Organisation of Eukaryotic Genome Flashcards
define genomes
the genetic material of an organism or virus; the complete complement of an organism’s or virus’s genes along with its non-coding nucleic acid sequences
(DNA or RNA, contains all the genetic information needed to direct the development and maintenance of that organism for all the proteins and RNA that the organism will ever synthesise)
what does the locus of a chromosome mean?
it is the fixed position at which any specific gene is located on the chromosome to specify a particular character of the organism
what does a complete eukaryotic genome comprise of?
one complete copy of genetic information carried by a haploid set of linear chromosomes in the nucleus (nuclear genome) mitochondrial genome (a single small circular DNA molecule) chloroplast genome (in photosynthetic organisms only, a single small circular DNA molecule)
what does the human nuclear genome comprise of?
around 3 x 10^9 DNA nucleotide base pairs
over 22 different autosomes (non-X/Y chromosome) and one of the two sex chromosomes, either X for female, Y for male
*being diploid, a human somatic cell (non-reproductive) contains 46 chromosomes. for humans, haploid is 23, and diploid is 46 (two of each, one per parent)
how is genome size normally expressed, and what is its relationship to the complexity of an organism?
total number of base pairs per HAPLOID genome (gamates), in kilobases (kb) or megabases (Mb)
direct relationship between complexity of organism and genome size
- more complex organisms (like eukaryotes) tend to have larger genome sizes compared to simpler organisms (like prokaryotes)
due to: more gene products required for development and maintenance, increase in proportion of regulatory sequences (eg. alternative splicing)
what is the correlation between biological complexity and the number of genes in organisms?
there is no correlation
what is the percentage of genome containing structural genes for prokaryotes and eukaryotes?
prokaryotes: 85-90%
eukaryotes: 1-5%
what DNA sequences (types of DNA) does a eukaryotic genome comprise of?
gene and gene-related DNA sequences: exons (only coding sequences), introns, promotor, terminator
intergenic DNA: regulatory sequences (distal control elements, enhancer and silencer), repetitive DNA (tandemly repeated DNA [centromere and telomere], transposon [ignore]), other non-coding DNA (eg. spacer, ignore)
describe the first level of condensation of chromosomes
10nm nucleosomes: a molecule of DNA coiled around an octamer of histone proteins (two of each H2A, H2B, H3, H4 histones)
“beads-on-a-string” look
nucleosome core, linker DNA, associated non-histone chromosomal proteins
what are histones, and how do they form an octamer and bond with DNA?
small proteins, high concentration of positively-charged residues (eg. lysine and arginine), form ionic bonds with negatively-charged sugar-phosphate backbone of DNA
octamer forms of core upon which 146 pairs of DNA is bound
describe the second level of condensation of chromosomes
DNA on histone octamer is further folded or coiled to produce 30nm chromatin fibre, known as solenoid (6 nucleosome per turn)
- H1 histone and linker DNA involved
describe the third level of condensation of chromosomes
scaffold proteins (non-histone proteins) condense the 30nm chromatin fibre to form looped domains in mitotic and meiotic chromosomes, looped domains themselves coil and fold, further compacting chromatin to produce metaphase chromosome (chromatid, before replication, is 700nm)
what is the role of condensation
- to organise and pack giant DNA molecules of eukaryotic chromosomes into structure that facilitate their segregation onto daughter nuclei (so DNA molecules of different chromosomes will not be entangled or break during separation at anaphase)
particular genes end up located at the same places, packing steps are highly specific and precise
what does a eukaryotic protein-coding gene require?
transcription unit
- exons
- introns
regulatory sequences
- control elements (promoter-proximal and distal)
- promoter
- untranslated regions (5’ UTR and 3’ UTR)
what is the transcription unit (in prokaryotes and eukaryotes)?
exons interrupted by introns, discontinuous coding DNA sequences
exon codes for particular amino acid sequence, introns not represented
number and sizes of introns per gene varies, more intron sequences than exons
for prokaryotes, genomes arranged in continuous operons, no introns
define regulatory sequences
regions of DNA sequence where gene regulatory proteins bind to control the rate of assembly of protein complexes required for gene expression
what is a promoter?
a series of DNA sequences located upstream of transcriptional start site
RNA polymerase and transcription factors bind to promoter to initiate transcription
what are control elements?
DNA segments that regulate the initiation and rate of transcription by binding particular proteins, promoter-proximal elements and distal elements (near and far from promoter respectively)
compare promoter-proximal elements and distal elements
- similarities: proteins bind to both
- differences:
promoter-proximal initiates transcription, distal controls rate of transcription
promoter-proximal has general / basal transcription factors bound to it, distal has activators bound to enhancers (increases transcription rate) and repressors bound to silencers (decreases transcription rate)
describe the 5’ UTR’s position and function, and what it is translated to on mRNA
starts at the +1 position on DNA template strand (non-coding) where transcription begins and ends one nucleotide before the start codon
transcribed into ribosome binding site on mRNA (regulates mRNA’s stability for translation)
describe the 3’ UTR’s position and function, and what it is translated to on mRNA
starts after stop codon
transcribed into polyadenylation signal on mRNA, for transcription termination
what does repetitive DNA refer to, and what percentage of it comprises of intergenic (non-coding) DNA?
sequences present in multiple copies in the genome
59%
what is tandemly repeated DNA (satellite DNA)
mostly relatively short sequences (1-500bp), repeated many times in tandem to form a long array or cluster in a localised area of genome
preferentially located in regions of heterochromatin including centromeres, telomeres, and specific locations within arms of certain chromosomes
three types, based on length of repeating unit: regular satellite DNA (centromeres), minisatellites (telomeres), microsatellites (ignore)
compare centromeres and telomeres (location and function, form of DNA packaging, nature of DNA sequence, type of satellite DNA)
centromere joins two sister chromatids in a replicated chromosome during cell division, telomere protects the two physical ends of linear chromosome)
both are heterochromatin and consists of tandem repetitive, non-coding satellite DNA
centromere made of regular satellite DNA, telomere made of minisatellite DNA
*prokaryotic genomes are circular and do not contain telomeres or centromeres
