Higher Biology Unit 1 Key area 2 Replication

Higher Biology Unit 1 Key area 2: Replication of DNA

Learning Intentions: To explore DNA replication

Success criteria: • I can • State DNA replicates through a semi-conservative replication. • Describe what is meant by the leading and lagging strand • Explain the function of DNA polymerase • Describe how the leading strand is replicated • Explain the function of DNA ligase • Describe how the lagging strand is replicated.

National 5 knowledge • With a partner, discuss what you can remember from National 5 about DNA replication

Meselson and Stahl were two scientists who, in 1958, carried out an ingenious experiment to solve this question.

Three possible hypotheses

Method of Replication • DNA replicates by a semi-conservative replication. • This means that after the first replication cycle, there are 2 strands, each of which has 1 original strand 1 new complementary strand.

Tip: • When you are copying these down into your jotter use 2 different color's so you can see what is being copied.

AT C G T A DNA parental strand composed of two complementary strands GC C G AT STEP 1: Hydrogen bonds between the bases break – separating the strands A T C G T A G C C G A T

STEP 2: Free nucleotides start to line up with complementary nucleotides A T C G T A G C C G A T T C A G C T A G

A T C G A T A G C C G A T T STEP 3: Sugar-phosphate bonds form. Two DNA molecules identical to the parental molecule have been formed.

Check Point: • With a partner: • Person 1: Explain the first steps • Person 2: Explain the next 2 steps.

Task: • Complete 2 cycles of Replication for the following strand of DNA: C G G G C T T A T G C C C G A A T A

Enzymes Involved • There are several enzymes which are involved in this process. • DNA polymerase controls the formation of the sugar-phosphate bonds. • However, DNA polymerase can only join nucleotides onto the 3’ end of a growing DNA strand. • This is where we need to remember what we learnt previous lesson.

A more detailed look at how the new strand is formed….

5’ end of DNA strand From this point on: We will refer to the 5’ to 3’ strand as the LEADING STRAND 3’ end of DNA strand And the 3’ to 5’ strand as the LAGGING STRAND 5’ end of DNA strand

5’ end of DNA strand 3’ end of DNA strand Step 1 -A DNA primer (a short stretch of complementary DNA) attaches to the start of the piece of DNA being copied. 5’ end of DNA strand 3’ 5’ Primer

5’ end of DNA strand 3’ end of DNA strand Step 2 - Free nucleotides continue on from the 3’ end of primer being joined together by DNA polymerase. 5’ end of DNA strand 5’ 3’ Primer DNA polymerase enzyme

5’ end of DNA strand 3’ end of DNA strand Step 3 - This continuous process until leading strand is copied. f o n o i t ion c e t r a i c D pli re 3’ end of DNA strand 5’ 3’ 5’ end of DNA strand DNA polymerase enzyme

The lagging strand • This is the more tricky one • because DNA polymerase attaches bases on from the 3’ end of the primer • This, on the lagging strand is not the end which is unzipping.

3’ end of DNA strand Many primers attach along the strand. These are extended by the DNA polymerase. The fragments are then joined by the enzyme ligase Primer 5’ DNA polymerase 3’ 5’ 3’ This is a discontinuous process creating the lagging strand. Ligase 5’ 3’ 5’ end of DNA strand

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)

When copying a long chromosome many replication forks operate simultaneously to speed up the replication process.

Replication bubbles and forks

Success criteria: • I can • State DNA replicates through a semi-conservative replication. • Describe what is meant by the leading and lagging strand • Explain the function of DNA polymerase • Describe how the leading strand is replicated • Explain the function of DNA ligase • Describe how the lagging strand is replicated.