Lecture 1: DNA basics Flashcards
Nucleic acids consist of
a deoxyribose sugar with a 5’ phosphoryl end and a 3 hydroxyl end, plus a nitrogenous base
DNA structure
two polynucleotid chains, each composed from 4 possible nucleotides adenine, guanine, cytosine, and thymine
nucleotides nitrogenous base and sugar are linked through a glycosidic bond
1 turn every 10 bp
The human genome is how long?
3.2x10^9 nucleotides distributed over 22 chromosomes
22 autosomal, 2 sex chromosomes
Each human cell contains ___ copies of each chromosome: ____ and ____ homologs, except…..
2 copies of each chromosome
maternal and paternal homologs, except germ line and RBCs
Ways of identifying chromosomes in the lab: producing a banding pattern with
“whole chromosome painting”
giemsa, allowed chromosomes to be identified and numbered
still used in cytogenetics laboratories for karyotype
painting the whole chromosome a distinct color and making a karyotype
nucleosome consists of
4 histone proteins (H2A, H2B, H3, H4) + 147 nt pair DNA helix wrapped around it
“linker” DNA = 53-80 nt
nucleosomes repeat every 200 nt pairs or so
Histones: common structural design
“structural motif called histone fold”
nucleosome core particle is disc shaped with DNA wrapped 1.7 turns
N terminal amino acid tail extends out from the core and are subject to covalent modifications important for chromatin regulation
Interactions between DNA and histones….the number of hydrogen bonds formed between DNA and histones per nucleosome
142
also contains hydrophobic interactions
salt linkages
lysine and arginine (possess positive charges) compromise more than 1/5 of histone residues: effectively neutralize negative charged DNA backbones
changes to a histone would be
lethal: highly, highly conserved. only 2 differences between a pea and cow H4
variant histone proteins add to various possible chromatin structures
DNA is ________ in the nucleosome 4/s
remains so for
unwrapped
10-15 milliseconds before tightening up
Chromatin remodeling allows for what?
further loosening of the DNA/histone contact
Chromatin remodeling complex
related to helicase: bind to both histones and DNA, causing change in the conformation of the DNA, making it less tightly bound
repeated cycles catalyze nucleosome sliding , making DNA more available to other proteins in the cell
catalyzed by ATP
Nucleosome packing
- forms dense fibrous structure/ 30 nm diameter
- unknown how fiber is formed
- Zig Zag model:
stacking facilitated by histone tails, especially H4 : linker histone H1 present in 1:1 ratio with nucleosome corses
Histone H1
linker histone; largest histone; less well conversed; contacts both DNA and protein, changes the path of DNA as it exists the nucleosome
