meiosis I part only

1. MEIOSIS I
TAWFEEQ HAARIS BUHARI.S.A

2. Meiosis is the production of gametes, which is a reduction division
which means a diploid gamete produces haploid gametes
Meiosis reduces the number of chromosomes by half and produces
two haploid cells from a diploid cell.
Meiosis results in four cells which are genetically different from
parent cell from each other.
At end of Meiosis are 4 Haploid (n) cells
INTRODUCTION

3. INTRODUCTION
Meiosis occurs in our germ cells that produce gametes (Sperm &
egg)
A gamete is a cell that is involved in reproduction
In males it occurs in testes (Spermatogenesis) its starts at puberty
In females it occurs in ovaries (Oogenesis).

4.  It is the fundamental basis of sexual reproduction
 Two haploid (n) gametes are brought together through
fertilization to form a diploid (2n) zygote.
 If egg and sperm had the same number of chromosomes
as other body cells then the offspring would have too
many chromosomes.
 Meiosis must reduce the chromosome number by half(n)
 Fertilization then restores the 2n number
Why Do We Need Meiosis

5. Why Do We Need Meiosis

6.  MEIOSIS I
 MEIOSIS II
Meiosis Has Two Distinct Stages

7. MEIOSIS I
Meiosis I consist of 5 Phases
 Interphase I
 Prophase I
 Metaphase I
 Anaphase I
 Telophase I

8. Cell build up energy
DNA Replication to make duplicated
chromosomes
Cell doesn’t change structure
Interphase I

9. During prophase I, DNA is exchanged between homologous
chromosomes in a process called homologous recombination.
This often results in chromosomal crossover.
The paired and replicated chromosomes are called bivalents
or tetrads.
The process of pairing the homologous chromosomes is
called synopsis.
At this stage, non-sister chromatids may cross-over at points
called chiasmata
Prophase I

10. Leptotene
Zygotene
Pachytene
Diplotene
Diakinesis
Prophase I
Prophase I of Meiosis divided into five sub type

11. The first stage of prophase I is the Leptotene stage
Leptotene also known as Leptonema from Greek
words meaning thin threads
During this stage, individual chromosomes begin
to condense into long strands within the nucleus
The two sister chromatids are still so tightly bound
that they are indistinguishable from one another
Leptotene

12. The zygotene stage, also known as zygonema,from
Greek words meaning “paired threads”
Zygotene, occurs as the chromosomes
approximately line up with each other into
homologous chromosomes.
The combined homologous chromosomes are said
to be bivalent
Zygotene

13. In pachynema, the homologous chromosomes become much more closely
associated. This process is known as synapses
The synapsed homologous pair of chromosomes is called a tetrad, because
it consists of four chromatids
It can't be observed until the next stage, but the synapsed chromosomes
may undergo crossing over in pachynema
 The chromosomes continue to condense
Pachytene

14. During the diplotene stage, also known as
diplonema,from Greek words meaning “two
threads”
The homologous chromosomes separate
from one another and these chromosome are
short & thicken.
The chromosomes themselves uncoil a bit,
allowing some transcription of DNA
Diplotene

15. Chromosomes condense further during the
diakinesis stage, from Greek words meaning
"moving through."
This is the first point in meiosis where the four
parts of the tetrads are actually visible.
In this stage, the homologous chromosomes
separate further, and the chiasmata terminalize .
Making chiasmata clearly visible.
Diakinesis

16. Metaphase 1 is the second phase of Meiosis
Nuclear membrane & nucleolus have
completely disappeared.
The tetrads from prophase I line up in the
middle of the dividing cell randomly
Spindle fibers attach to the tetrads from
both ends of the cell & arranged in
equatorial region
Metaphase I

17. Anaphase I begins when the two
chromosomes of each bivalent separate and
start moving toward opposite of the poles.
In anaphase I the sister chromatids remain
attached at their centromeres and move
together toward the poles.
Nuclear membrane & Nucleolus completely
absent
Anaphase I

18. The homologous chromosome pairs reach
the poles of the cell.
The homologous chromosome pairs
complete their migration to the two poles
A nuclear envelope & membrane reforms
around each chromosome set,and the
spindle fibres disappears.
Telophase I

19. T H A N K S