2.S: Chromosomes, Mitosis, and Meiosis (Summary)
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- Chromosomes are complex and dynamic structures consisting of DNA and proteins (chromatin).
- The degree of chromatin compaction involves proteins and varies between heterochromatic and euchromatic regions and among stages of the cell cycle.
- Chromosomes can be distinguished cytologicaly based on their length, centromere position, and banding patterns when stained dyes or labeled with sequence-specific probes.
- Homologous chromosomes contain the same series of genes along their length, but not necessarily the same alleles.Sister chromatids initially contain the same alleles.
- Chromosomes are replicated by DNA polymerases and begin at an origin. Replication is bi-directional. Eukaryotes have multiple origins along each chromosome and have telomerase to replicate the ends.
- Mitosis reduces the c-number, but not the n-number.Meiosis reduces both c and n.
- Homologous chromosomes pair (sysnapse) with each other during meiosis, but not mitosis.
- Several types of structural defects in chromosomes occur naturally, and can affect cellular function and even evolution.
- Aneuploidy results from the addition or subtraction of one or more chromosomes from a group of homologs, and is usually deleterious to the cell.
- Polyploidy is the presence of more than two complete sets of chromosomes in a genome.Even-numbered multiple sets of chromosomes can be stably inherited in some species, especially plants.
- Endopolyploidy is tissue-specific type of polyploidy observed in some species, including diploids.
- Both aneuploidy and structural defects such as duplications can affect gene balance.
- Organelles also contain chromosomes, but these are much more like prokaryotic chromosomes than the nuclear chromosomes of eukaryotes.
origin of replication
prophase (I, II)
metaphase (I, II)
anaphase (I, II)
telophase (I, II)
syanapse, pair up
salivary gland chromosome