1. Key area 2: Replication of DNA
Higher Biology
Unit 1
Key area 2: Replication of DNA

2. Learning Intentions: To explore DNA replication

3. 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.

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

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

6. Three possible hypotheses

7. 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 and 1 new complementary strand.

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

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

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

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

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

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

14. 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.

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

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

17. 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.
3’ end of DNA strand
5’ end of DNA strand 3’ 5’
Primer

18. 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.
3’ end of DNA strand
5’ end of DNA strand 3’
DNA polymerase enzyme 5’
Primer

19. Direction of replication
5’ end of DNA strand
3’ end of DNA strand
Step 3- This continuous process until leading strand is copied.
Direction of replication
5’ end of DNA strand
3’ end of DNA strand 3’
DNA polymerase enzyme 5’

20. 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.

21. Many primers attach along the strand.
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
This is a discontinuous process creating the lagging strand.
Primer 5’
DNA polymerase 3’ 5’ 3’
Ligase 5’ 3’
5’ end of DNA strand

22. 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)

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

24. Replication bubbles and forks

25. 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.