Reproduction!
C-11-1
There are many types of reproduction...
~ binary fission - done by bacteria
~ asexual - just split in half - amoebas
~ fragmentation - pieces split off and each
piece becomes a new organism - starfish
~ budding - whole organism grows off another
and then splits off - hydra
~ parthenogenesis - female makes a viable egg
that grows without being fertilized water fleas
~ sexual - 2 parents give genetic material to
make offspring that are genetically
different from them - most eukaryotes
* advantage - genetic diversity!
Germ cells give rise to gametes - sex cells
* when gametes from the parents combine, the
result is called a zygote
gametes = haploid = 1 of each type of chromosome
(1n)
zygote = diploid = 2 of each type of chromosome
(2n)
so for humans, in each gamete the 1n = 23
in each diploid cell the 2n = 46
* Remember that homologous chromosomes are
chrom. of the same type
ex: chrom. 1 from mom is homologous to
chrom. 1 from dad
* Also remember that autosomes are the
chromosomes that are NOT sex chromosomes!
We have already looked at making more body cells
through mitosis...
Let's look at how to make the gametes through
Meiosis!
Meiosis
C-11-2
Meiosis is cell division that makes daughter cells
with half the number of chromosomes than the
parent cell.
2 divisions take place:
1.) Meiosis I - first division
~ Prophase I
- chrom. condense
- nuclear envelope breaks down
- homologous chrom. pair up
- crossing over can occur
~ Metaphase I
- spindle fibers move homologous chrom.
to middle of cell
- homologues remain together
~ Anaphase I
- homologous chrom. move to opposite
poles
~ Telophase I
- chrom. gather at poles
- cytoplasm starts to divide
~Cytokinesis - cytoplasm divides
Now we have 2 cells with 1/2 the # of
chromosomes...
BUT we still have 2 chromatids on each
chromosome!! We now need to split these up.
2.) Meiosis II
~ Prophase II
- new spindles form
~ Metaphase II
- chrom. line up in middle
~ Anaphase II
- chrom. divide at centromeres and chrom.
move to either pole
~ Telophase II
- nuclear envelope forms around each set
of chrom.
- spindles disappear
- cell begins to divide
~ Cytokinesis - cell divides
This results in a total of 4 new haploid cells that
are genetically different from the parent cell!
When this process occurs, the homologous
chromosomes will be randomly distributed
~ called independent assortment
Also, the fertilization of gametes is random as
well!
~ there are about 64 trillion combinations of
chromosomes!!
Multicellular Life Cycles
C-11-3
Diploid life cycle:
* most animals have this
* most of life is spent in this state
* all cells except gametes are diploid
~ to make gametes, meiosis must occur...slightly
different in males and females:
Spermatogenesis
Oogenesis
Haploid life cycle:
* happens in most fungi and some protists
* most of life spent in haploid state
* only diploid structure is the zygote
* zygote goes through meiosis to make more
haploid cells
* new haploid cells go through mitosis to make
multicellular haploid individuals
Alternation of Generations:
* some org. alternate between diploid and
haploid phases
* plants:
- the multicell. diploid phase = sporophyte
- produce spores through meiosis
- spore forms a multicell. gametophyte
which is haploid
- gametophyte makes gametes which when
fertilized give rise to the sporophyte!