1. DNA Replication and Protein Synthesis

2. DNA Replication

3. DNA replication and protein synthesis happens mainly during S phase.

4. How DNA Replication Occurs
DNA replication is the process by which DNA is copied in a cell before a cell divides by mitosis, meiosis, or binary fission
One copy moves to each new cell
Since the two strands are
complementary, each strand
serves as a template to make a
new strand

5. Steps of DNA Replication
Helicases separate the DNA Strands
These move along the DNA molecule breaking the hydrogen bonds between the complementary bases
This allows the two strands to separate
The Y shaped region where the strands are separating is called the replication fork (not spoon)

6. Steps of DNA Replication
DNA polymerases add complementary nucleotides to each of the original strands
Nucleotides float freely in the nucleus
As nucleotides are added, bonds form between the sugar of one nucleotide and the phosphate group of the next
Hydrogen bonds form between the bases

7. Steps of DNA Replication
DNA polymerases are finished and fall off
- Result is two molecules of DNA identical to the original
- Molecules are ready to move into new cells during division
- Called semi-conservative replication because each of the new DNA molecules has kept (or conserved) one of the two (or semi) original DNA strands.

8. Action at the Replication Fork
DNA synthesis occurs in different directions on each strand
As the replication fork moves along the original DNA, synthesis follows the movement of the replication fork
Synthesis on the other strand moves in the opposite direction, away from the replication fork
This leaves gaps in the newly synthesized strand, which are later joined by an enzyme DNA ligase

9. Prokaryotic and Eukaryotic Replication
Circular chromosome
Replication begins at one place along the chromosomes
Two replication forks are formed and proceed in opposite directions
Replication continues along each fork until they meet an the whole molecule is copied

10. Prokaryotic and Eukaryotic Replication
Each chromosome is long, but not circular
At the rate a polymerase adds nucleotides (50/sec), it would take 53 days to replicate the largest human chromosome
Instead, replication begins at many points or origins along the DNA, and two forks move in opposite directions
In a fruit fly, replication begins simultaneously at about 3500 sites in a DNA molecule

11. Errors in DNA Replication
DNA replication usually occurs with great accuracy (only one error for every billion nucleotides added)
How is this done???
Polymerases have repair or “proofreading” functions in the same way the computer checks for spelling errors
For example, if A is paired with C, the polymerase removes the C and replaces it with T
Click picture for video

12. Errors in DNA Replication
When mistakes do occur, the base sequence of the newly formed DNA differs from the original
A change in the nucleotide sequence of a DNA molecule is called a mutation
These can have serious effects on the function of a gene – therefore disrupting cell function

13. DNA Replication and Cancer
Mutations can arise and lead to altered cells and organisms
The changes allow individuals to survive and reproduce better
Sometimes un-repaired mutations can cause cancer
Tumors are caused by mutations that affect cell division

14. How is the exact replication of DNA ensured?
By complementary base pairing and proofreading by DNA polymerases

15. What are replication forks?
Areas of DNA where the double helix separates prior to replication

16. Protein Synthesis

17. Three Basic Steps to Protein Synthesis
Transcription-DNA code is copied by mRNA (messenger RNA). Happens in nucleus.
Translation-Ribosomes use information from mRNA to produce proteins. Happens in cytoplasm.
Protein Synthesis- ribosomes make proteins with tRNA anticodons

18. Transcription Decoded
During transcription, RNA polymerase (enzyme) binds to DNA and splits the DNA strands.
RNA polymerase then uses one strand of DNA as a template from which nucleotides are assembled into a strand of mRNA.

19. How does RNA polymerase “know” where to start and stop making an RNA copy of DNA?

21. Promoters
RNA polymerase will bind only to regions of the DNA known as promoters, which have specific base sequences.
Base sequences AUG means “start” in DNA codon language

22. Transcription video clip now!!!

23. The genetic code
Proteins are made by joining long chains of amino acids called polypeptides (proteins).
Each polypeptide contains a combination of 20 amino acids.

24. Genetic Code
The genetic code is read three letters at a time (a codon), so that each “word” of coded messages is three bases long.
Example
UCG-CAC-GGU
Stands for serine- histidine - glycine

27. Translation & Protein Synthesis
mRNA code is copied by tRNA in sets of 3 bases at a time (codons).
Protein Synthesis-tRNA delivers amino acids to ribosomes which link amino acids together to make a protein.

28. tRNA
amino acid
attachment site
anticodon U A C

29. Watch translation Video clip now!!!!!