Lecture 10 The Central Dogma of Molecular Biology

Lecture: 10 The Central Dogma of Molecular Biology Dr. Q. M. I. Haq

The Central Dogma of Molecular Biology p p First proposed by Francis Crick (1958) He said that the sequence involved in the expression of hereditary characteristics is: ØThe Central Dogma involves three major players, namely DNA, RNA, and proteins. ØIt also involves three processes, namely DNA replication, transcription, and translation through protein synthesis. ØIt forms the basis of storage, transmission, and expression of hereditary information.

The Central Dogma of Molecular Biology

DNA Replication §The DNA duplication. §The transfer the genetic information from a parent to a daughter cell. §The DNA base sequences are precisely copied.

Replication: 1 st step p Unwind DNA n helicase enzyme p unwinds part of DNA helix p stabilized by single-stranded binding proteins Replication: 2 nd step § Build daughter DNA strand u u add new complementary bases DNA polymerase III Replication p Adding bases n can only add nucleotides to 3 end of a growing DNA strand p need a “starter” nucleotide to bond to n strand only grows 5 3

Okazaki Leading & Lagging strands Limits of DNA polymerase III u can only build onto 3 end of existing DNA strand 5 ki fra Okaza 3 5 3 5 growing replication fork s gment 3 5 5 5 Lagging strand ligase 3 Leading strand 3 Lagging strand u Okazaki fragments u joined by ligase § “spot welder” enzyme 3 5 3 DNA polymerase III Leading strand u continuous synthesis

RNA primer u built by primase u serves as starter sequence for DNA polymerase III DNA polymerase I u removes sections of RNA primer and replaces with DNA nucleotides Telomeres Repeating, non-coding sequences at the end of chromosomes = protective cap Telomerase u enzyme extends telomeres u different level of activity in different cells

DNA polymerases p DNA polymerase III n 1000 bases/second! n main DNA builder p DNA polymerase I n 20 bases/second n editing, repair & primer removal Roger Kornberg 2006 Arthur Kornberg 1959

DNA Replication (SEMICONSERVATIVE and SEMIDISCONTINOUS) 2 old strands §It is SEMICONSERVATIVE or the 2 old strands will serve as the template for the formation of 2 new strands. §It is SEMIDISCONTINOUS or the new strands formed are attached by fragments (Okazaki fragments). 1 new strand 1 old strand Matthew Meselsson and Franklin Stahl

Example of DNA Replication 5’ TAC CGG AAT GCA ATG CAT ATG 3’ OLD 3’ ATG GCC TTA CGT TAC GTA TAC 5’ OLD 5’ TAC CGG AAT GCA ATG CAT ATG 3’ OLD 3’ ATG GCC TTA CGT TAC GTA TAC 5’ NEW 5’ TAC CGG AAT GCA ATG CAT ATG 3’ NEW 3’ ATG GCC TTA CGT TAC GTA TAC 5’ OLD

DNA Replication p Large team of enzymes coordinates replication