THE PROCESS OF MEIOSIS
OBJECTIVES
1. What is the product of meiosis? (MCA-II standard)
2. What happens during each step of meiosis?
INTRODUCTION
1. Meiosis is a process that reduces the number of chromosomes in new cells to 1/2 the number.
2. The halving of the chromosome number counteracts the division of cells in reproduction.
3. Meiosis produces gametes (sex cells) (germ cells), which are haploid reproductive cells.
√ A haploid (1N) cell has only one set of chromosomes.
4. Human gametes are sperm cells and egg cells.
5. Human sperm and egg cells each have 23 chromosomes.
6. The fusion of a human sperm cell and an egg cell results in a zygote that has 46 chromosomes.
7. The resultant zygote is said to be a diploid cell.
√ A diploid (2N) cell has two sets of chromosomes.
8. Cells preparing for meiosis first enter interphase, the phase meaning “between cell divisions.”
9. Interphase is subdivided into 3 phases: G1, S, and G2.
10. Replication of DNA occurs during which of the interphase subdivisions? S phase
MEIOSIS
chromosome
chromosome
chromosome
chromosome
cell membrane
nuclear envelope
centriole
centromere
aster
spindle
1.
The nuclear envelope
PROPHASE I
is starting to disappear .
1.
How many chromosomes do you see?
4
2.
Each chromosome has 1 or 2 chromatids? 2
3.
Each chromosome lines up with its matching
chromosome, called a homologue.
4.
Because there are two sets of chromosomes
present, the cell is said to be diploid (2N).
5.
Sometimes chromatids can break off and attach
to adjacent chromosomes – a process called
crossing - over.
METAPHASE I
1.
How many chromosomes do you see?
4
2.
Each chromosome has 1 or 2 chromatids? 2
3.
The chromosomes line up very similar to
of mitosis but with one key difference:
line up as homologous pairs
4.
Spindle fibers have formed in
between the centrioles and the chromosomes.
5.
The newly formed spindle fibers latch onto
ANAPHASE I
the centromeres of the chromosomes.
1.
How many chromosomes do you see?
4
2.
Each chromosome has 1 or 2 chromatids? 2
3.
The chromosomes move very similar to
mitosis but with one key difference:
separating homologous pairs rather than sister
chromatids
TELOPHASE I
1.
What structure is re-forming around the
chromosomes?
2.
Nuclear membrane
How many chromosomes do you see in each
re-forming nucleus? 2
3.
Each chromosome has 1 or 2 chromatids? 2
4.
Because there is now only one set of chromosomes
in each nearly-formed cell, the cells are said to
PROPHASE II
be haploid (1N).
1.
At this point meiosis has created 2 cells.
2.
Each cell has 2chromosomes.
3.
Each chromosome has 1 or 2 chromatids? 2
3.
Each chromosome has 2 chromatids because
DNA was copied in the S phase of interphase.
4.
Note the re-appearance of spindle fibers
between the centrioles.
METAPHASE II
1.
Each cell has 2 chromosomes.
2.
Each chromosome has 2 chromatids.
3.
Note that the spindle fibers have reached
and attached to the centromeres.
4.
Note that metaphase II is very similar to
metaphase I of meiosis with one difference:
ANAPHASE II
chromosomes now line up individually rather than as homologous pairs
1.
What is pulling the chromosomes apart?
Spindle fibers
2.
Each cell now has 4 chromosomes.
3.
Each chromosome has 1 chromatids.
4.
Note that anaphase II is very similar to
anaphase I of meiosis with one difference:
sister chromatids are pulling apart rather than homologous pairs
1.
What structure is re-forming around the
chromosomes?
TELOPHASE II
2.
Nuclear membrane
How many chromosomes do you see in each
reforming nucleus?
2
3.
Each chromosome has 2 chromatids.
4.
List one similarity to telophase I of meiosis.
Chromosomes have separtated
5.
State how telophase II differs from telophase I
in two different ways:
a.
2 chromosomes in each cell rather than 4
b.
4 cells formed rather than just 2
KEY CONCEPTS
1. Meiosis produces reproductive cells called gametes (sperm and egg).
2. Gametes are specialized cells that have 1/2 the number of chromosomes of the parent cell.
3. Gametes have only one set of chromosomes or are said to be haploid (1N) cells.
4. Gametes are genetically unique because of the prophase I process called crossing over.