Unit 1: DNA and the Genome Key area 2: Replication of DNA

CFE Higher Biology DNA and the Genome How is DNA replicated? Now you know the structure of DNA... …how is it copied?

CFE Higher Biology DNA and the Genome Why do cells need to copy their DNA? DNA is copied during cell division (mitosis) to ensure that new cells have the same number of chromosomes and to ensure that all cells have the same genes.

CFE Higher Biology DNA and the Genome Hypothesise… Take a 5 minutes to discuss in groups ways in which DNA might copy itself. Use colouring pencils to put your ideas on the idea sheet. Original DNA New Copied DNA Key

CFE Higher Biology DNA and the Genome Three possible hypotheses

CFE Higher Biology DNA and the Genome Meselson and Stahl Meselson and Stahl were two scientists who, in 1958, carried out an ingenious experiment to solve this question.

CFE Higher Biology DNA and the Genome This is their experiment: They grew E. coli bacteria in “Heavy” nitrogen. This nitrogen was used to make DNA. They grew the cells first in “heavy nitrogen” then switched them to “light nitrogen”.

CFE Higher Biology DNA and the Genome When you spin DNA with “Heavy nitrogen” in a ultracentrifuge it sinks to the bottom. “Light nitrogen” stays at the top. DNA with LIGHT nitrogen only DNA with HEAVY nitrogen only DNA with both heavy AND light nitrogen In groups fill in a results prediction sheet. Each group will be allocated either: Conservative, Semi-conservative or Dispersive.

CFE Higher Biology DNA and the Genome The results… Generation Result 1 2 LighterHeavier So which model was correct?

CFE Higher Biology DNA and the Genome Summary In your jotter, create a summary of: The 3 models of DNA replication Medelson & Stahl’s experiment design Their results.

CFE Higher Biology DNA and the Genome

CFE Higher Biology DNA and the Genome DNA is a unique molecule because it can direct its own replication and reproduce itself. DNA replicates by semi-conservative replication.

CFE Higher Biology DNA and the Genome ACTGCAACTGCA TGACGTTGACGT DNA parental strand composed of two complementary strands STEP 1: Hydrogen bonds between the bases break – separating the strands ACTGCAACTGCA TGACGTTGACGT

CFE Higher Biology DNA and the Genome STEP 2: Free nucleotides start to line up with complementary nucleotides ACTGCAACTGCA TGACGTTGACGT T A C G C A T G T G A A C T C G

CFE Higher Biology DNA and the Genome ACTGCAACTGCA TGACGTTGACGT TGACGTTGACGT ACTGCAACTGCA STEP 3: Sugar-phosphate bonds form. Two DNA molecules identical to the parental molecule have been formed.

CFE Higher Biology DNA and the Genome Enzyme control of DNA replication DNA replication is a complex process involving many enzymes. The enzyme DNA polymerase controls the formation of the sugar-phosphate bonds when making the new strand.

CFE Higher Biology DNA and the Genome The leading strand DNA polymerase can only join nucleotides onto the 3’ end of a growing DNA strand. Therefore…

CFE Higher Biology DNA and the Genome DNA polymerase enzyme 3’ end of DNA strand Primer 5’ end of DNA strand Start of complementary strand of replicated DNA Direction of replication Leading strand of replicated DNA Replication of the leading strand of DNA

CFE Higher Biology DNA and the Genome 1.After the hydrogen bonds break, the DNA unzips. 2.A DNA primer (a short stretch of complementary DNA) attaches to the start of the piece of DNA being copied. 3.DNA polymerase the attaches free nucleotides to the 3’ end of the primer. 4.This continuous process till leading strand is copied.

CFE Higher Biology DNA and the Genome 5’ end of DNA strand 3’ end of DNA strand Primer DNA polymerase Ligase Replication of the lagging strand of DNA

CFE Higher Biology DNA and the Genome DNA polymerase can only add onto the 3’ end of a primer. So for the other strand: Many primers attach along the strand. These are extended by the DNA polymerase. The fragments are then joined by the enzyme ligase This is a discontinuous process creating the lagging strand.

CFE Higher Biology DNA and the Genome Replication bubbles and forks

CFE Higher Biology DNA and the Genome When copying a long chromosome many replication forks operate simultaneously to speed up the replication process.

CFE Higher Biology DNA and the Genome Requirements for DNA replication For DNA replication to occur, the nucleus must contain: DNA (to act as the template) Primers A supply of the 4 types of nucleotide DNA polymerase and ligase enzymes A supply of ATP (energy)