1. DNA- Structure and Replication

2. Discovering DNA
For a long time, scientists thought cells were only composed of carbs, lipids, and proteins
It was obvious that in reproduction SOMETHING inside the cells transferred from parent to offspring, but what?
In the late 1800s, Meischer discovered “nuclein” – a fourth macromolecule found in the nucleus, later renamed the “Nucleic Acid”
Nucleic Acids come in two forms – DNA and RNA

3. DNA Structure
DNA (a nucleic acid) is made up of monomers called nucleotides
each nucleotide is made up of three parts:
1. 5-carbon sugar (deoxyribose)
2. phosphate group
3. nitrogenous base
FOUR types of nitrogenous bases
adenine (A) & guanine (G) – purines
cytosine (C) & thymine (T) – pyrimidines

4. Nitrogenous bases purines (A & G): two rings on the nitrogen base
pyrimidines (C & T): one ring on the nitrogen base

5. But how do the Pieces fit together?

6. Chargaff’s Rules

7. Chargaff’s Rules
Erwin Chargaff analyzed the DNA of many different species and noticed this pattern
# of A = # of T
# of G = # of C
# of purines = # of pyrimidines

8. Franklin’s Picture
Scientists were RACING to try and be the first one to figure out what DNA looked like
Rosalind Franklin used X-Ray technology to take this picture of DNA which she shared with…

9. DNA “double helix”
…Watson & Crick – the scientists who used the picture to discover the shape of DNA!
DNA consists of two strands wound around each other like a twisted ladder

10. DNA “double helix”
base pairing between nitrogen bases in the middle explains Chargaff’s Rules
A always bonds with T (“points”) – double bond
C always bonds with G (“curves”) – triple bond

11. DNA “double helix”
The nitrogen bases pair with a hydrogen bond that is very WEAK! They can easily be pulled apart (or “unzipped”) and then put back together.

12. So if you have one side of DNA that has these bases,
Cytosine (C)
Adenine (A)
Thymine (T)
Guanine (G)
--(G) Guanine
--(T) Thymine
--(A) Adenine
--(C) Cytosine
These bases will make the other side.

13. DNA and Cell Division

14. DNA Forms and Function
DNA is the instructions for cell. It contains the information for how to make the proteins that carry out homeostasis to stay alive.
DNA is a double helix, and that helix is further twisted to create other forms like chromosomes that are easier to move during divisions
Some cells contain a LOT of DNA! Organization is key
Before each cell division ALL of the DNA must be copied.

15. DNA in different cells
prokaryotes have a SINGLE, circular chromosome in the cytoplasm (no nucleus)

16. DNA and Chromosomes
prokaryotes have a SINGLE, circular chromosome in the cytoplasm containing their DNA
eukaryotes have MANY chromosomes contained in a nucleus

17. Making a Chromosome
Before division, the DNA helix is very tightly wound around proteins called histones to make a super coiled structure called a chromosome. Chromosomes cannot be used to make proteins, but they are easier to move around during division.
Normally, DNA is found in chromatin form

18. Before chromosomes, what happens during the S phase of the cell cycle?
Think back!
Before chromosomes, what happens during the S phase of the cell cycle?
DNA replicates!

19. DNA Replication

20. DNA Replication C—G G-- --C T-- --A A-- --T G-- --C G-- --C
to make a copy of itself, DNA “unzips” the pairs
C—G
G C
T A
A T
G C
G C

21. Notice these strands are the same.
DNA Replication
new bases come in to make a new complementary strand
C—G
G--C G --C
T--A T--A
A--T A--T
G--C G--C
G--C G--C
Notice these strands are the same.

22. DNA Replication Enzymes
Teams of enzymes copy the DNA very rapidly, within several hours
Helicase: unzips the DNA at several locations
DNA Polymerase: bring in the new base pairs to build the new strand
Ligase: proofread the new DNA

23. Your turn
copy and complete the DNA strand
C--
G--
T--
A--

24. Does it look like this?
C--G
G--C
T--A
A--T

25. “Unzip” and copy it! C—G G-- --C G-- --C T-- --A A-- --T A-- --T

26. Does it look like this? C—G G--C G--C G--C G--C T--A T--A A--T A--T