3.2 Flashcards
Difference between chromosomes and chromatin
Chromosomes have tightly packaged DNA, found only during cell division, DNA not used for macromolecule synthesis.
Chromatin have unwound DNA, found in interphase and DNA used for macromolecule synthesis.
Difference between heterochromatin and euchromatin
Heterochromatin has condensed structure and is inactive for transcription.
Euchromatin has loose structure and is active fro transcription.
Level 1 DNA packaging
DNA double helix coils around octamer of histones in a nucleosome. Binding of H1 histone to core nucleosome forms chromatosomes.
H1 located at entry and exit point of DNA
Distance of 200 bp between each nucleosome.
Histones
Histones have 102-135 amino acids.
H3,H4,H2A and H2B from the histone octamer.
2 x H3 and H4 combine together to form a tetramer the same goes for H2A and H2B. Both tetramers join forming the nucleus of the nucleosome.
Nucleosomes
Nucleosomes attached to each other give rise to an 11nm chromatin fiber.
Level 2 dna packaging is chromatin fibres containing 6 nucleosomes per turn.
Genes that are actively transcribed are in a less condensed state.
Level 3 DNA packaging
The degree of packaging of DNA reachers 10k times.
30nm fiber forms loops linked at base of protein structure. Structure formed by condensins Sc1 and Sc2.
Chromatin loops attach to scaffold by rich AT sequences called SAR. Resulting in 300nm fiber.
Histone and DNA interaction
Interaction must be dynamic.
amino terminal end of histone can undergo modifications.
Histone acetylation is associated with transcriptional activation.
Methylation is associated with active and repressed chromatin
Chromosomes
Replicated DNA are held together by centromere.
Two chromatids make up sister chromatids.
Sister chromatids are held together by cohesins.
Centromere divides chromatids in 2 arms and ends of chromatids are called telomeres.
Chromosome Classification
Metacentric - chromosome divided in same length of arms.
Submetacentric - centromere divides into two uneven arms.
Submetacentric with satellites - uneven arms but has a satellite.
Acrocentric - centromere near one end making very small p arm.
Telocentric. - centromere at an extreme, p arm practically non existent.
Centromere Region
contains sister chromatid association sites,
binding site for the mitotic spindle. Kinetochore formed by association of proteins to centromere. microtubules bind to these proteins.
Telomere
Made up of DNA and Proteins.
Maintain structural integrity of chromosomes
position each chromosome in nucleus and insure complete DNA replication. (TTTAGGG)
Structure of a telomere
Telomeric DNA forms loop on itself to from circular structure with shelterin that protects the end of chromosomes against degradation.
Enzyme that replicates telomeric DNA is Telomerase.
Telomerase maintenance important in life expectancy and reproductive capacity of cells.
