DNA structure

Properties of genetic material:

  • Contains information

  • Replicate accurately

  • Capable of change (mutation)

 

Chromosomal theory of inheritance - discovery of chromatin and chromosomes:

  • Mid 1800s - Flemming chromatin found in nucleus - using microscopes

  • Distinct chromosomes seen during cell division, only when the divide can you see them

  • Function was unclear in 1800s

 

  • 1868 - Friedrich Miescher isolated 'nuclein' (nucleic acid) from puss

  • Nucleic acids contained - hydrogen, oxygen, nitrogen and phosphorus

  • Later chromosomes/chromatin found to contain proteins and nucleic acids

 

Chromosomal theory of inheritance - Boveri:

Chromosomes responsible for inheritance - eggs had half no. of chromosomes
chromosomes are linear structures with genes located at specific sites along them

  • Consistent with the ideas of Mendel

 

Griffith:
Bacteria due to streptococcus pneumoniae

Found two stains

  • Rough (IIR) - mouse lives (harmless)

IIR + heat killed strain > bacteria dies

  • Smooth - mouse dies (virulent) - bacteria lives

Can mutate into each strain - spontaneously

Something in dead IIIS had transformed IIR into IIIS

 

Avery, MacLeod and McCarty (1944)

First idea that DNA was important - used enzymes

If DNA was degraded = no transformation - therefore showing that DNA is needed to change bacteria

People didn’t believe as everyone believed that proteins controlled everything and didn’t trust their enzymes

 

Hershey and Chase (1953):

Bacteriophage - contains only DNA and protein (attacks bacteria)

Grew phage in

  • Radioactive phosphorus (DNA - phosphate group)

  • Sulphur (proteins - disulphide bridges)

  • Then infected bacteria

Radioactive phosphorus, but not sulphur found progeny phages = DNA important

 

Identification of the structure of DNA:

Wilkins (1940)- showed a long, thin DNA molecule using x-ray crystallographic equipment

Wat + Crick (1953)- contained repeating phosphate and deoxyribose sugar groups,  A and T + G and C appeared in ratios of 1:1, helical shape (phosphates on outside) - corkscrew shape

 

Franklin and Goslings sister paper:

 

Properties of genetic material:

Contains info

Replicate accurately

Capable of change

 

Structure of nucleic acids:

  • DNA (deoxyribonucleic acid) - a stable double-stranded polymer of nucleotides

  • Each nucleotide composed of deoxyribose sugar, phosphate, bases (A,T,G,C)

  • Sugar phosphate backbone - 5' and 3' ends

  • RNA - ribose sugar, typically single-stranded, phosphate, bases (A,U,G,C)

 

Structure of bases:

  • Sequence of different bases that identifies nucleic acid and function

  • Purines (2 rings) - adenine and guanine

  • Pyrimidines (1 ring) - thymine and cytosine

 

Complementary base paring:

  • Large base (purine) pairs with small base (pyrimidine)

  • Adenine + thymine (2 H bonds)

  • Guanine + cytosine (3 H bonds)

  • Many H bonds along helix

  • Two strands of DNA can be denatured (separated) by heat

 

Implications of structure:

  • Base pairs are planar (flat) with hydrophobic stacking interactions between adjacent bases

  • Phosphate groups negatively charged

  • Two strands run antiparallel

 

Writing DNA sequences:

  • Sequence of bases

  • 5' to 3'

  • Only bases - if we know coding strand then we know template strand

  • Easily stored on computer

 

Structure provides mechanism for heredity:

  • Genetic material must:

  • Be stable over time

  • Complementary strands suggest method of replication

  • Suggests how change can occur

 

Eukaryote genomes:

  • Plant, animal, fungi, protist

  • Genome = all DNA in nucleus of cell

  • Haploid human nuclear genome - 3 Gb (gigi bases)

  • Human mitochondrial genome - 15kb (kilo bases)

  • Kilo base = 1000 bp

  • Mega base = 1 million bp

  • Giga = 3 million bp

 

When cell is not dividing you cannot see chromosomes (not condensed)

Heterochromatin (more compact) - form of DNA:

  • Peripheral heterochromatin around outside of nucleus

  • Contains DNA that is not transcribed

 

Euchromatin:

Less compact DNA form contains genes that are frequently expressed

 

Nucleolus - where RNA is transcribed

 

Identification of structure of DNA:

Karyotype - complete set of chromosomes

Autosomes (22 pairs )and sex chromosomes (1 pair)

 

Features:

  • Centromeres (point of restriction) at different places in different chromosomes

  • P arm (petite) - short arm

  • Q arm - long arm

  • Telomeres - structures at ends of chromosomes

  • G-bands (giemsa banding) - visible in each chromosome: used to name chromosome regions

 

G-bands are used to name different regions of the chromosome

They are numbered in order going away from centromere

 

Packing chromosomes:

Chromosome is about 4.80 cm long with 240 million nucleotides in each strand

 

DNA packaging - chromatin:

Nucleosomes observed with electron microscope

Observed as beads on a string

 

DNA warps around histones to form nucleosomes - linker DNA between

Histones positively charged - made out of 5 different proteins

 

H1 (histone protein) - brings nucleosomes together to form chromatin fibre (coiled nucleosomes)

 

Scaffold proteins:

Chromatin further condensed by scaffold proteins

Scaffold protein are not histone proteins

 

Plants:

Nucleus - linear chromosomes

Chloroplast and mitochondria have circular DNA

 

Prokaryotes:

E.g. bacteria

  • No nucleus

  •  chromosomes - single circular DNA

  • Smaller - 5Mb

  • Packaged by super coiling (dense clump called nucleoid)

  • Plasmids - important in antibiotic resistance