Lecture 2 Flashcards
(19 cards)
Genome
The total genetic content of a cell, most is in the nucleus but the mitochondria and chloroplasts all contain genetic information.
Describe bacterial and viral chromosomes
Single nucleic acid molecule
Largely devoid of associated proteins
Smaller than eukaryotic chromosomes
Describe bacterial chromosomes
Double stranded DNA compacted in a nucleioid. DNA in bacteria may be associated with HU and H1 DNA binding proteins.
What is a chromosome
A linear array of genes which are functional units of inheritance.
Genes are separated by non-coding structural chromatin.
The sizes of genes vary largely as well as the gaps of non-coding DNA between them
Mitochondrial genome
The size of the mitochondrial genome varies largely between species (50 to millions per cell).
Mitochondria grow and divide independently of the cell
Chloroplast genome
The chloroplast genome is much larger than the mitochondrial genome.
They range from 0-60 per cell.
Chloroplasts grow and divide independently of the cell.
DNA supercoiling
DNA double helix is twisted in space around its own axis, controlled by topoisomerase enzymes.
This compacts DNA.
Topoisomerases can cut one or both DNA strands to wind/unwind the helix before resealing the ends.
Chromatin
A complex of DNA and chromosomal proteins
What are the main chromosomal proteins
Histones: abundant, basic proteins with a positive charge that binds to DNA.
5 main types: H1, H2A, H2B, H3, H4.
Structural and involved in wrapping up DNA into chromosomes
Non-histones: all other proteins associated with DNA.
Involved in regulation and gene expression
Describe the packing of DNA into chromosomes
Level 1: winding of DNA around histones to create a nucleosome structure
Level 2: Nucleosome connected by strands of linker DNA
Level 3: Packing of nucleosome into a 30nm chromatin fibre
Level 4: formation of looped domains
Describe the different histone proteins
Small basic positive proteins that are highly conserved H1: rich in lysine H2A: rich in lysine H2B: rich in lysine H3: rich in arginine H4: rich in arginine
Describe chromatin
50% DNA
50% protein (50:50 of histone to non-histone)
Heterochromatin
Euchromatin
Heterochromatin
Tightly packed/condensed form of DNA= condensed and inactive
Remains highly condensed in all phases of the cell cycle
Genetically inactive
Found in eukaryotes only
Euchromatin
Uncoiled and active.
Often under active transcription.
Changes between an extended fibre and highly condensed chromatin.
Found in prokaryotes and eukaryotes.
Histone modification
Acetylation (lysine residues), methylation (lysine residues) and phosphorylation (serine residues).
Summative histone modifications
Acetylation = shift from heterochromatin to euchromatin to aid gene expression
Methylation = silence gene expression
Phosphorylation - required for meiosis/mitosis
Centromeric DNA
Centre of the chromosome, specialised sequences function with the microtubules and spindle apparatus during mitosis and meiosis
Telomeric DNA
At the ends of the chromosome and maintain stability.
Consist of tandem repeats and play a role in DNA replication
What are the variations in the organisation of DNA
Polytene chromosomes have distinct banding patterns and represent paired homologs. They are composed of many DNA strands. These chromosomes have puff regions where the DNA has uncoiled
Lamp brush chromosomes: large, have lots of DNA looping. Found in oocytes in the diplotene stage of meiosis. Involved in active transcription of genes.
