Structure & Replication DNA Structure & Replication

DNA Facts A single strand of human DNA = about 1 meter long (uncoiled). The total DNA in each cell = about 2m. The nucleus of a typical cell has a diameter of 0.00034mm = 0.000013in A single strand of DNA contains about 3 billion base pairs. If the DNA sequence of a person was compiled in books, it would take the equivalent of 200 telephone books, each 1000 pages.

Function of DNA Stores & transmits genetic info from one organism to next generation Instructs cells on work to do

DNA in other organelles? Why?

DNA Structure Deoxyribonucleic Acid Building blocks - nucleotides 5-Carbon Sugar (deoxyribose) Phosphate Group Nitrogen Base

4 nitrogen bases Adenine (A) & Guanine (G)  purines Cytosine (C) & Thymine (T)  pyrimidines

DNA Strand Nucleotides bond together  polymer Sugar & Phosphate attached by covalent bonds Nitrogen bases point outward & attach to each other by hydrogen bonds (weaker)

DNA Double Helix Nucleotide Hydrogen bonds Sugar-phosphate backbone Key Adenine (A) Thymine (T) Cytosine (C) Guanine (G)

Erwin Chargaff Same amount of A&T and C&G A pairs only w/T – (2 H-bonds) C pairs only w/G – (3 H-bonds) Complementary base pairing rule Source of DNA A T G C Streptococcus 29.8 31.6 20.5 18.0 Yeast 31.3 32.9 18.7 17.1 Herring 27.8 27.5 22.2 22.6 Human 30.9 29.4 19.9 19.8

Twisted Ladder B/C Sugar & Phosphate groups do not line up exactly

Double Helix Discoveries Rosalind Franklin – X-ray diffraction showed DNA helix was 2-3 strands Watson & Crick – 1953 – exact structure double helix held together by nitrogen bases

Anti-parallel The 2 strands of DNA are arranged in opposite directions (anti-parallel). The strands are identified (named) by the numbered carbon located at the end of the strand. 5’ is read as 5 prime, 3’ is read as 3 prime

Chromosome Supercoils Coils Nucleosome Histones/proteins DNA double helix

More DNA Facts If DNA were the thickness of a clothesline, the length would be 8km (5mi) long. In bacteria, nucleotides are replicated at a rate of about 500 per second; in mammals, about 50 per second. During DNA replication, only about one error occurs for every one billion nucleotides made.

ENZYMES made of Proteins 100s to 1000s of chemical rxn in human body Enzymes speed up rxn by millions and even billions of times Single enzyme molec. acts on about 1000 substrate molecules/second.

IMPORTANCE OF ENZYMES Enzymes are biological catalysts. Lower the amount of energy needed for rxn Not changed by rxn Not used up by rxn

They lower the amount of energy needed for a reaction. Reaction pathway without enzyme Activation energy Activation energy with enzyme Reactants Products

Responsible for metabolism: Catabolism: breaking apart molecules (hydrolysis) Anabolism: building molecules (dehydration synthesis/condensation rxn)

Most enzymes are named after the substrate they work on (usually ending in “-ase”). Lipase-lipids Protease-proteins Sucrase-sucrose

DNA Replication Cells copy info for each new cell Occurs in nucleus 3 steps

1. Unzip DNA Helicase unzips (uncoils) part of DNA strand, breaking H-bonds btwn bases

2. Complementary Base Pairing DNA Polymerase pairs up free nucleotides with complementary bases (forming new H-bonds)

3. Joining nucleotides Ligase – joins nucleotides (S+P) w/covalent bonds  2 identical DNA strands, each w/1 new strand & 1 old strand Semi-conservative replication

Checking for Errors DNA polymerases are also “proofreaders” Only add nucleotides to growing chain, if previous base is correctly paired Backtracks to correct mistake Only about 1 error for every one billion nucleotides made

DNA Replication Animation

Chromosome Supercoils Coils Nucleosome Histones/proteins DNA double helix