1. Molecular Biology DNA Structure and Function
Chapter 9
Part A

2. Chromosome Review Chromatin Chromosome DNA and associated proteins
Unduplicated and duplicated
Sister chromatids
Centromere
centromere

3. The Structure of DNA Francis Crick and James Watson, 1953
Used key pieces of information to determine the structure of DNA
Four kinds of monomers
Adenine, Guanine, Thymine, and Cytosine
Chargaff Rules
The amount of adenine and thymine is the same
The amount of guanine and cytosine is the same
Rosalind Franklin’s X-ray crystallography
Basic shape, size, and spacing of DNA

4. The Structure of DNA Francis Crick and James Watson, 1953
DNA consists of two antiparallel chains of nucleotides coiled into a right-handed double helix

5. The Structure of DNA Nucleotides
There are three parts to each nucleotide
Phosphate group
A five carbon sugar called deoxyribose
One of four different nitrogenous bases
Adenine
Guanine
Thymine
Cytosine

6. The Structure of DNA Nucleotide chains
Nucleotides bond to each other creating a long chain
The phosphate group of one nucleotide makes a covalent bond with the sugar of the next nucleotide
This creates an alternating phosphate-sugar backbone for the chain (rails of the ladder)
The bases extend out from the backbone (inside the helix)
The order of the bases is tremendously variable and codes for all inherited traits in all living organisms

7. The Structure of DNA Nucleotide chains
Two chains of nucleotides are held together by hydrogen bonds
The hydrogen bonds occur between the internally positioned bases (creating the “rungs” of the ladder)
Complementary base pairing
A-T T-A
G-C C-G

8. The Structure of DNA Nucleotide chains
The two strands are anti-parallel
Oriented in opposite directions
One strand has the 3’ carbon in the upward position
The other strand has the 5’ carbon in the upward position

9. The Structure of DNA
DNA consists of two antiparallel chains of nucleotides coiled into a right-handed double helix

10. Questions Who were the scientists that described the structure of DNA?
DNA molecules are long chains of _____.
What are the three parts of a nucleotide?
What are the four nitrogenous bases used in DNA?
Which complementary base pairings form?
What type of chemical bond holds the two chains of DNA together?

11. The Structure of RNA RNA = ribonucleic acid
Very similar to DNA with three distinct differences
The 5 carbon sugar molecule is ribose instead of deoxyribose
RNA is single stranded
The base uracil is used instead of thymine
Complementary base pairing rules
A-U
T-A
G-C
C-G
DNA
RNA

12. The Structure of RNA Ribonucleic acid (RNA)
There are three types of RNA transcribed
Messenger RNA (mRNA)
Transcribed from genes that encode proteins
Used to carry the instructions for synthesizing proteins from the nucleus to the cytoplasm
Ribosomal RNA (rRNA)
Major portion of the ribosomes
the structures used to assemble polypeptide chains
Transfer RNA (tRNA)
Delivers amino acids one by one to ribosomes, in the order specified by the mRNA
Amino acids = polypeptide monomers

13. How DNA is arranged in a cell
DNA is protected and packaged in very specific ways
Must be able to be
replicated when a cell is ready to divide
“read” to produce proteins to carry out cellular functions
packed into a small space

14. DNA Replication
Chromosomes typically have only one molecule of DNA (double helix)
When a cell reproduces, each chromosome needs two DNA molecules
One for each of the future offspring
DNA replication during the S stage of the cell cycle copies the DNA of each chromosome before cell division
One unduplicated chromosome becomes one duplicated chromosome

15. DNA Replication
DNA helicase unwinds DNA starting at the origin of replication
Eukaryotic: multiple origins of replication
Prokaryotic: one origin or replication

16. DNA Replication
DNA helicase unwinds DNA starting at the origin of replication
Two replication forks are formed
Extend in both directions as replication proceeds

17. DNA Replication New bases are added to the parental strands
Each original strand is a template
Complementarity makes it possible to recreate the other strand
DNA polymerase matches base pairs
A-T and G-C

19. DNA Replication DNA polymerase requires a “primer” as a starting point
Short fragment of RNA which is removed later

20. DNA Replication
DNA polymerase can only construct nucleotide strands in the 5’  3’ direction
The leading strand is synthesized continuously
The lagging strand is synthesized in short fragments (Okazaki fragments)

21. DNA Replication Finishing steps
Primers are removed and replaced with DNA nucleotides
Backbone is sealed by DNA ligase

22. DNA Replication Two identical sister chromatids are created
Each has half of the original DNA molecule and half new nucleotides
Replication is semiconservative

23. DNA Replication Telomere replication
Ends of chromosomes
Chromosome ends get progressively shorter with each replication
Lagging strand has no place for a primer to be made
Associated with aging
Telomerase is used in some cells to maintain chromosome ends

24. Questions Why do cells need two molecules of DNA when they reproduce?
What does DNA helicase do?
What does DNA polymerase do?
What does DNA ligase do?
Make the single-stranded DNA double stranded by using complimentary base pairings. ATGCCGA
Is replication conservative, semi-conservative, or non- conservative

25. DNA Repair Errors can be introduced into DNA
DNA polymerase can make mistakes (“typos”)
Environmental conditions: radiation and mutagenic chemicals

26. DNA Repair Repairs DNA polymerase proofreads and corrects mistakes
Mismatch repair
Uncorrected replication mistakes

27. DNA Repair Repairs Nucleotide excision repair Thymine dimers
Caused by ultraviolet light
If not removed they lead to DNA mutations

28. DNA Repair Mutations Uncorrected mistakes may result in a mutation
Mutation = a permanent change in the DNA sequence
Serious consequences like cancer
Positive consequences like a new adaptive trait

29. Summary The structure of DNA How DNA is arranged in cells
History
Chemical structure
How DNA is arranged in cells
DNA replication
DNA repair