BISC 101 - DNA Structure & Function - STUDENT COPY PDF

Dr. Onkar S. Bains BISC 101  What is DNA and its structure? DNA is a nucleic acid – DNA = deoxyribonucleic acid Made of long chains (polymers) of nucleotides – Each nucleotide consists of a: Phosphate group Deoxyribose sugar group Nitrogen-containing...

Dr. Onkar S. Bains BISC 101  What is DNA and its structure? DNA is a nucleic acid – DNA = deoxyribonucleic acid Made of long chains (polymers) of nucleotides – Each nucleotide consists of a: Phosphate group Deoxyribose sugar group Nitrogen-containing base  DNA has four kinds of bases: Thymine (T) Cytosine (C) Adenine (A) Guanine (G) PYRIMIDINES PURINES  DNA is a double-stranded helix  Strands are antiparallel to each other – The two strands run side by side to each other but in opposite directions – The 5’ end has a free phosphate group while the 3’ end has a free hydroxyl (OH) group OH phosphodiester bond HO  Complementary base pairing (Watson-Crick): – Thymine and adenine (2 hydrogen bonds) – Cytosine and guanine (3 hydrogen bonds)  One turn of the helix occurs every 10 base pairs Negatively charged phosphate groups face out  Useful mnemonics for purines and pyrimidines:  Amazing DNA facts!! DNA from a single diploid human cell extends in a single thread for almost 2 meters long!!! – DNA is “packaged” by coiling around a core of proteins known as histones – Histones are positively charged You could fit 25,000 strands of DNA side by side in the width of a single adult hair  If you unravelled all of your DNA from all of your cells and laid out the DNA end to end, the strand would stretch from the Earth to the Sun and back about 333 times!! – The distance between the Earth and Sun is 150 billion kilometres…so distance from Earth to Sun and back to Earth is 300 billion kilometres – It is estimated that the human body contains about 50 trillion cells – Most of these cells are diploid, so there is about 100 trillion meters of DNA per person – 100 000 billion kilometers DNA per person / 300 billion kilometres = 333  Central Dogma of Biology Describes the flow of genetic information from DNA to RNA to proteins DNA REPLICATION TRANSCRIPTION NUCLEUS RNA TRANSLATION Protein CYTOPLASM  Origins of Replication Origin of replication = DNA sequence where replication is initiated A replication bubble is formed (each bubble has two replication forks) A eukaryotic chromosome may thousands of replication origins DNA replication is semi-conservative (one strand is from parental DNA while other strand is the newly synthesized daughter strand)  Mechanism of DNA replication The initial nucleotide strand is an RNA primer – RNA primase synthesizes primer on DNA polymerase III adds nucleotides to primer DNA strand DNA replication is catalyzed by DNA polymerase III which needs an RNA primer DNA polymerase I replaces RNA DNA polymerase I degrades the RNA primer and replaces it with DNA primer with DNA nucleotides DNA polymerases add nucleotides to 3’ end of growing strand – Synthesis of new strand is 5’ to 3’ – Reads template strand 3’ to 5’  Proteins for DNA replication – DNA helicases unwind the double helix by breaking hydrogen bonds between strands – Single-stranded DNA binding proteins prevent DNA strands from rejoining after helicase separates strands – RNA primase catalyzes the synthesis of short RNA primers – DNA polymerase III extends the strand from primer in the 5’-to-3’ direction; also has proofreading activity – DNA polymerase I degrades RNA primers and replaces it with DNA nucleotides (synthesized in 5’- to-3’ direction); also has proofreading activity – DNA ligase joins the short DNA fragments on lagging strand (Okazaki fragments) into a continuous daughter strand (introduces a phosphodiester bond between two fragments) Leading strand = CONTINUOUS synthesis Lagging strand = DISCONTINUOUS synthesis  Proteins for DNA replication Topoisomerase binds to DNA and relieves torsional stress experienced further upstream along helix that occurs as a result of unwinding due to helicase activity – Cuts phosphodiester bond from one or both strands of double helix to relax DNA, and then cut strand(s) is/are reannealed  Central Dogma of Biology Describes the flow of genetic information from DNA to RNA to proteins DNA REPLICATION TRANSCRIPTION NUCLEUS RNA TRANSLATION Protein CYTOPLASM