DNA Replication

Student Assessment:

First we will be taking the Ch. 12 Test on Chromosomes and Inheritance

DNA Replication

Sometimes when we talk about microbiology we start to forget where we are in the cell. Lets remember that when talking about DNA and DNA replication we are talking about processes that take place in the nucleus of the cell.

Sometimes when we think of DNA copying itself we think of chromosomes but that is not correct. Chromosomes are only used for cell division known as mitosis. DNA replication happens well before mitosis even begins. DNA replication happens during the second phase of Interphase, known as S-phase. During S-phase the DNA is still in the shape of chromatin. Remember that we do not see chromosomes until prophase of mitosis.

5 prime and 3 prime

Most students struggle with understanding what 5 prime and 3 prime means.

The backbone of a DNA strand is made of a phosphate group and a deoxyribose sugar. Ribose is a 5 carbon sugar and whenever we draw a sugar we draw one corner for every carbon. So ribose has 5 corners looking like a pentagon. To make it easier to talk about the direction of DNA we number each carbon (each corner) of the sugar. The direction that the 5th carbon is facing we call 5' (5 prime) and the direction that the 3rd carbon is facing we call 3' (3 prime). During DNA replication the enzymes can only move in a 3' to 5' direction.

When DNA is copied the two parent strands are separated and a new daughter strand is built connected to each parent strand. This means that every copied DNA strand has one old strand and one new strand. We call this a semi-conservative process.

There are many enzymes involved in the process of DNA replication.

-First a DNA strand develops a replication bubble. This will be the sight of DNA replication.

-DNA Helicase unzips the DNA at the replication fork and opens up the replication bubble.

-Topoisomerase holds the double helix and breaks it when the tension gets to high.

-RNA Primase lays down an RNA Primer which signals the DNA Polymerase to start creating the daughter strand

-DNA Polymerase III uses free nucleotides to create the new daughter strand

-DNA Polymerase I replaces the RNA Primer with DNA nucleotides.

-Ligase stitches the nucleotides replaced by DNA Polymerase I to the strand that was created by DNA Polymerase III

-The leading strand is the strand that is following the DNA Helicase

-The lagging strand is the strand that is being built in the opposite direction of the DNA helicase

-Because the lagging strand is being built moving away from Helicase, the daughter strand has to be built in fragments.

-The fragments on the lagging strand are known as Okazaki fragments

-A nuclease enzyme goes around and cuts out any improperly placed nucleotides. It works like a DNA spell check.

Student Assessment:

Next class session you will have a replication bubble quiz

For your quiz you will take out a blank sheet of paper and have to draw a replication bubble with all the enzymes labeled.

Here is what must be included and labeled on your drawing

-Replication bubble

-Parent strand with nucleotides

-Daughter strand with matching nucleotides

-DNA Helicase

-DNA Polymerase III

-DNA Polymerase I

-RNA Primase

-The RNA Primer

-Ligase

-The Leading Strand

-The lagging Strand

-The 5' to 3'

-The 3' to 5'

-The Okazaki Fragments