DNA & Molecular Structure of Chromosomes- Genetics

Genetic Material must

replicate (genotypic function),

control the growth and development of the organism (phenotypic function)

allow the organism to adapt to changes in the environment

chromosomes contain

genes, proteins and nucleic acids

genetic information is encoded in

DNA

DNA is usually

- double stranded
- adenine paired with thymine
- guanine paired with cytosine

RNA is usually

single stranded, uracil instead of thymine

DNA structure

DNA is a nucleic acid
Composed of nucleotides
5-carbon sugar called deoxyribose
Phosphate group (PO4)
Attached to 5′ carbon of sugar
Nitrogenous base
Adenine, thymine, cytosine, guanine
Free hydroxyl group (—OH)
Attached at the 3′ carbon of sugar

the chain of nucleotide has a WHAT orientation

5' to 3'

nucleotide

a sugar, a nitrogenous base, and phosphate group

nucleoside

a sugar and nitrogenous base only

chargaffs rule

A=T and C=G, always equal amount of proportion between the paired nucleotides

erwin chargaff determined

chargaffs rule

Rosalind Franklin

discovered DNA 3D shape

james watson and francis crick- 1953

determined it was a double helix

supercoiling

-dna molecules in vivo are supercoiled
- occurs when DNA molecules of one or both strands are cleaved

negative supercoiling

if it rotates in opposite direction of DNA (left handed), many biological function can only be carried out if DNA is negatively supercoiled

polymerization of a DNA strand

-2 strands are polymers of nucleotides
- phosphodiester backbone- repeating sugar and phosphate units joined by phosphodiester bonds
- 2 strands wrap around 1 axid
- antiparallel- 1 strand runs 5'-3' and the second strand runs 3'-5'

phosphodiester bond

- bond between adjacent nucelotides
- formed between the phosphate group oe one nucelotide and the 3'- OH of the next nucleotide

eukaryotic chromsomes contain

huge molecules of DNA that are highly condensed during mitosis and meiosis

centromeres and telomeres of eukaryotic chromosomes have

unique structures

each chromosome contains

a single large molecule of DNA

Chromatin

complex of DNA, chromosomal proteins, other chromosomal constituents

histones

basic (positively charged at neutral pH)

non-histone chromosomal proteins

acidic (negatively charged at neutral pH)

histones also

play a major structural role in chromatin, histones form nucleosomes with DNA

nucleosomes

- packaged DNA exists in three forms
- nucleosome cores, which contain 146 nucleotide pairs, as well as linker DNA
- nucleosome core is protected by histones from endonucleases

nucleosomes play a role in transcription

- structure of nucleosomes changes between transcriptionally active and inactive regions
- tails of histone molecules protrude from the nucleosome, accessible to enzymes that add or remove methyl/acetyl groups

Human Metaphase Chromosomes

- Non-histone chromosomal proteins form a scaffold that condenses into into 30 nm chromatin fibers
- subculture of 30 nm fibers is 11nm nucleosomes interacting and condensing with each other

scaffold

is the central core of a chromosome