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Dna replication, Lecture notes of Biochemistry

DNA REPLICATION

Typology: Lecture notes

2015/2016

Uploaded on 03/22/2016

javeria.zaheer
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Download Dna replication and more Lecture notes Biochemistry in PDF only on Docsity! DNA Replication “Replication“ is the process in which each strand of the original double-stranded DNA molecule serves as template for the reproduction of the complementary strand. Two identical DNA molecules have been produced from a single double-stranded DNA molecule Characteristics Semi-conservative Bi-directional Iniatiates at specifics origins Catalyzed by DNA polymerases DNA Replication-Correlate with Cell Cycle DNA replication occurs in the nucleus during S phase of eukaryotic cell cycle The two identical sister chromatids separate during mitosis when cell divide. Prokaryotic and Eukaryotic Replication Prokaryotic DNA is a closed circular double stranded molecule with single origin of replication Eukaryotic DNA is a long linear molecule with multiple origins of replication DNA POLYMERASE Enzymes that synthesize nucleic acids by forming phosphodiester bonds PROKARYOTIC DNA POLYMERASE: DNA Polymerase I, DNA Polymerase III EUKARYOTIC DNA POLYMERASE: DNA polymerase α synthesizes lagging strand Unwinding of double helix Unwinding of parental strands is brought about by action of Helicase Helicases break Hydrogen bonds holding the two strands of DNA using energy from ATP The two strands begin unwinding and form two replication forks Stabilization of single stranded DNA Single stranded DNA binding proteins (SSB) bind to each of the single stranded DNA molecule SSB stabilize, preventing them from re-associating. SSB prevents the single strands from degradation by nucleases Formation of RNA Primer DNA polymerase is unable to initiate DNA synthesis RNA primers are thus required to begin synthesis at the 5’ end Primase synthesizes a small 10 nucleotide long RNA Primer. DNA polymerase then extend the 3’ end of the preformed primer Elongation DNA polymerase III synthesizes DNA in 5’-3’ direction The newly synthesized strand is complementary and anti-parallel to the template Two types of DNA strands are synthesized Leading strand Lagging strand LEADING AND LAGGING STRANDS Leading strand is synthesized continuously as one long, single piece Lagging strand is synthesized discontinuously as series of small fragments. Each fragment on Lagging Strand is 1000 nucleotide long called Okazaki fragments. Each fragment is made in 5’-3’ direction Leading and lagging strands Removal of RNA Telomers Telomers are short sequences at the end of linear eukaryotic molecule. DNA polymerase can not complete synthesis at 5’ end of each strand With each round of replication telomers are shortened Telomer shortening is thought to be responsible for aging. Telomerase Eukaryotic enzyme used to maintain telomers. Telomrase has: Reverse transcriptase activity (hTRT) Short RNA template complementary to DNA telomer sequence Telomerase activity is presents in germ cells and stem cells Somatic cells do not have telomerase activity correlates with aging.
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