Sexual reproduction & meiosis
Asexual reproduction -- offspring are identical copies – “clones”
Sexual production -- produces offspring that are different
In sexual reproduction -- two parents are needed -- each parent
gives ½ the chromosomes offspring
In human father’s sperm cell -- 23 chromosomes
mother’s egg cell -- 23 chromosomes
after fertilization
total 46 chromosomes
In plants flowers or cones – accomplish sexual reprod.
pollen (♂) & ovary (♀)
Sex cells or gametes -- are special cells that contain only ½ # of
Chromosomes
Plants -- are pollen & egg / ovary
Animals – are sperm & egg cells
The process of producing these gametes or sex cells -- MEIOSIS
Occurs in organisms that sexually reproduce –
in ♀ animals -- ovaries
in ♂ animals -- testes
These gametes -- have ½ the # of chromosomes of normal cells -called haploid (n)
In human gametes -- a set of 23 chromosomes
Normal cells – full set of chromosomes -- called diploid (2n)
In human cells – 46 - 23 pairs of chromosomes
Each chromosome has a similar sized chromosome
that it pairs with – called a homologous pair
Meiosis -- produces haploid cells
Mitosis -- produces diploid cells
Steps of meiosis -- start with 1 diploid cell end with 4 haploid cells
1. prophase I
2. metaphase I
3. anaphase I
4. telophase I
5. prophase II
6. metaphase II
7. anaphase II
8. telophase II
Interphase -- normal activity -- (in this ex. cell has 4 chromosome
is diploid)
1st step - prophase I 1. chromosome coil up
2. homologous pairs of chromosomes
– pair up – swap sections of DNA –
called CROSSING OVER -produces variety
2nd step - metaphase I 1. homologous pairs line up at equator
of cell
2. chromosomes attach to spindles
in mitosis all chromosomes just line up at equator
3rd step - anaphase I 1. homologous pair split – move to
opposite poles
4th step - telophase I 1. cell split into 2 cells
2. nucleus forms
5th step - prophase II - now the 2 cells are haploid
(only 2 chromosomes in each)
1. nuclei disappear
2. centrioles to opposite sides
6th stage -- metaphase II 1. chromosomes line up at
equator
2. attach to spindles
7th stage -- anaphase II –
1. chromosomes split and
move toward opposite sides
8th stage -- telophase II -1. cells divide into 4 cells
2. nucleus appear
3. All 4 cells are genetically
different!!
after telophase II - in ♂ -- all 4 cells develop into sperm cells - quickly
in ♀ – only 1 cell develops into an egg
other 3 degenerate - process takes 15 to 45 yrs.