L18: chromosomes

what does packaging of DNA into chromatin fibres allow

selective gene expression + faithful replication/transmission of genome to progeny cells

what is the karyotype

organised representation of all the chromosomes in a eukaryotic cell at metaphase

how are chromosomes organised in the nucleus

individual chromosomes occupy distinct subnuclear territories

what is a chromosome

highly coiled fibre of chromatin

what is chromatin

supercoiled array of nucleosomes

what is the diameter of a chromatin fibre

30 nm

name the major histone families and their functions

H2A, H2B, H3, H4 → core histones, form histone octamer surrounded by DNA, N terminal tails project out of nucleosome + interact with other proteins to facilitate regulation of chromatin structure + function

H1, H5 → linker histones, strap DNA onto histone octamers to limit movement of DNA + stabilise 20nm fibre formation

describe the purpose of a fractal globule

globules within globules that reversibly condense + decondense without knotting. maintain polarity + allow condensing of chromatin in interphase cells

name the specialised DNA sequences in chromosomes and describe their function

telomeres → at each end + prevent loss of DNA during replication

replication origins → hundreds, allow formation of a replication fork

centromere → keeps sister chromatids together

describe the structure of a centromere

contain alpha-satellite DNA repeats that readily form condensed chromatin with histone octamers containing unusual subunits

describe the structure + function of kinetochores

inner plate proteins binding chromatin containing alpha-satellite DNA

outer plate proteins binding protein components of mitotic spindle (e.g. microtubules)

part of mechanism for ensuring faithful segregation of sister chromatids at cell division

describe the structure of the kinetochore in yeast

basket linking a single nucleosome of centromeric chromatin to a single microtubule

inner plate binds H3

describe the composition of the human genome

half repeated sequences: majority of this transposons (LINEs, SINEs, retroviral-like elements, DNA only transposons). small amount of simple sequence repeats and rest segmental duplications

half unique sequences: only 20% genes, and only 1.5% protein coding (rest of gene for introns). rest nonrepetitive DNA that is neither introns nor codons (gene regulatory)

what is increasing biological complexity accompanied by

increasing number of protein coding genes + increasing amounts of non-protein coding DNA for regulating transcription + organising access to protein coding genes

name the 3 types of transposons

DNA transposons,

retroviral retrotransposons,

non-retroviral polyA retrotransposons

describe how DNA transposons move

cut and paste mechanism without self duplication.

transposase monomers bind to short inverted repeated sequences, synapse into a transpososome, and break the DNA. they then insert the transposon into the target chromosome at a target sequence

name 3 examples of transposons and where theyre found

P-element (fly)

Activator-Dissociator (maize)

Tn3/Tn10 (E. coli)

describe how retroviral retrotransposons move DNA

replicate via RNA intermediates to produce new DNA copies integrating at new genomic locations using self encoded reverse transcriptase

describe how non-retroviral PolyA retrotransposons move DNA

abundant in vertebrae genomes. replicate via RNA intermediate using own retrotransposon encoded reverse transcriptase. integrate products of RT directly into genome without need to package into virus-like particle

name examples of nonretroviral retrotransposons

LINEs + SINEs

Human L1 elements (LINE-1 elements)

Human Alu elements

Mouse B1 elements

what is the reason for the expansion of non-retroviral retrotransposons

genomic expansion + diversity (more raw material for evolution)

adaptation to new environments (novel phenotypes accelerating rate of adaptation + speciation)

gene regulation

chromosomal rearrangements (formation of new genes + gene families)

genomic stability (stabilise chromosome structures)