Lecture series D7
“Multicellular biology” Part I
“Germ cells”
notes based on Alberts et al 4th ed. (2002) Chapter 20
prepared by T. J. Newman, November 20-22, 2005
revised by T. J. Newman, November 14, 2006
this document not
for public use – all images copyright Garland Science Publishing 2002
GERM CELLS AND
FERTILIZATION
·
Most higher
organisms rely on sexual reproduction in order to create new individuals
·
This is in
contrast to prokaryotes, and many simpler eukaryotes which create new
individuals by asexual reproduction
o
e.g. bacteria divide into two cells
o
Hydra
create new individuals by budding
o
many insect
species and some lizard species are exclusively female and create new
individuals by parthenogenesis
·
The sexual
reproduction cycle is simple in principle, and fairly universal (but there are
exceptions, esp. in fungi)
o
haploid generations of cells alternate with diploid generations of cells
§
two haploid cells
combine (fuse) to form a diploid cell
§
later, a
descendent of this diploid cell divides by meiosis
to create haploid cells
·
in meiosis, the
genes on homologous chromosomes
are recombined
§
these haploid
cells will join with those from another diploid cell line to form a new diploid
cell
o
in short: haploidy, fusion, diploidy, meiosis
·
One can think of
a complex multicellular organism as a device to create haploid cells and
optimize their successful fusion with other haploid cells from a different
individual of the same species
·
Haploid cells
involved in fusion are called gametes
o
typically, there
are two types:
§
a larger, less
motile cell, termed the ovum (egg)
§
smaller, highly
motile cells, termed spermatozoa
·