DeoxyriboNucleic Acid DNA DeoxyriboNucleic Acid

DeoxyriboNucleic Acid DNA DeoxyriboNucleic Acid

DNA is made of units called nucleotides The DNA Code is formed by Nitrogen bases

Nucleotides are in 3 parts Nitrogen Base Phosphate Sugar (deoxyribose)

There are 4 Nitrogen Bases in DNA that make up the code A – Adenine T – Thymine G – Guanine C – Cytosine

There are 2 types of Nitrogen Bases A) Purines – Adenine and Guanine double ring B) Pyrimidines – Thymine and Cytosine single ring

The nitrogen bases bond together with weak hydrogen bonds A only bonds to T C only bonds to G A - T and C - G are base pairs

Example sequence A - T - C - G -

Example sequence A - T T - C - G -

Example sequence A - T T - A C - G -

Example sequence A - T T - A C - G G -

Example sequence A - T T - A C - G G - C

The chains of nucleotides form a sequence which is the genetic code

DNA Structure DNA forms a twisted ladder structure called a double helix

Watson and Crick Discovered the Double Helix DNA structure

DNA “Ladder” Rungs (steps) of the ladder are the Nitrogen base pairs connected with weak hydrogen bonds Sides of the ladder are the phosphates and the sugars (deoxyribose) connected with strong covalent bonds like a backbone

DNA Replication DNA duplicates itself before mitosis This occurs in the S phase of cell cycle

Semi conservative model The two DNA strands split before replication

A-T has 2 hydrogen bonds C-G has 3 hydrogen bonds

Replication The weak hydrogen bonds “unzip” and new nucleotides are attached on each side according to base pair rules. A-T C-G

DNA Replication Video http://www.mcb.harvard.edu/Losick/images/TromboneFINALd.swf Show beginning

Each nucleotide has 2 ends named for the carbon on the sugar it attaches to, Either 3’ or 5’. The 3’ end always attaches to the 5’ end with a phosphate in between

The sides of the DNA molecule are STRONG So the DNA Code will stay intact

PERFECT COPIES? DNA must be replicated perfectly so the new cells that form are identical There are proofreader enzymes to ensure copies are made correctly

ENZYMES Helicase – Breaks apart the DNA strands Primase – Builds a short RNA primer to start the chain Polymerase – Elongates DNA strand by Attaching new nucleotides can only read in 3’ to 5’ direction Elongates in the 5’ to 3’ direction

Direction of DNA Leading strand - Side made continuously in one direction Lagging strand – Side made away from replication fork

Okazaki Fragments Short segments of DNA that make lagging strand

Ligase Enzyme that connects Okazaki fragments

End of replication problem Removal of RNA primer leaves gap Telomeres – non coding repetitions at end Telomerase enzymes that lengthen telomeres

Telomeres PROTECT ends of Chromosome If telomeres did not wear away, some scientists think it would prolong life

DNA Replication Video http://www.mcb.harvard.edu/Losick/images/TromboneFINALd.swf DNA rap – Blame it on the DNA http://www.youtube.com/watch?v=gqvYOr78THo&feature=email