DNA structure and function

Discovery of chromatic/chromosomes

Mid 1800’s → Fleming looked at plants under a microscope

• saw distinct chromosomes at stages of cell division

• Function was unclear in 1800’s

Isolating nucleic acids

1868→ Friedrich Miescher isolated nucleic acid from pus

• Contained H, O, N and phosphorus

• Later found that chromosomes contained nucleic acids and proteins

Chromosomal theory of inheritance

Late 1800’s→ Boveri and Sutton (1902)→ idea that chromosomes carry inherited info between parents and offspring

• studied nematode worms and grasshoppers

• Realised that egg cells have ½ chromosomes of an adult cell

• Said that chromosomes are linear structures with genes at specific points along them

Genetic transformation

1928→ Griffith

• streptococcus pheumoniae cause pneumonia

• Found 2 strains in mice

• Rough= Harmless to mouse, Smooth = deadly

What did Griffiths do with mice

1. Injected with rough = mice alive

2. Injected with smooth = mice dead

3. Injected with heat killed smooth = mice alive

4. Injected with heat killed S and live R = mice died, and live S recovered from mouse

Concluded that something in the heat killed S must’ve caused it.

A substance from the dead S was transferred to the live R, changing its genetic makeup.

Further strep experiments

1944→ Avery, McLeod and McCarthy

• transformation experiments

1. Degrade S proteins. Incubate with rough

2. Degrade S carbohydrates, incubate with R

3. Degrade S DNA, incubate with R

• only the DNA one didn’t transform S to R

Must mean DNA plays a role in transformation.

Bacteriophage experiments

1953→ Hershey and Chase

• Bacteriophage (only DNA and proteins)

Grew phage in:

• radioactive phosphorus (in DNA)

• Sulphur (in proteins)

Only radioactive phosphorus was passed onto offspring

Strong evidence DNA carries genetic info

First DNA structure images found

1940-48→ Wilkins

• first clear images of DNA with X-ray crystallography

• Sperm isolated from squid, showed a long thin McClure (fuzzy DNA)

Double helix structure found

1953→ Crick and Watson

• used info from Rosalind Franklin’s X-ray data

• Found DNA contains repeating phosphate and deoxyribose groups

• A&T and C&G always occurred in 1:1 ratio

• Suggested a helical/corkscrew shape

Franklin and Goslings sister paper

High resolution image of Beta-DNA (hydrated)- lots more spots

Enabled calculation of basic DNA dimensions:

• Probably helical

• Phosphate groups on the outside

• Two coxial molecules

• Estimated diameter

Consistent with Crick and Watson estimates

Forms of DNA

A = dehydrated

B = hydrated, most common

Z = transient form found in living things

Basic DNA structure

Deoxyribonucleic acid

• nucleotide = sugar, phosphate and base

• RNA = ribose sugar, DNA = deoxyribose sugar

• 5’ and 3’ end

• 5’ to 3’ = coding strand

• 3’ to 5’ = complementary

Bases

Purines = Adenine and Guanine

• 2 rings

Pyrimidines = Thymine and Cytosine (Uracil for T in RNA)

• one ring (pyramid)

A&T = two H bonds

C&G = three H bonds

Antiparallel implications

Major groove = opportunities for proteins to bind to sequence in a specific manner

Minor groove = opportunities from proteins to bind

• -ve charged phosphates associate with water and histone proteins

• Base Paris are planar with hydrophobic stacking interactions between bases

Prokaryote (bacterial)

• no nucleus- single circular DNA= nucleoid

• Small, about 5Mb

• Have plasmids (tiny DNA circles as small as 1000bp) important in antibiotic resistance

Eukaryote

• plants, animals, fungi, protists

• Genome = all DNA in nucleus and other organelles e.g. mitochondria

• Nucleus = linear chromosomes

• Mitochondria and chloroplasts = circular DNA

• Haploid human nuclear genome = 3Gb (3 billion bases)

• Human mitochondrial genome = 15Kb (Kb 1000)

Hetero/euchromatin

Heterochromatin = more compact

• Contains DNA that is not transcribed

• Peripheral hetero = around outside of nucleus

Euchromatin

• less compact DNA form

• Genes that are frequently expressed

Chromosome arrangement

→ a very large DNA-protein complex

• most human cells have 46 (22 pairs autosomal and 1 pair sex)

• XY = male

• XX = female

• Gamete have 23 chromosomes

Q arm = long, P = small

Centromere = point of constriction

Q telomere = Q end, P telomere = P end

Geisma (G) bands used to name different regions of chromosomes

DNA packing

• observed as ‘beads on a string’

• DNA. Wraps 1.65 times around the 8 core histone to form nucleosomes

• Histones are +ve charged DNA is -ve

• Core Histones = H2A, H2B, H3, H4

• H1 = brings nucleosome together to form a chromatin fibre

• Chromatin further condensed by scaffold proteins

• 10,000 fold shorter than extended length