BIOLOGY
1111
CHAPTER 10
MEIOSIS
ASEXUAL
REPRODUCTION
one
parent alone passes on a duplicate of all its genes to each new individual
genes-
specific sections of a chromosome (DNA)
produces
clones
genetically
identical copies of the parent
ex:
orchids, flatworms, aphids
SEXUAL
REPRODUCTION
both
parents (each with pairs of genes) pass on one of each gene to their offspring
thus
the first cell of a new individual also ends up with pairs of genes
pairs
of genes may not be identical
molecular
structure of genes change (mutation)
ALLELES
pairs
of genes determine individual traits
each
molecular form of the same gene is called an allele
one
or both alleles determine the traits or characteristics of an individual
ex:
1 allele may code for blonde and another for red hair
VARIATION
all
members of sexually reproducing species do not “look” alike
through
sexual reproduction, offspring end up with new combinations of alleles, and
these lead to variations in their physical and behavioral traits
the
resulting variation among offspring may be acted on by agents of natural
selection
GAMETES
meiosis
divides the chromosomes in a nucleus twice before cell division
variation in traits is a foundation for evolutionary change
meiosis
is the first step leading to gamete formation
gametes-
sex cells
ex:
sperm, ova, egg, pollen
gametes
arise from germ cells found in reproductive organs
germ
cells have the same chromosome number as the rest of the body cells
the
normal number of chromosomes [diploid (2N)]
in which their are two of each type of chromosome
if
two chromosomes have the same length, shape,
genes dealing with the same traits and they line up with each other at
meiosis they are homologous
HOMOLOGOUS
CHROMOSOMES
humans
------ 23+23 in germ cells
after
meiosis gametes end up with half the number of chromosomes [haploid (N)]
human
sperm and egg each have only 23 chromosomes (one of each type)
meiosis
reduces the parental chromosome number by half to the haploid number
KEY
EVENTS OF MEIOSIS
two
key events of meiosis which contribute to new combinations of alleles and thus
variations in traits are:
1)
crossing over
2)
chromosome
alignment
CROSSING
OVER
occurs
in the first stages of meiosis as chromosomes begin to condense
corresponding
segments of homologous chromosomes are exchanged
genes
for the same characteristics have slightly different forms (alleles)
each
crossover is a chance to swap different alleles
crossing
over leads to genetic recombinations which in turn lead to variation
CHROMOSOME ALIGNMENT
major
shuffling of the entire chromosome
alignment
of homologous chromosome at the equator is random
human
cell contains 23 chromosomes inherited from the mother (maternal) and23
chromosomes inherited from the father (paternal)
3
chromosome results in 8 possible combinations 23 = 8
random
migration of maternal and paternal chromosomes to opposite poles insures a
unique gene combination possibility for each gamete
humans
have 23 pairs of chromosomes
223
= 8,388,608 possible combinations
MAGNITUDE
OF POSSIBILITIES
magnitude
of possible chromosome combinations in the formation of gametes and the gene
shuffling occuring during crossing- over gives rise to an unlimited number of
possible gene make-ups for gametes
GAMETE
FORMATION
1)
spermatogenesis
formation
of male gametes
2)
oogenesis
formation
of female gametes
SPERMATOGENESIS
1)
diploid germ cell enlarges as an immature primary spermatocyte
2)
primary spermatocyte undergoes meiosis and divides into 4 haploid daughter cells
called spermatids
3)
spermatids mature into a flagellated, motile sperm
cells
OOGENESIS
1)
diploid germ cell develops into immature oocyte
2)
oocyte undergoes meiosis and divides into 4 unequally sized cells
cytoplasm
is conserved in one single egg
3
remaining non-viable cells are called polar
bodies
polar
bodies have the correct number of chromosomes but lack enough nutrients and
metabolic machinery to be successfully fertilized
FERTILIZATION
the
parental chromosome number is restored at fertilization
nuclei
of the two haploid gametes fuse
meiosis
preceeding the union insures a correct number of chromosomes in each new
generation
fertilization
contributes to variation in offspring
ex:
humans
1)
2-3 crossovers for each chromosome
2)
random positioning of pairs of parental chomosomes results in 223 =
8,338,608 possibilities
3)
possible number of male and female gametes that are produced and the resulting
chance that any two will actually fuse is staggering
MEIOSIS
AND MITOSIS COMPARED
Mitosis
1)
single celled organisms use mitosis in asexual reproduction
2)
multicellular oganisms use mitosis during growth and tissue repair
Meiosis
1)
basis of gamete formation and sexual reproduction
promotes
variations in traits among offspring
END