2.S: Chromosomes, Mitosis, and Meiosis (Summary)

• 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.

Key Terms

chromosome core histones nucleosome 30nm fiber histone H1 scaffold proteins heterochromatin euchromatin satellite DNA chromatid centromere metacentric acrocentric telocentric holocentric telomere homologous non-homologous chromatid sister chromatid non-sister chromatid interphase mitosis prophase metaphase anaphase telophase DNA polymerase origin of replication telomerase riboprotein Hayflick limit HeLa cells cytokinesis

meiosis gametes prophase (I, II) metaphase (I, II) anaphase (I, II) telophase (I, II) cytokinesis meiocyte bivalent syanapse, pair up synaptonemal complex reductional division equational division leptotene zygotene pachytene diplotene diakinesis crossing over chiasma (chiasmata) polar bodies G1 G2 S M G0 interphase n c replicated chromosome karyotype/karyogram autosome sex-chromosome homogametic

heterogametic aneuploidy monsomic trisomic Down syndrome deletion duplication insertion inversion translocation non-disjunction chromosome breakage polyploidy x monoploid sterile tetravalent octoploid hexaploid triploid endoreduplication endopolyploidy salivary gland chromosome polytene gene balance cellular network chloroplast mitochondria endosymbiont endosymbiont theory organellar chromosome mtDNA