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Organisation & Control of Eukaryotic Genome (1) - Coggle Diagram
Organisation & Control of Eukaryotic Genome (1)
1) structure of eukaryotic chromosome
nucleosome is the basic level of the eukaryotic chromosome organisation
bead-like structures composed of double-stranded DNA (2nm) wrapped around a core of histone proteins (=10nm)
nucleosomes are joined by linker DNA, giving rise to the beads-on-string structure
further folding & coiling of nucleosomes & linker DNA results in solenoid structure (appears as 30nm chromatin fibre)
looped domains are approximately 300nm thick, formed by folding of the 30nm fibres into loops which are attached to non-histone proteins / scaffolding proteins
700nm fibres (level of packaging at which metaphase chromosomes are organised) are formed when 300nm fibres further fold to form 1 of the 2 chromatids during metaphase
1 nucleosome = 8 histone proteins + 146 nucleotide base pairs of DNA
histone proteins are absent in prokaryotes
histone proteins contain high levels of positively charged lysine & arginine residues, which associate closely with the negatively charged sugar-phosphate backbone of DNA
2) organisation of eukaryotic genome
Gene density refers to the no. of genes present in a specific no. of base pairs
Eukaryotes have lower gene densities than prokaryotes due to large amt. of non-coding DNA present in eukaryotic genomes
coding DNA sequence
protein-coding genes
encode a.a sequences of proteins
RNA genes
encode functional RNA such as rRNA & tRNA
Ribosomes & tRNAs are needed in such large quantities that 1 copy of the gene cannot fulfil the celll's needs -> these genes are amplified to ensure adequate gene products
non-coding DNA sequence
include control elements, introns, untranslated regions (UTR), centromeric DNA & telomeric DNA
(i) control elements
non-coding DNA sequences which transcription factors can bind to & regulate gene expression
core promoter region (promoter)
includes TATA box & transcriptional start site
TATA binding protein recognises & binds to the TATA box -> recruits general transcription factors & RNA polym. to form transcription initiation complex
activity of core promoter results in basal rate of transcription
enhancer
where activators bind to
increases interaction btw promoter & RNA polym. (DNA-protein interaction) -> increasing rate of t/c
occurs by looping the DNA -> further stabalizes the t/c initiation complex through protein-protein interaction
activators have DNA binding domains
silencer
where repressors bind to
silencers are usually close to / overlapping the promoter
when repressors bind to silencers, rate of t/c is decreased
block & impede RNA polym.'s progress
recruit enzymes which increase compaction level of chromatin
recruit proteins which bind to general t/c factors & destabalize t/c initiation complex
prevent binding of activators to enhancer
repressors have DNA binding domain
(ii) centromere
structure
consists of highly repetitive (satellite) DNA sequence
constricted region of a mitotic chromosome which holds 2 sister chromatids tgt
wrapped around the centromere is kinetochore
made of proteins & is the interface btw the microtubules of spindle fibre & DNA of centromere
in higher organisms, kinetochore contains proteins & some RNA in a trilaminar structure
kinetochore allows microtubules to bind to centromere
function
centromere & kinetochore are involved in chromosomal movement during mitosis & meiosis
alignment of chromosomes during metaphase
separation of sister chromatids during anaphase
holds 2 sister chromatids tgt
(iii) telomere
telomeres are ends of a linear eukaryotic chromosome
telomeres become progressively shorter after each round of DNA replication due to end replication problem
structure
consists of multiple repetitions of 1 short nucleotide sequence
has a 3' overhang which can be degraded by 3' exonucleases
3' single-stranded overhang folds back & forms a T-loop -> hiding the ss DNA overhang
the overhang hybridises with an earlier repeat of the complementary sequence on the opposite strand
telomere capping proteins bind at the T-loop juncture to maintain the stability of this structure -> preventing degradation
function
prevent the ends of chormosomes from being degraded by exonucleases
a cell detects the open end of a DNA molecule as DNA damage & may trigger signal transduction pathways -> cell cycle arrest / cell death
protect important genes near telomeres by delaying degradation of genes
prevent ends of diff. chromosomes from accidentally fusing with each other
when a cell replicates too many times, the telomere gets too short to generate a loop / allow capping proteins to bind -> exposes the end of a ds DNA which is very unstable & may fuse, creating joined chromosomes
provide a counting mechanism for the no. of cell division a cell has undergone
as a cell replicates more times, its telomeres become shorter -> eventually trigger signal transduction pathways -> cell cycle arrest / apoptosis
prevents unlimited proliferation of cells
maintenance of telomere length
in germ cells, problem of shortening telomeres do not occur due to telomerase
telomerase is a ribonucleoprotein containing RNA & protein subunits (enzyme)
functions as a reverse transcriptase -> synthesizes a dsDNA from a ssRNA template
binds to telomere via complementary base pairing btw 3' overhang of telomere & RNA component of enzyme
3' overhang is extended using the telomerase RNA as a template -> DNA nucleotides complementary to the telomerase RNA are added
telomerase translocates towards the 3' end of the newly added sequence for further extension
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