1. 3.3 and 3.4 DNA Structure and Replication

2. 3.3 Assessment Statements
3.3.1 Outline DNA nucleotide structure
3.3.2 State the names of the 4 bases in DNA
3.3.3 Outline how DNA nucleotides are linked together
3.3.4 Explain how a DNA double helix is formed using complementary base pairing
3.3.5 Draw and label a simple diagram of the molecular structure of DNA

3. A little bit of DNA History

4. Francis Crick and James Watson

5. The Discovery of DNA (& other stuff)

6. DNA and RNA Structure In 1868, Meischer first isolated
deoxyribonucleic acid
from cells in pus and
from fish sperm.
No one knew its
function.

7. In 1928, Griffith was working with smooth
(S,virulent) and rough (R, non-virulent) strains
of a pneumonia-causing bacterium.

8. In 1944, Avery
showed that the
substance was DNA.

9. Hershey and Chase
confirmed that DNA is
the hereditary material
(not protein).

10. 3.3.1 Outline DNA nucleotide structure

11. 3.3.1 DNA Nucleotide Structure

12. DNA is composed of four kinds of
nucleotides.
Each consists of
a five carbon sugar (deoxyribose)
a phosphate group (PO4-)
one of four bases
adenine (A)
thymine (T)
guanine (G)
cytosine(C)

13. 3.4.2 Name the 4 bases in DNA The nucleotides are similar, but
T and C are
single ringed
pyrimidines
A and G are
double ringed
purines.
3.4.2 Name the 4 bases in DNA

14. Edwin Chargaff, in 1949,
noted two critical bits of
data.
The four kinds of
nucleotide bases making up a DNA molecule differ in relative amounts from species to species

15. 2) The amount of A = T, and the amount of C = G.

16. 3.3.4 Explain how a DNA double helix is formed using complementary base pairing

17. Rosalind Franklin used x-ray diffraction
techniques to produce images of DNA
molecules.

18. She concluded:
DNA exists as a long, thin
molecule of uniform diameter
The structure is highly repetitive
DNA is helical

19. Franklin’s Story

20. Watson and Crick used numerous sources of data to build models of DNA.

21. The following features were incorporated into their model:
The bases were hydrogen bonded (a weak
bond) together in the center of the helix
T (a pyrimidine) had two hydrogen bonds with
A (a purine)
C (a pyrimidine) had three hydrogen bonds
with G (a purine)

22. The nucleotides are joined together using phosphodiester bonds

23. Each sugar of the backbone is covalently bonded to the nitrogenous base off of carbon 1 of the ribose sugar.
covalent bond
carbon 1

24. There are 10 base pairs per turn of the helix.

25. The two sides are anti
parallel, meaning that the
sugar and phosphates are
running in opposite directions.
Each side ends in a phosphate
(5’ end) and a sugar (3’ end)
Since the sides are anti
parallel, one side goes in the
3' to 5' direction, and the other
goes in the 5' to 3' direction.

26. 3.3.5 Draw and label a simple diagram of the molecular structure of DNA

27. 3.3.5 Draw and label a simple diagram of the molecular structure of DNA

28. 3.4 Assessment Statements
3.4.1 Explain DNA replication in terms of
unwinding of the double helix and separation of
strands by helicase followed by formation of
new complementary strands by DNA
polymerase
3.4.2 Explain the significance of complementary
base pairing in conservation of the base
sequence of DNA
3.4.3 State that DNA replication is
semiconservative.

29. 3.4.1 Explain DNA replication
Helicase
Breaks hydrogen bonds between bases, unzips and unwinds the double helix.
Is an enzyme (a protein that speeds up chemical reactions)
Is made during G1

30. Overview of DNA replication
DNA replication in real time!

31. DNA polymerase adds DNA nucleotides in a 5’
to 3’ direction. The enzyme is very specific and
can only add nucleotides to the 3’ end.
5’ 3’
Helicase enzyme unzips the double helix
DNA polymerase adds nucleotides using the original strand as a template
replication fork
Free nucleotides from cytoplasm are added using complementary base pairing
3’ 5’
3’ 5’

32. 3.4.3 State that DNA replication is semiconservative.
The mechanism of semi-conservative replication
produces two descendent double helices that
each contain one of the original polynucleotide
chains

33. Meselson and Stahl carried
out an experiment which showed
the semi conservative nature of
DNA replication.

34. 3.4.2 Explain the significance of complementary base pairing
The DNA molecule is copied precisely from one cell generation to the next.
In a unicellular organism this means that the total genome is successfully copied into each new generation.
In a multicellular organism all cells contain an exact copy of the total genome (even though not all genes are expressed in every cell).
Genes are faithfully passed from one generation to the next.

35. 1.Which molecules form the nucleotide marked in the diagram?
A. phosphate, deoxyribose and nitrogenous base
B. phosphorus, ribose and nitrogenous base
C. phosphorus, deoxyribose and guanosine
D. phosphate, ribose and guanine

36. 2. Which of the following are connected by hydrogen bonds?
A. Hydrogen to oxygen within a molecule of water
B. Phosphate to sugar in a DNA molecule
C. Base to sugar in a DNA molecule
D. Hydrogen to oxygen between two different molecules water

37. 3. (a) Draw a labelled diagram to show how two nucleotides are joined together in a single strand of DNA (3)

38. 4. What principle is necessary to preserve the sequence of DNA during replication?
A. Base pairing is complementary.
B. One gene codes for one polypeptide.
C. Substrates are specific to enzymes.
D. The genetic code is universal.

39. 5.(a) Explain why DNA must be replicated before mitosis and the role of helicase in DNA replication (4)

40. (b) Explain how the base sequence of DNA is conserved during replication (5)