Download Section 8.1

Section 8.1
Chromosomes
Chromosome structure
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Chromosomes are visible during cell
reproduction only
Chromatin – the same material in
between cell divisions (uncoiled)
Each chromosome is made of 2 parts:
A single DNA molecule tightly coiled
around
Proteins called histones
Each chromosome has:
Two identical halves
 Each half is called a chromatid
 Chromatids are made before cell division
 Chromatids are joined close to the center
at a spot called the centromere
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Chromosome structure
Types of chromosomes
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Sex chromosomes – determine the sex of an
organism; may also carry other information
In humans sex chromosomes are X and Y
Normal males have XY; normal females have
XX
Autosomes – all the other chromosomes
(body chromosomes)
In humans there are 44 autosomes (46 total)
Homologous chromosomes
In sexually reproducing organisms, every
cell has two copies of each autosome
 One copy comes from each parent
 These are called homologous
chromosomes or homologues
 They are the same size and shape, and
carry information about the same traits
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Karyotypes
A picture of the chromosomes taken
during cell reproduction
 All the chromosomes are arranged in
homologous pairs
 Can be used to detect genetic defects,
such as incorrect chromosome number
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Chromosome number
Diploid cells – 2 sets of chromosomes
 2 of each autosome + 2 sex chromosomes
 Represented with 2n (humans n=23; 2n=46)
 Haploid cells – 1 set of chromosomes
 Only one autosome of each homologous pair + only 1
sex chromosome
 Represented with 1n
 Egg and sperm cells are haploid  after fertilization
the new cell is diploid
 All eggs have one X chromosome; sperm have either
X or Y  sperm determine the sex of the child
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Chromosome number