Book : Molecular Biology
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Posted by: CHELSEA
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Writer CHELSEA

DNA Replication

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DNA Replication is Semi-Conservative

DNA replication of one helix of DNA results in two identical helices. If the original DNA helix is called the "parental" DNA, the two resulting helices can be called "daughter" helices. Each of these two daughter helices is a nearly exact copy of the parental helix (it is not 100% the same due to mutations). DNA creates "daughters" by using the parental strands of DNA as a template or guide. Each newly synthesized strand of DNA (daughter strand) is made by the addition of a nucleotide that is complementary to the parent strand of DNA. In this way, DNA replication is semi-conservative, meaning that one parent strand is always passed on to the daughter helix of DNA.

Replication Forks and Origins of Replication

The first step in DNA replication is the separation of the two DNA strands that make up the helix that is to be copied. DNA Helicase untwists the helix at locations called replication origins. The replication origin forms a Y shape, and is called a replication fork. The replication fork moves down the DNA strand, usually from an internal location to the strand's end. The result is that every replication fork has a twin replication fork, moving in the opposite direction from that same internal location to the strand's opposite end. Single-stranded binding proteins (SSB) work with helicase to keep the parental DNA helix unwound. It works by coating the unwound strands with rigid subunits of SSB that keep the strands from snapping back together in a helix. The SSB subunits coat the single-strands of DNA in a way as not to cover the bases, allowing the DNA to remain available for base-pairing with the newly synthesized daughter strands.

Terms

deoxyribonucleoside triphosphate - The building blocks of DNA replication. A five-membered, oxygen- containing ribose sugar ring that has three phosphate groups attached to its 5' carbon and either an adenine, cytosine, guanine, or thymine base group attached to its 1' carbon. 

Base-pair excision - One class of DNA repair system. Recognizes and removes single nucleotide mutations that result from unnatural bases. Daughter strand - Refers to the newly synthesized strand of DNA that is copied via the addition of complementary nucleotides from one strand of pre-existing DNA during DNA replication. 

DNA Helicase - The enzyme responsible for separating the two strands of DNA in a helix so that they can be copied during DNA replication. DNA Ligase - The enzyme responsible for sealing together breaks or nicks in a DNA strand. Responsible for patching together Okazaki fragments on the lagging strand during DNA replication. 

DNA Polymerase - The enzyme responsible for catalyzing the addition of nucleotide substrates to DNA both during and after DNA replication. Primase - The enzyme responsible for initiating synthesis of RNA primers on the lagging strand during DNA replication. Holoenzyme - A term used to describe a collection of different enzymes that work together in a given process such as DNA replication. 

Hydrolysis - The process in which water is chemically added to a molecule. Lagging strand - In DNA replication, the strand of pre-existing DNA that is oriented in the 5' to 3' direction with respect to the direction of replication on which synthesis is discontinuous. Leading strand - In DNA replication, the strand of pre-existing DNA that is oriented in the 3' to 5' direction with respect to the direction of replication on which replication is continuous. 

Mismatch repair - One class of DNA repair system. Recognizes and removes mutations that result from base-pairing that is not complementary. Okazaki fragment - Short stretches of newly synthesized DNA found on the lagging strand during DNA replication. Origin of replication - Site of initiation of DNA replication. Short, usually internal stretch in a DNA helix that opens so that each strand is separate for DNA replication. 

Parent strand - In DNA replication, refers to the pre-existing single strand of DNA that is copied into a new strand of DNA via complementary base pairing. Pyrophosphate - A two phosphate-containing molecule. In DNA replication, it is released from a 2' deoxyribonucleoside triphosphate during its addition to a growing, newly synthesized DNA strand. Its subsequent hydrolysis provides the energy for the addition reaction. 

Replication fork - Term used to describe the junction at which nucleotide substrates are being added to a growing DNA chain during DNA replication. Its shape resembles a "Y" where the two branches represent single stranded daughter strands of DNA and the base represents helical DNA. RNA Primer - Short stretches of ribonucleotides (RNA substrates) found on the lagging strand during DNA replication. Helps initiate lagging strand replication and are later removed. 

Semi-conservative - Refers to the fact that after the replication of one DNA helix each of the two daughter helices that result contain one newly-synthesized and one pre- existing strand of DNA. Short-patch excision - One class of DNA repair system. Recognizes and removes short stretches of DNA that surround mutations resulting from large adducts on a DNA strand that impede DNA replication. 

Single-stranded binding protein - A protein involved in helping to keep strands of DNA that have been separated by DNA helicase from recoiling in a helix. It works by coating the single strands in such a way as not to cover the bases, allowing them to remain free for base pairing. Thymine dimer - A form of DNA damage that results from radiation. Adjacent thymines on the same strand of DNA form a bond that results in a bulky adduct that can impede DNA replication. 

Tautomerization - A process in which a molecule undergoes an electron rearrangement that results in a slightly different organization of the same molecule. The two forms of the same molecule are called "tautomers" of each other.



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