Highlights Lecture #17 Spring 2017

Highlights DNA Replication II

1. Telomerase (the enzyme that makes telomeres) is an example of an enzyme that is a reverse transcriptase because it copies RNA and makes DNA from it. The RNA it copies is the template it carries with it to extend the ends of linear chromosomes. Tumor cells are a cell type with an active telomerase. This probably is a factor that enables them to be “immortal”.

2. p53 is a eukaryotic protein that performs quality control for DNA replication. It determines whether or not a cell can proceed to go forward with division after replication is complete. It works as follows – if replication is OK, p53 lets the cell progress through the cell cycle and divide. If there is a problem with replication, p53 stops the cell cycle and induces the cell to synthesize DNA repair proteins. If the repair proteins are able to repair the DNA, then p53 sends the cell through the cell cycle and division proceeds. If the damage can’t be repaired, p53 induces the cell to commit suicide. This is called apoptosis. Mutations that damage the function of p53 can give rise to a cancer.

3. Ultraviolet light can act as a mutagen (an agent that causes mutations). It does this by creating cross-links between adjacent thymines in a strand of DNA. If these are not properly repaired, a mutation can result. It is for this reason that people who tan/burn in tanning booths often develop skin cancer later in life. Repairing this damage is performed by the system known as nucleotide excision repair. In this system, a nuclease excises a portion of the strand with the damage, DNA polymerase fills in the gap, and DNA ligase seals everything up.

4. Thymine dimers are probably the most common damage that occurs to DNA. They arise from exposure to UV light (tanning booths, excessive sun tanning) and result in a covalent bond formed between adjacent thymine bases in DNA. If they are not repaired, thymine dimers can result in mutation. The more exposure you have to UV light, the more likely you will develop skin cancer.

5. DNA can be damaged by other means. Oxidation is a common one. When oxidation of guanine occurs, for example, 8-oxo-guanine is created. This base can form stable base pairs with adenine. If DNA containing 8-oxo-guanine is allowed to replicate, adenine will be inserted in place of cytosine, causing a mutation. A repair mechanism called base excision repair helps to fix this.

6. Another repair mechanism in E. coli (called mismatch repair) is that of the Mut S / Mut H / Mut L system, which recognizes a mismatch that occurs in DNA as a result of an error in polymerization and proofreading. Eukaryotic cells have a system very much like the bacterial one.

Highlights Transcription

1. Transcription is the making of RNA using DNA as a template. Transcription requires an RNA polymerase, a DNA template and 4 ribonucleoside triphosphates (ATP, GTP, UTP, and CTP). Prokaryotic cells have only a single RNA polymerase. Transcription occurs in the 5′ to 3′ direction. RNA polymerases differ from DNA polymerases in that RNA polymerases do NOT require a primer.

2. Transcription requires DNA strands to be opened to allow the RNA polymerase to enter and begin making RNA. Transcription starts near special DNA sequences called promoters.

3. Promoters in E. coli have two common features. The first is a sequence usually located about 10 base pairs “upstream” of the transcription start site (the transcription start site is the location where the first base of RNA starts). This sequence is known as the “-10″ sequence or the Pribnow box”. It has the consensus sequence 5′-TATAAT-3′.

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