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What are polytene chromsosomes? Lampbrush chromsomes?
- Polytene chromsomes: paired homologues that are comprised of large numbers of identical DNA strands. They undergo replication without strand separation or cytoplasmic division. Puffs are created when the bands separate during transcription.
- Lampbrush chromosomes: meiotic chromsomes. Tend to extend (not condense) during meiosis. Loops, similar to polytene puffs, indicate DNA being used in transcription.
What is a nucleosome? Give details of the nucleosome structure
- Histones: positively charged proteins associated with DNA
- nonhistone proteins: less positively charged...
- Histones are able to bond electrostatically to the negatively charged phosphate groups of nucleotides, and occur regularly (~every 200 bp) along DNA.
- Nucleosome: DNA coils around an octamer of histones (H2A)2+(H2B)2 and (H3)2+(H4)2. This is the first level of packing, reducing DNA to ~1/3 its original length.
What are the different types of histone proteins associated with eukaryotic chromatin
- H1: further packs nucleosomes into solenoids (second level of packing, 6x DNA compaction)
- (H2A)2+(H2B)2: tetramer responsible for 1/2 of the nucleosome histone complex
- (H3)2+(H4)2: tetramer responsible for 1/2 of the nucleosome histone complex
What is the difference between heterochromatin and euchromatin?
- heterochromatin: genetically inactive due to lack of genes or repressed genes. Replicate later during the S phase than euchromatin. eg. telomere, centromere, much of Y chromosome
- euchromatin: genetically active, less dense areas of the chromosome ("normal chromatin")
What is satellite DNA?
- Differs from main-band DNA in its molecular composition.
- Consists of short repetitive sequences.
- Found in heterochromatic centromeric regions of chromsomes.
- Shows as an additional peak when analzying DNA composition.
- Not present in prokaryotes
What are the different types of repetitive DNA found in eukaryotic chromsosmes?
- Highly repetitive sequences: (Centromeric and Telomeric DNA)
- Centromeric DNA: found in the centromere
- Telomeric DNA: found in the telomere
- Middle repetitive sequences: (VNTRs and STRs) most prominent are noncoding sequences
- Variable number tandem repeats: (minisatellites) found within and between genes, variations in length originally used for DNA fingerprinting
- Short tandem repeats: (microsatellites) disperesed throughout genome, vary based on number of repeats at a given site
- Repetitive Transposed Sequences: (SINEs and LINEs) interspersed individually throughout the genome, can be short or long, are transposable
- Short interspersed elements: <500bp long, may be present 500000 times in the human genome.
- Long interspersed elements: ~6kb long, may be present 850000 times
What are transposable sequences?
- Transposable sequences are mobile sequences that have the potential to move to different locations within the genome. 1) transcription into RNA molecule
- 2) RNA serves as template for DNA complement using reverse transcriptase
- 3) integration into chromsome at new site
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