Sexual Reproduction And Meiosis
Gametogenesis: the formation of gametes (eggs and sperms)
Chromosomes exist in pairs in body cells.
Each species has a characteristic number of chromosomes.
homologous chromosomes: the 2 members of a pair of chromosomes
-- contain genes for the same traits
diploid number: the number of chromosomes found in the double set of chromosomes
found in all body cells (2n)
** Eggs and sperm contain only 1/2 the diploid number of chromosomes.
haploid (monoploid) number: 1/2 the diploid number of chromosomes -- found
in the egg and sperm cells of a species
gametes: eggs or sperm
gonads: specialized organs in higher animals where the gametes are made
ovaries: female gonads
testes: male gonads
Meiosis: process which involves a 1/2 reduction division of the chromosome number
-- The chromosome numbers of a cell produced by meiosis are reduced by 1/2
-- haploid nuclei are formed which contain only one chromosome of the original
homologous pair.
** Meiosis ONLY occurs in the gamete producing cells of the gonads.
** The diploid chromosome number of a species is maintained from one
generation to the next through the processes of MEIOSIS and FERTILIZATION.
Process of Meiosis
-- formation of sex cells occurs
-- involves two divisions
-- the first is a reduction division -- the second a mitotic division
First Meiotic Division
1. Each single stranded chromosome is replicated during the non dividing resting period
-- this results in double stranded homologous chromosomes being formed.
2. Chromosomes shorten and thicken with the spindle apparatus being formed.
3. Homologous chromosomes pair up side by side in a double line at the equator. (center
of the cell) These homologous chromosome pairs are called tetrads. It is here that
synapsis and crossing over occurs.
synapsis: the meeting of two homologous chromosome pairs
tetrads: a 2 pairs of homologous chromosomes = 4
crossing over: the exchange of chromosome pieces in the tetrad during crossing over
Crossing over increases the variability of the offspring. This is why the
offspring of sexual reproduction show many variations.
** Changes in the chromosome number of an individual usually results from nondisjunction.
polyploidy: having an entire extra set of chromosomes Will result in the 3n or 4n
number of chromosomes.
4. Homologous pairs of chromosomes separate and move toward opposite poles of the
cell.
disjunction: the separation of the homologous chromosome pairs during the first division
of meiosis
nondisjunction: the failure of homologous chromosomes to separate
** Down's syndrome results from the non-disjunction of chromosome 21 -- this results in
the afflicted individual having an extra 21st chromosome. (an extra chromosome overall
as well)
** Changes in the chromosome number of an individual usually results from
Non-disjunction.
5. Cytoplasmic division of the cell occurs: ends the first stage of meiosis (reduction
division)
-- each daughter cell contains the haploid number of chromosomes.
Second Meiotic Division
-- similar to mitosis but no chromosome replication occurs
-- the diploid number of chromosomes is reduced to
the monoploid number in this division
1. Spindle apparatus is synthesized with homologous
chromosomes lining up at the equator.
2. Centromeres are replicated and each single stranded chromosome separates
and moves toward opposite poles of the cell.
3. Cytoplasmic division of the cell occurs.
Results of Meiosis
1. The diploid number (2n) of chromosomes is reduced by 1/2 to the monoploid number.
(n)
2. Specialized reproductive cells (eggs and sperms) are formed.
3. The distribution of homologous chromosomes is random -- thus variations frequently
occur.
** Variations rarely occur in MITOSIS.
** Variations often occur in MEIOSIS.
** Mitosis is associated with growth and asexual reproduction. Meiosis is associated
with sexual reproduction.
spermatogenesis -- the production of sperm (one successful meiotic division produces 4
sperm)
Sperm structure: smaller than the egg or ovum but capable of locomotion with its
flagellum
oogenesis -- the production of ova or eggs (1 egg and three polar bodies are produced in
one meiotic division
Meiosis and fertilization are necessary for sexual reproduction.
Fertilization: egg and sperm (head and mid piece) unite (tail of sperm disintegrates)
egg (n) + sperm (n) = zygote (2n)
Reproduction and Development in Vertebrates
I. Aquatic vertebrates: (fish and amphibians)
-- fertilization and development is external (in the water)
-- lay many eggs with no parental care
Most amphibians undergo metamorphosis (a change in body form during development)
II. Terrestrial vertebrates:
(a.) Reptiles: fertilization occurs internally/development occurs externally
-- lay many eggs with little parental care
(b.) Birds: internal fertilization/external development
-- lay few eggs with much parental care
(c.) Mammals: internal fertilization in all
-- internal development in placental mammals
-- external development in marsupials and monotremes
placental mammals: young develop within the mother/ have a birth or naval cord
marsupial mammals: young develop in a pouch outside the mother
ex. kangaroo, opossum
monotremes: egg laying mammals
ex. duck-billed platypus
Internal Mammalian Development
uterus (womb): organ in the female where the zygote becomes attached and the embryo
implanted
placenta: organ which temporarily attaches the embryo to the inner wall of the uterus (a
maternal and embryonic tissue)
** Gas & food exchange is accomplished between the mother and the embryo by
diffusion and active transport through the placenta. No direct blood link exists
between the mother and the fetus.
umbilical cord: attaches the embryo to the placenta -- contains 2 arteries and a vein
(embryonic tissue)
amnion -- contains fluid which protects the embryo against (mechanical or physical
injury)
mammary glands -- produce milk for nutrition in the newborn
afterbirth: discharge of the umbilical cord and the placenta after birth (1/2 to 2 hours)
Embryonic Development
cleavage: rapid cell division of the zygote after fertilization until the blastula stage is
reached -- no increase in zygote size occurs -- since mitosis forms these cells, each cell
formed will have the same genetic information
1. Fertilized ovum -- 2 -- 4 -- 8 -- 16 cell stages
2. Blastula: hollow ball surrounded by a single layer of cells
3. Gastrula: a 2 layered and then a 3 layered structure which results from indentation
(gastulation) of one side of the blastula
** Cell differentiation begins to occur after gastrulation.
*** The gastrula gives rise to the three primary germ layers or embryonic layers of the
body.
ectoderm (outer layer)
mesoderm (middle layer)
endoderm (inner layer)
After this small cluster of cells forms in humans, tissues begin to form. During the first
three months of human development, organs begin to form. All organs and body features
are developed by the end of the sixth month. During the last three months of pregnancy,
organs and features develop well enough to function after birth.
The embryo may encounter risks from faults in its genes and from its mother's exposure
to environmental factors such as inadequate diet, use of alcohol, tobacco, drugs, other
toxins, or infections.
Growth involves both an increase in cell numbers and cell size.
Patterns of development vary between different plants and different animals.
Aging: complex series of developmental changes which occur with the passage of time -caused by both heredity & environment
Death: irreversible cessation (end) of all brain functions
HUMAN REPRODUCTION
Male System
1. Testes: make and store sperm
(scrotum) -- saclike pouch which houses the testes, is 1-2 degrees celsius below normal
body temperature
2. Seminiferous tubules: carries/stores sperm from the testes
3. Epididymis: temporary storage organ for sperm at the beginning of the Vas Deferens
4. Vas Deferens: tube which carries the sperm past 3 lubricating glands
(vasectomy) -- removal or tie up of the Vas Deferens
5. Cowpers gland, Seminal vesicle, prostate gland:
-- 3 lubricating glands
-- prostate produces an alkaline fluid which neutralizes the urine in the urethra
-- prostate cancer kills many over 70 males
-- treated with radiation and estrogen
6. Urethra: opening through the penis
7. Penis: releases the semen
semen: liquid loaded with sperms
ejaculation: the release of semen ( the bombers are ready)
Female System
-- functional from puberty until menopause
1. Ovaries: 2 near the midline of the back
Fully developed by puberty -- function is to
produce eggs
2. Oviducts (Fallopian Tubes):
-- muscular contractions of the oviduct and
beating of its cilia draw ovum into the oviduct
-- Fertilization and initial cleavage occurs in the
upper oviduct.
** Usually only one egg is released at a time.
3. Uterus (womb): spongy bed in which the
zygote will become implanted and develop -gastrulation and further embryonic development
occurs here
4. Cervix: narrow neck of the uterus
** Cancer of the cervix is common -- PAP
smear detects.
5. Vagina: opening to the outside of the body
Key Reproductive Hormones
1. testosterone: produced by the testes -stimulates sperm production and secondary
sex characteristics development in the male
2. progesterone: prepares the uterus for
pregancy produced by the ovaries
3. estrogen: also produced by the ovaries
-- stimulates the development of female
secondary sex characteristics
4. FSH (follicle stimulating hormone)-produced by the pituitary
-- stimulates the development of the
follicle (pouch containing the egg in the ovary)
5.LH (luteinizing hormone)-- stimulates the
development of the corpus luteum
-- produced by the pituitary
Menstrual Cycle…from puberty to
menopause
Stages:
1. follicle stage (10-14 days)
-- production of ova occurs in tiny cavities in the
ovary called follicles
-- enlarging follicle produces estrogen which
causes the uterus to get ready
for embryo implantation (thickens its lining)
2. ovulation (1 day)
-- follicle enlarges & ruptures ovary wall
-- egg is released to the oviduct (usually only 1
is released at a time)
3. corpus luteum stage (10 -14 days)
-- yellow tissue fills the follicle after ovulation
-- called the corpus luteum "yellow body"
-- secretes progesterone which readies the uterus
for pregnancy
4. menstruation (3-5 days)
periodic shedding of the thickened lining of the
uterus which occurs if fertilization does not
occur
Menstrual Graphic with Hormonal
Interactions
gestation period: time period of pregnancy
in vitro ferilization: outside the uterus
fertilization
diploid - a cell that contains two of each type of
chromosome, such as a human somatic cell.
chromosomes - discrete structures which occur in
eukaryotic cell nuclei, contain one or two DNA
double helices (in their unreplicated and replicated
forms respectively), and are associated with protein
especially when condensed.
gametes - reproductive cells; sperm and egg cells in
animals.
Gametogenesis: the formation of gametes (eggs and
sperms)
homologous chromosomes: the 2 members of a pair
of chromosomes (contain genes for the same traits)
haploid (monoploid) number: 1/2 the diploid
number of chromosomes (found in the egg and
sperm cells of a species)
gonads: specialized organs in higher animals where
the gametes are made
Meiosis: process which involves a 1/2 reduction
division of the chromosome number