Chapter 8
Cell Reproduction
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Chromosomes are DNA that is coiled into very compact structures.
o They are rod shaped and made of DNA and protein.
o Each chromosome is a single DNA molecule wrapped around a protein called
histones.
o Chromosomes consist of 2 identical halves. Each half is called a chromatid.
o The chromatid are held together by a protein disc called a centromere.
o When cells divide each new cell gets one chromatid from each chromosome.
o Between cell divisions regions of DNA uncoil so it can be read and the
information can be used to direct cellular activities.
o The less tightly coiled regions of DNA are called chromatin.
Chromosome Numbers
o Each species of living organisms has a characteristic number of chromosomes in
each cell.
 Examples:
 Humans-46
 Gorilla-48
 Sand Dollar-52
 Dog-78
 The number of chromosomes a species has will be dictated by the
number of genes on each chromosome.
 Humans have many genes on each chromosome
Sex Chromosomes vs. Autosomes
o Sex chromosomes are chromosomes that determine the sex of the organism
 They also carry genes for other characteristics.
 Sex chromosomes are either X or Y
 Females have 2 X chromosomes (XX)
 Males have an X and a Y (XY)
o Autosomes are all the chromosomes except the sex chromosomes
 There are 44 autosomes
 Every cell of an organism produced by sexual reproduction has 2 copies
of each autosome, one from each parent.
 The 2 copies of each autosome are called homologous chromosomes or
homologues. They are the same and shape and carry genes for the same
traits.