Vocabulary:

1

• Vocabulary
• Homologous
• Diploid
• Haploid
• Meiosis I and II
• Tetrad
• Crossing-over
• Gametes

Key Concepts What happens during the process of
meiosis? How is meiosis different than mitosis?
10-2 Meiosis
2
Genes, Chromosomes, and Numbers

• ? Organisms have tens of thousands of genes that
  determine individual traits
• ? Where are genes??...
• ? lined up on chromosomes!
• ? a typical chromosome can contain a thousand or
  more genes

3
(No Transcript)
4
(No Transcript)
5
Chromosomes and Genes

• ? in the body cells of animals and most plants,
  chromosomes occur in pairs
• ? one chromosome in each pair came from the
  female parent, and the other came from the male
  parent

6
VOCABULARY

• ? Homologous chromosomes paired chromosomes 1
  came from mom, and the corresponding chromosome
  came from dad
• ? Diploid has 2 sets of chromosomes (2n) all
  body cells are diploid
• ? Haploid one set of chromosomes (n)
• -in the gametes (sex cells)
• -when egg joins with sperm, the diploid of
  chromosomes is restored!

7

• ? MEIOSIS is a specialized type of cell division
  that occurs in the formation of gametes such as
  egg and sperm.
• ? Meiosis appears much more complicated than
  mitosis
• -it is really just two divisions in sequence
• -each one of which has strong similarities to
  mitosis.

Meiosis
8
(No Transcript)
9
INTERPHASE

• ? Interphase prior to meiosis is
• identical to interphase prior to
• mitosis
• ? This includes the copying of
• DNA in the S phase!
• ? Meiotic division will only occur in cells
  associated with male or female sex organs.

10
MEIOSIS I

• ? Meiosis I, the first of the two divisions, is
  often called reduction division
• ? it is here that the chromosome number is
  reduced from 2n (diploid) to n (haploid).
• Ex In humans, the
• diploid number 46
• (haploid 23)

11
PROPHASE I

• ? Prophase I of meiosis is similar to prophase in
  mitosis
• ? chromosomes condense and become visible
• ? spindle fibers develop
• ? nuclear envelope breaks down

12
Prophase I - Important Events

• ? Homologous chromosomes pair up to form a TETRAD
• ? The sister chromatids press together at points
  along their length
• ? It is during this alignment that chromatid arms
  may overlap and temporarily fuse and exchange
  segments resulting in crossing over
• ? Result chomosomes (consisting of 2 sister
  chromatids) are no longer just from mom or
  dadthe chromosomes are a mixture!!!

13
More VOCABULARY

• Tetradeach chromosome pairs with homologous
  chromosome
• 4 sister chromatids!
• Crossing over when chromosomes are in tetrads,
  they may exchange parts
• This happens in Prophase I
• Result increased variety in offspring (a good
  thing)

14
(No Transcript)
15
Result INCREASED genetic variety!!!
16
METAPHASE I

• Here is where the critical difference occurs
  between Metaphase I in meiosis and metaphase in
  mitosis.

17
METAPHASE I

• In MITOSIS, all the chromosomes line up in single
  file on the metaphase plate in no particular
  order.
• In Metaphase I (of Meiosis), the chromosome PAIRS
  are aligned on either side of the metaphase
  plate.

18
ANAPHASE I

• ? During Anaphase I the spindle fibers shorten
• -the homologous pairs
• are pulled away from
• each other toward
• each pole of the cell.

19
TELOPHASE I

• ? spindle fibers break down
• ? the nuclear membrane (envelope) may or may not
• reform, and the chromosomes do not
• disappear.
• ? At the end of Telophase I
• ? each daughter cell has a single set of
• chromosomes
• ? half the total number in the original
• cell where the chromosomes were
• present in pairs.
• ? chromosomes are still in doubled
• state (1 chromosome 2 sister
• chromatids)

20

• ? While the original cell was diploid (2n), the
  daughter cells are now haploid (1n).
• ? This is why Meiosis I is often called reduction
  division.
• Example if the
• -Diploid 4, then the
• -Haploid 2

It was 4, now it will be 2 in each cell!!
21
MEIOSIS II

• ? Meiosis II is quite simple in that it is simply
  a mitotic division of each of the haploid cells
  produced in Meiosis I.
• ? There is no interphase between Meiosis I and
  Meiosis II
• ? Meiosis II begins with

Can You Guess?
22
(No Transcript)
23
PROPHASE II

• ? A new set of spindle fibers forms
• ? the chromosomes begin to move toward the center
  of the cell
• ? no longer with their homologous partner!

24
METAPHASE II

• ? The chromosomes in each haploid cell align in
  the center of the cell this time in single file!!

25
ANAPHASE II

• ? The centromeres split
• ? The spindle fibers shorten
• -pulls the single chromosomes toward each pole
  of the cell.

26
TELOPHASE II

• ? the nuclear envelope reforms
• ? the chromosomes begin to uncoil, converting
  back to chromatin
• ? total of four daughter cells, each with half
  the total number of chromosomes as the original
  cell.
• ? followed by CYTOKINESIS

27
(No Transcript)
28
NOVA video

• http//www.pbs.org/wgbh/nova/baby/divi_flash.html

29

• ? MALE GAMETES
• -all four haploid
• cells will eventually
• develop into mature,
• functional sperm
• cells.

30

• ? FEMALE GAMETES
• (as seen in life cycles in
• higher organisms)
• -the cytoplasm and cellular
• organelles are divided
• unequally
• -three of the cells will
• typically abort
• -leaves a single cell to
• develop into a mature egg cell
• usually much larger than a typical sperm cell.

31
Gametes
32
(No Transcript)