MITOSIS and MEIOSIS (Biology 1010)

chromosome - a large quantity of double-stranded DNA complexed with nucleoproteins, 
  which goes through a cycle of coiling (condensation); occurs in the nucleus; 
  carries the genes (hereditary information); genes are located at sites on the 
  chromosomes called loci and can occur in different versions called alleles, 
  which result from mutations, which are rare mistakes in DNA replication.

chromatin - condition of the chromosomes when they are uncoiled during Interphase. 

structural gene - a portion of double-stranded DNA that carries instructions for 
  making a specific protein or polypeptide

regulatory gene - controls the expression of structural genes and/or the course of 
 									development 
centromere - constricted region on the chromosome; contains no genes, but functions 
  in movement of chromosomes during nuclear division (mitosis and meiosis); it is 
  the region that does not replicate during prophase, but later, at the beginning 
  of anaphase. 

chromatid - one unreplicated chromosome

sister chromatids - identical copies of the DNA of a particular chromosome, joined 
  at their centromeres; created by DNA replication, separated from each other 
  during mitosis (or meiosis II). 

homologs - two versions of a chromosome, which occur in non-gamete cells of 
  diploid organisms.  A pair of homologous chromosomes will carry two copies of 
  each gene, which may or may not be identical copies.  Different versions of these 
  genes are called alleles and code for proteins which are slightly different from 
  each other in primary structure.  Homologues pair (synapse) with each other 
  during Prophase of Meiosis I and may exchange pieces of their DNA (non-sister 
  chromatids) before the "reduction division" resulting in haploid daughter cells.  
  If a variant allele is transferred to a non-sister chromatid of its homologue, 
  recombination is said to have occurred (a "new combination" of alleles on that 
  chromosome).  The physical process that accomplishes this exchange during pairing 
  involves breakage and "misrepair" and is called crossing over.  Homologues do 
  not pair during mitosis. 

haploid - a nucleus contains only 1 version of each chromosome.  N = 1 chromosome 
  complement.  Gametes (sperm and egg) are haploid; many lower organisms consist 
  entirely of haploid cells. 

diploid - a nucleus containing 2 versions of each chromosome (2 alleles of each 
  gene).  2N = 2 chromosome complements.  Most higher organisms are entirely 
  diploid except for their gametes.  Zygotes (fertilized eggs) are always diploid. 

autosome - a chromosome that is not a sex chromosome

sex chromosome - in most higher organisms, one pair of chromosomes determine sex of 
  the individual (e.g. X and Y in mammals); usually, one sex (e.g. male mammals, 
  XY) is heterogametic, that is, its pair of sex chromosomes are not homologous and 
  do not pair in Meiosis; the homogametic sex (females in mammals) lacks the Y-
  chromosome genes, but has 2 "doses" of the X-linked genes.  Structural genes 
  carried on the sex chromosomes code for traits that are said to be sex-linked. 

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