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Chapter 8
Cell Reproduction
8-1
Chromosomes
DNA
• Long, thin molecule that stores genetic
information
• Instructions for Life
Chromosomes
• DNA coiled up
• This happens during
cell division
Histones
• Proteins that the DNA wraps around to
make chromosomes
Chromatid
• Each half of a
chromosome
• “sister chromatids” =
chromosome
Centromere
• Holds chromatids together until they
separate during cell division
Chromatin
• Less tightly coiled DNA/protein
• Happens when the cell is NOT dividing so
the information can be used
Chromosome Numbers
• Each species has its
own number of
chromosomes in each
cell
• Ex: Fruit Flies = 8
Carrot = 18
Cat = 32
Human = 46
Two Types of Chromosomes
• 1. Sex chromosomes
• 2. autosomes
Sex Chromosomes
• Chromosomes that
determine if an animal
is a male or a female
• Females = X and X
• Males = X and Y
Autosome
• All other chromosomes that do not
determine the sex of an animal
• Each cell has two copies of each
autosome
• Example: DNA for eye color
Homologous Chromosomes
• The two copies of
each autosome
• Same size and shape
• Carry genes for the
same traits (for
example blue eye
color)
Karyotype
• A picture of
homologous
chromosomes
Diploid
• Cells with two sets of
chromosomes
Haploid
• One set of
chromosomes
• Ex: human sperm and
egg cells
Chapter 8.3
Meiosis
Types of Reproduction
• Asexual reproduction
– Produce offspring with only ONE parent
– “babies” are identical to parent
• Sexual reproduction
– Produce offspring by meiosis and the joining
of the sperm and egg
– “babies” are NOT identical to parents
• Meiosis
– New cells have half the number of
chromosomes
• Gametes
– Reproductive cells (sperm and egg)
Key Points
• Meiosis must happen so when the sperm
and egg join to create a new organism, it
has the correct number of chromosomes.
• Examples:
– Human = 46
– Egg = 23
– Sperm = 23
– Egg + Sperm = baby
– 23 + 23 = 46
Meiosis I. (very similar to
mitosis)
• Prophase I.
– DNA coils into chromosomes
– Each chromosome lines up with its
homologue (the similar chromosomes)
– Synapsis: pairing of homologous
chromosomes
– Each pair is called a tetrad
– Crossing-over: may occur where pieces of
one chromosome break off and attach to the
other chromosome  genetic recombination
• Metaphase I.
– Tetrads line up along the middle of the cell
• Anaphase I.
– Each homologous chromosome of each
tetrad moves to opposite ends of the cell
• Telophase I.
– Cell divides creating 2 new cells each with a
haploid number of chromosomes (1/2 the
original number of chromosomes, but 2
copies of each)
Meiosis II. (DNA does NOT get
copied first)
• Prophase II.
– Chromosomes begin to move
• Metaphase II.
– Chromosomes move to the cell’s midline
• Anaphase II.
– Chromatids separate and move to opposite
ends of the cell
• Telophase II.
– Four new cells form with ½ the original
number of chromosomes
• **Meiosis occurs ONLY IN
REPRODUCTIVE ORGANS.
• See Figure 8-12 page 155
• Spermatogenesis:
– Production of sperm cells
• Oogenesis:
– Production of egg cells