Bell Ringer
• What is karyotyping and how is it useful?
Nondisjunction
• The homologous chromosomes, or sister chromatids do not separate
properly during cell division.
• This can happen during Meiosis 1 or Meiosis 2
Objectives
• By the end of today you should be able to
- Know the difference between somatic cells and gametes
- Identify the difference between a diploid cell and a
haploid cell.
- Compare the processes of Mitosis and Meiosis
You have two types of cells in your body
• Somatic Cells
-body cells
- Make up most of your
body tissues and organs
• Germ cells
-Cells in your reproductive
organs that develop into
gametes (sex cells)
• Every species has a characteristic number of chromosomes
per cell.
Examples: Humans= 46 chromosomes or 23 pairs
Dog= 78 chromosomes or 39 pairs
Husk Tomato= 24 chromosomes or 12 pairs
Chromosomes 1-22 make up
your autosomes ( characteristics
not related to your sex)
The 23rd pair of chromosomes
are your sex chromosomes. (XX
or XY)
Sexual Reproduction
• Involves the fusion of two gametes, resulting in offspring that are a
genetic mixture of both parents.
• This fusion is called fertilization. The nuclei of the sperm and egg
cell fuse together to form one nucleus.
• Since you are taking DNA from 2 cells and creating one you need half
the amount of DNA
Diploid and Haploid Cells
• Diploid Cells
- The cell has two copies of
each chromosome; one from the
mother , and one from the father
- Body cells
-represented as 2n
- Humans cells the diploid
number is 46
• Haploid Cells
-A cell has only one copy of
each chromosome
-Gametes (sex cells)
-Each human egg or sperm
cell has 22 autosomes and 1
sex chromosome
Organism
Human
Diploid (2n)
Haploid (n)
46
23
Chimpanzee
24
Elephant
56
Garden pea
14
Turkey
Polar Bear
Pineapple
40
74
25
Meiosis
• Germ cells in reproductive organs undergo the process of meiosis to
form gametes.
• Meiosis is a form of nuclear division that divides a diploid cell into a
haploid cell.
Objectives
- What is the difference between somatic cells and gametes
- Identify the difference between a diploid cell and a
haploid cell.
- Compare the processes of Mitosis and Meiosis
Objectives
• Illustrate the process of meiosis 1 and describe what is
happening
• Describe how crossing over can alter the composition of
chromosomes
Meiosis l
• Prophase l
-Chromatin  Chromosomes
-nuclear membrane breaks down
-Homologous chromosomes pair up
-Crossing over may occur
Meiosis l
• Metaphase l
-Homologous chromosome pairs are randomly lined up along the
middle
-This mixes up the chromosomal combinations and helps create
genetic diversity
Meiosis l continued
• Anaphase l
-Paired homologous chromosomes separate
-Sister chromatids stay together during meiosis l
Meiosis l
• Telophase l
-Nuclear membrane forms again
-Spindle fibers disassemble
- Cell undergoes cytokinesis
End Result is two cells with unique combinations of 23 chromosomes
Objectives
• Illustrate the process of meiosis 1 and describe what is
happening
• Describe how crossing over can alter the composition of
chromosomes
Objectives
• Illustrate the process of meiosis and describe what is happening
• Describe how nondisjunction can alter the number of chromosomes
in a cell
• Distinguish between mitosis and meiosis.
• Compare and contrast spermatogenesis with oogenesis.
There is no Interphase:
Chromosomes do NOT duplicate
Meiosis ll
• Prophase ll
-Nuclear membrane breaks down
Chromosomes condense
-Centrioles move to opposite sides of cell
-spindle fibers assemble
• Metaphase ll
-Spindle fibers align the 23 chromosomes in the
middle
-Each chromosome still has 2 sister chromatids
Meiosis ll
• Anaphase ll
-The sister chromatids are pulled apart
to opposite sides of the cell
• Telophase ll
-nuclear membranes reform
-spindle fibers break apart
-The cell undergoes cytokinesis
Cytokinesis
The end result is four haploid cells with a combination of chromosomes
from both the mother and the father.
Spermatogenesis
Oogenesis
Nondisjunction- The homologous chromosomes, or sister
chromatids do not separate properly during cell division.
Meiosis 1
Meiosis 2
Objectives
• Illustrate the process of meiosis and describe what is happening
• Describe how nondisjunction can alter the number of chromosomes
in a cell
• Distinguish between mitosis and meiosis.
• Compare and contrast spermatogenesis with oogenesis.