The Nucleus Flashcards

Question

What are the 4 steps in DNA replication?

Answer 1

1. Unwinding of DNA - Double stranded DNA needs to be separated into single strands to begin replication - DNA replication occurs at regions in the DNA called the Origin of Replication (Orc) - A group of proteins binds to the Orc to begin replication, the most important being DNA Helicase - DNA Helicase unwinds DNA into two single strands, forming a structure that is called the replication fork - where the rest of replication occurs - Note that this step requires energy 2. RNA Primers Bind to Unwound DNA - Now that the DNA is unwound, the logical next step is to begin copying the freshly exposed single strands of DNA - The enzyme that accomplishes this is DNA polymerase - It can only copy a stand of DNA if it has a short piece of RNA cound to it - Therefore, before the cell can copy DNA it must add a primer that is made up of RNA -it does this by using the enzyme primase 3. Elongation by DNA Polymerases Leading Strand (3' to 5' direction) - DNA polymerase can only add nucleotides to the 3' end of a DNA strand and therefore can only move along the parent strand of DNA in the 3' to 5' direction, creating a new strand that is in the 5' to 3' orientation - DNA polymerase catalyzes the new phosphodiester bond between an incoming nucleotide and the existing nucleotide on the backbone - it also ensures that the incoming nucleotide is the correct base pair match to the parent strand Lagging Strand (5' to 3' direction) - DNA polymerase can't move along it to begin synthesis of a new strand until it is able to read the strand in the 3' to 5' orientation - To mitigate potential damage to the lagging strand while it is not yet being replicated, single stranded binding proteins (SSBPs) bind to the single stranded DNA - However, they cannot ward off damage indefinitely, and still need to be synthesized as soon as possible 4. Termination of DNA Replication - Now that all RNA primers have been removed, there is a new problem: the primers that were at the tip of the leading and lagging strand have been removed, but there is no upstream piece of RNA for DNA polymerase to start synthesis of DNA to replace them - If this were to be left as it is, there would be a very small region of parent strand DNA that is left unreplicated; this would be degraded, since single strand DNA is unstable - Therefore, after every cycle of DNA replication, there would be a gradual loss of DNA at this end, causing chromosomes to shrink over time, which would be very damaging to a cell - -> The solution to this problem in the cell are structures that extend the end of DNA called telomeres - -> Telomeres are long, noncoding sections of DNA at the ends of the DNA molecule - -> Since they are non functional, they can be degraded over cell cycles without affecting cell function

Answer 2

- The lagging strand loops out and many RNA primers are synthesized on a 3' carbon on the DNA - The DNA polymerase then synthesizes DNA in patches along these primers; these patches are called Okazaki fragments - These fragments are why replication is considered semi discontinuous - the leading strand is synthesized in one continuous process, but the lagging strand is synthesized in many discontinous chunks - An enzyme goes through and replaces the RNA primers in both strands with DNA - These new fragments of DNA where the RNA primers used to be are not bound to the other fragments - An enzyme called a ligase goes through the new strand of DNA and catalyzes these bonds - Once all of the RNA pimers have been replaced with DNA and ligase had joine the backbones of the Okaziki fragments

Answer 3

1. The enzyme that adds telomeres called telomerase. It is a unique and interesting enzyme, and it is quite different from the other enzymmes involced in DNA replication. It carries a short piece of RNA within the enzyme that binds to the 3' overhang and extends past the DNA. 2. This RNA is then used as a template and a DNA polymerase adds a corresponding DNA strand to this extension. 3. There is now an extra long 3' overhand that contains repetitive. non-functional DNA. An RNA primer is added, and DNA polymerase adds DNA like usual until there is not enough of a 3' lead for the enzyme to proceed. 4. Telomerase repeats this process several thousand times, creating the long ends of the chromosomes.

Answer 4

DNA replication is not perfect, mistakes occur frequently. It is estimated that approximately 6 errors occur every time the cell replicates. DNA can also be damaged by other factors like ultraviolet light, oxidation or DNA breaks.

Answer 5

When DNA damage occurs, the result is a mutation in the DNA. A mutation is any change in the DNA structure that results in a variant form that can be passed on to subsequent generations. Mutations are the underlying cause of many illness and disease. However, they are also how a species can evolve and an essential part of life.

Answer 6

DNA repair is important becasue DNA contains all of the information that cells inherit from their ancestors. DNA damage, which could include loss of segments of DNA, could remove some of this information, reducing a cell's abiliy to perform vital functions. Because DNA is so essential, DNA integrity is a top priority for cells. Cells use multiple strategies to protecting DNA from damage repairing damage that occurs, and correcting errors in DNA sequence.

Answer 7

1. Repair during DNA replication: DNA polymerase has a proofreading function and can chekc that it is not making mistakes 2. Repair throughout the cell cycle: DNA repair proteins are continually scanning DNA for errors and making repairs; some check immediately after replication, fixing the newly synthesized strand of DNA, whereas others check at different points throughout the cell cycle

The Nucleus Flashcards

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