily tell what the other half should look like

Review• Base pairs match up to

only one other base pair– Adenine to Thymine– Guanine to Cytosine

• This is because of the structure of the hydrogen bonds of the DNA

Review

GuanineCytosineAdenineThymine

CytosineGuanineThymineAdenine

DNA Replication Steps

1. Unzip DNA2. Add Base Pairs to

both unzipped strands

3. Proofread the 2 new strands

DNA Replication

• Before a cell divides, it goes through a process called DNA replication

• This is where a cell copies its DNA to create new complementary strands

• Complementary strands are the DNA that is created from the existing DNA strands

DNA Replication• DNA replication is

carried out by a series of enzymes

• These enzymes “unzip” a molecule of DNA base pairs

• Each half of the unzipped pairs serve as a template for the new strand

DNA Polymerase

• DNA Polymerase is the principal enzyme involved with DNA Replication

• It has 2 main functions.• It “polymerizes” or creates a

complex compound by linking together nucleotides

• It also proofreads and corrects any mistakes that happen in the DNA strands

DNA Polymerase

• DNA polymerase serves a second function of checking the new strand for errors

• “Proof reads” the DNA strand– Reduces mess-ups in

copying DNA– Maximizes the odds that the

copied DNA will be an exact copy

• What might mess-ups be called?

DNA Helicase

• DNA Helicase is an important enzyme that is involved in DNA replication

• This enzyme unzips the DNA strand

• Its job is to separate the DNA strand and make sure it does not reattach

DNA Helicase

• DNA base pairs hold each other together by hydrogen bonds

• Helicase separates the DNA strand by taking away the hydrogen bonds that hold DNA together

Enzymes At Work

• There are a host of other enzymes working in DNA replication

• They all serve the purpose to copy DNA with complete accuracy

Enzymes At Work

In Summary

• DNA is a long series of base pairs that must be coiled in order to be stored

• The structure of DNA is allows it to be easily copied

• DNA that is copied is unzipped, held open and then completed

DNA Replication

Animation Animation 2