1. Human Cells 2 Structure and replication of DNA
Higher Human Biology
Human Cells
2 Structure and replication of DNA

2. Learning Intentions Revise DNA structure
Learn more detail on structure of DNA
Learn some of the detail on DNA replication

3. Getting started What is DNA? What is DNA made from?
Why is DNA a unique molecule?
Draw a section of DNA?

4. DNA – DeoxyriboNucleic Acid
double helix
It is the molecule of inheritance
It can direct its own replication
It is the code to ensure proteins are produced
It is unique in everyone

5. Chromosomes and DNA
Chromosomes are made of DNA tightly coiled along with some associated proteins called histones

6. DNA sub units Sub units of DNA are called nucleotides
1 = P it is an phosphate molecule
2 = deoxyribose sugar
3 = a nitrogenous base molecule
(4 possible ones – guanine, cytosine, adenine and thymine)

7. Joining Nucleotides
Nucleotides are joined together between the phosphate group
and the deoxyribose sugar (sugar – phosphate bonds)

8. DNA the whole structure
Double stranded helix
Strands run antiparallel
Complimentary base pairing across the strands (G-C and A-T)
Strands joined by hydrogen bonds
Each strand exposes a 3’ and a 5’ carbon end

9. How does DNA code for life?
There are approximately 3 billion base pairs in our DNA
The DNA is organised into genes
Genes contain a sequence of bases that code for specific proteins
The sequence of bases are unique
Humans have 20,000 to 25,000 essential genes
This only makes up a small part (1 - 2%) of all the DNA
Scientists are still working on what the other DNA does

10. Consolidate Read pages 16 to 23 in the text book
Complete the questions on page 23
Draw a labelled diagram of DNA showing the structure

11. DNA replication Replication is unique to DNA. It requires:
An original piece of DNA as a template
Primers (short lengths of nucleotides)
A supply of the 4 different nucleotides
ATP
Enzymes (e.g. DNA polymerase and DNA ligase)

12. DNA and replication basic

13. DNA replication more detail
DNA uncoils
Hydrogen bonds are broken (enzyme ). Molecule unzips exposing the nitrogenous bases
A short primer molecule (a few nucleotides joined together) joins to the 3’ end.
New bases can only be added at the 3’ end
Bases pair in a complimentary way (A-T, G-C) from the primer on the one strand.
These join with sugar – phosphate bonds using DNA
polymerase
Bases then join to other bases with hydrogen bonds

14. DNA and replication

15. DNA replication more detail
On the antiparallel strand new nucleotides align with the exposed bases and short fragments of DNA are formed. This is called the lagging strand
These short fragments are joined together by DNA ligase
This process for each strand continues until both sides have been copied.
Each DNA molecule will then coil into the double helix
This process is semi conservative because each new DNA molecule has a strand from the original plus a new strand making the double helix

16. DNA and replication

17. When is DNA replicated?
DNA is replicated just before cells divide by mitosis and meiosis.
Mitosis produces 2 diploid somatic cells that all contain the same number of chromosomes as the original parent cell.
Meiosis produces 4 haploid gametes that all contain 50% of the original parent cell chromosome number.

18. Consolidation Read pages 24 to 31 in the text bock
Answer the questions on page 31/32
Go over the section in scholar
Complete Chapter 2 in the matching and multiple choice book
Extended response - Homework
Give an account of the structure and replication of DNA