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Unformatted text preview: MCB 121 - Lecture 2 Outline: I. Chromosome structure (DNA) II. Chromatin structure (proteins) III. Heterochromatin/ Euchromatin IV. Methods of Chromosome Biology Chromosome : nucleic acids and associated proteins required to replicate and segregate genetic material Karyotype of human cell:- cell is at metaphase (most compact stage) * at interphase (the chromsomes are more diffuses)- 22 pairs of autosomes and 1 pair of sex chromsome (X = female, Y= male)- for each pair; one comes from dad, another comes from mom Homologous Chromosomes: are chromosomes in a biological cell that pair during cell division during the creation of gametes- one from mom, another from dad - contains the sequence required for proper segregation: centromeres, telomeres (maintance), origins of replications => note these are not coding region of the genome There are two copies of each chromosome = sister chromatids; each chromatid is one molecule DNA = one continuous strain without interruption along the length Cell cycle: After division, we have G1 (diploid = 2n) -> S phase (DNA replication) -> G2 (4n= 4 copies of genome) - > M phase (mitosis occurs, chromosome segregate from each other) -> create two G1 cells Conversion of diploid genome to haploid gamete: Gametes (1n, egg and sperm) fuse -> 2n zygote -> mitosis -> G1 (2n) G1 (2n) -> meiosis -> gametes (1n) When cells are not in mitosis => interphase => G1, S, G2 When cells are in mitosis => mitotic chromosomes are in most compacted phase => M Interphase of nucleus = several levels of packages First level: beads on the string = 10nm fiber DNA wraps twice around the histone proteins (nucleosome) * nucleosome is about 145bp followed by 60 bp linker * this about 10X level of compaction compared to B-form DNA Second level: solenoid (loop) = 30 nm fiber * neuclosomes pack with each other * about 6 nucleosome per loop * this is about 30X level of compaction Third level: loop/scaffold * each of the loop is 30nm fiber, each loop is about 40-80kb * this is about 1000X level of compaction * this is the level where chromosomes start to condense Forth level: coiled loops * metaphase chromosomes * 10000X level of compaction * most compact form During meiosis, which are the divisions that form haploid germ cells from a diploid parent, chromosome structure is spatially and temporally dynamic. The progression of chromosome forms through meiosis can be appreciated through this figure of meiosis in Petunia. Chromosomes are stained with a DNA intercalating dye 4',6-diamidino-2-phenylindole (DAPI) which appear blue when excited with UV light. - DAPI binds to the bases of the chromosomes - they emit light under UV light and can be observed under fluoresce microscope - Different chromosomes are individually labeled (painted with different color) using DNA probe (using nucleotide that have different fluoresce labels on it.) - Homologous chromosomes are not necessary in proximity with one another - Each chromosome seems to occupy it's own territory in the nucleus...
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