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Quiz 3 Study Guide (Ch. 6)

by: Tiffany Norris

Quiz 3 Study Guide (Ch. 6) Biol 201

Marketplace > Loyola Marymount University > Biology > Biol 201 > Quiz 3 Study Guide Ch 6
Tiffany Norris
Loyola Marymount University

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Chapter 6 Study Guide, Exam 2, Quiz 3
Cell Function
Yiwen Fang
Study Guide
Cell, function, Biology, DNA, replication
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This 4 page Study Guide was uploaded by Tiffany Norris on Sunday October 2, 2016. The Study Guide belongs to Biol 201 at Loyola Marymount University taught by Yiwen Fang in Fall 2016. Since its upload, it has received 16 views. For similar materials see Cell Function in Biology at Loyola Marymount University.

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Date Created: 10/02/16
Cell Function Quiz 3: Chapter 6 Study Guide [DNA REPLICATION & REPAIR] DNA Replication Replication is important because cells must make a copy to pass on genes and perpetuate the species • Fidelity = the copy must be exactly like the original o Fidelity is ensured by base pairing, using a complementary strand. o DNA replication is a semi-conservative style, meaning there is one new and one original copy of DNA. This also helps prevents mistakes § Meselson-Stahl experiment was used to prove that replication is semi-conservative. Different types of Nitrogen isotopes were used. 3 STEPS OF DNA REPLICATION 1. INITIATION The process of DNA replication is begun by initiator proteins that bind to the DNA and pry the two strands apart, breaking the hydrogen bonds between the bases. o The positions at which the DNA is first opened are called replication origins, and they are usually marked by a particular sequence of nucleotides. o Bases A & T have 2 hydrogen bonds, therefore they’re easier to break. There is a high amount of A & T’s at the origin, which allows the process of replication to begin more easily. o There are multiple origins in eukaryotes, while bacterial cells only have 1 origin. § In humans, beginning DNA replication at many places at once greatly shortens the time a cell needs to copy its entire genome. 2. ELONGATION i. Helicase opens the helix ii. DNA molecules in the process of being replicated contain Y-shaped junctions called replication forks. At these forks, the replication machine moves along the DNA, opening up the two strands of the double helix and using each strand as a template to make a new daughter strand. • The forks move very rapidly—at about 1000 nucleotide pairs per second in bacteria and 100 nucleotide pairs per second in humans. • The forks are bidirectional and asymmetrical: two replication forks are formed at each replication origin, and they move away from the origin in opposite directions, unzipping the DNA as they go. DNA polymerase synthesizes new DNA using one of the old strands as a template. • DNA is synthesized in the 5’-to-3’ direction o This enzyme catalyzes the addition of nucleotides to the 3’ end of a growing DNA strand by forming a phosphodiester bond between this end and the 5’ phosphate group of the incoming nucleotide. • Okazaki fragments or the “lagging strand,” is a chain of short DNA fragments that are synthesized discontinuously. They are later joined together to form a continuous new strand. The other strand of DNA, which is synthesized continuously, is called the leading strand. • DNA synthesis is primed by an RNA polymerase called primase, which makes short lengths of RNA (primers) that are elongated by DNA polymerase. § Leading strand requires one primer § Lagging strand requires multiple primers Challenges with DNA Replication • Our DNA is linear so we have a hard time replicating the last okazaki fragments. o Telomerase fixes this problem. It is a special DNA polymerase containing an RNA. § Bacterial DNA has no end (because it is circular) therefore it does not have telomerase. 3. TERMINATION Nuclease removes the primer Ligase links the fragments together like glue DNA Proof Reading • DNA Polymerase is self- correcting: using 3' → 5' exonuclease activity § The overall accuracy of DNA replication, including mismatch repair, is one mistake per 10 nucleotides copied. DNA Damages Spontaneous vs. Induced Damage I. Induced: induced by chemicals or radiation a. The ultraviolet radiation in sunlight is damaging to DNA; it promotes covalent linkage between two adjacent pyrimidine bases, forming, for example, the thymine dimer à II. Spontaneous: naturally occurring in a cell o During the time it takes to read this sentence, a total of about a trillion purine bases (A and G) will be lost from DNA in the cells of your body by a spontaneous reaction called depurination. § Depurination does not break the phosphodiester backbone but, instead, gives rise to lesions that resemble missing teeth. o Another common reaction is the spontaneous loss of an amino group from cytosine in DNA to produce the base uracil. This is called deamination. DNA Repair Basic Repair Pathway: 1. Recognize the damage and remove the damaged DNA a. usually by nuclease (cleave the covalent bond) which leaves a small gap on one DNA strand 2. A repair DNA polymerase (DNA polymerase I) fills the gap 3. DNA ligase seals the nick Example: Mismatch repair, nucleotide excision repair etc. Mismatch Repair System: to remove replication errors that escape proofreading, thus reducing the error rate § A particularly dangerous type of DNA damage occurs when both strands of the double helix are broken, leaving no intact template strand to guide proper repair. Two ways to repair double-strand breaks: • In human somatic cells, the most common means of repairing double-strand breaks is by a mechanism called non- homologous end-joining. In this process, the two broken ends are simply brought together by a specialized group of enzymes and rejoined by DNA ligation. o Although this mechanism repairs the break, nucleotides are usually lost at the site of repair • Another solution is to use the genetic information provided by an entirely separate DNA duplex to repair the break accurately. This strategy is carried out by a set of reactions collectively known as homologous recombination. It flawlessly repairs DNA damage. o Homologous recombination is conserved in all organisms


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