Meiosis and Sexual Reproduction
Asexual Reproduction (review)
Single parent gives rise to new offspring by mitotic cell division.
Each new individual receives a set of chromosomes identical to the parent chromosomes.
No variation of hereditary information.
I. Sexual Reproduction
Two parents give rise to new offspring by the fusion of nuclear materials from two different cells.
Offspring are NOT identical to the parent.
Variations exist, which increases the species ability to adapt to the changing environment.
(A)
Gametes - Sex cells
1. Males – sperm
2. Females – ovum (egg)
(B)
Fertilization - The fusion of the nuclei of one sperm with one ova (egg) to produce a zygote.
(C)
Haploid - Also known as monoploid. Represented by: n
The gametes contain half the number of chromosomes. Remember, gametes are sex cells that
combine to form new offspring. Therefore gametes are haploid and once they fuse (combine),
they form a zygote that is diploid.
(D)
Diploid - Contain the full number (set) of chromosomes. Represented by: 2n
Homologous Chromosomes
Pairs of similar chromosomes
Humans have 46 chromosomes:
22 pairs of homologous chromosomes
1 pair of sex chromosomes
(II)
Meiosis - Meiosis is a type of cell division in which the daughter cell receives only half the number of
chromosomes present in the parent cell.
23 Pairs of chromosomes of a human cell
The chromosomes labeled X and Y are the sex chromosomes.
XX = female
XY = male
Two important things happen during Meiosis I that lead to much of the genetic variation we see in the offspring
of sexually reproducing organisms. They are crossing over and independent assortment. Both of these
processes result in the recombination of genes during sex cell formation.
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(III) Stages of Meiosis
A. Crossing Over
Definition : the exchange of pieces of homologous chromosomes during meiosis
The pairing of homologous chromosomes is called synapsis. It results in a group of four similar
chromatids held together.
Crossing over occurs during synapsis, when the chromosomes are in this group of 4
 Offspring will have new combinations of genes = increasing variation within the overall population
 This is good for evolution and survival of the species)
B. Independent Assortment - the random arrangement and separation of chromosomes during meiosis
increasing genetic variation in the sex cells
c. Recombination (genetic recombination) - After synapsis & crossing over, the homologous
chromosomes line up in the center of the cell.
The order in which the homologous chromosome pairs line up is random. This random ordering leads to
different combinations of chromosomes in the resulting sex cells, increasing genetic variation in the
offspring.
gene for
brown eyes
gene for
black hair
gene for
blue eyes
gene for red
hair
gene for
black hair
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Summary of Meiosis
 Meiosis begins with a diploid (2n) cell. Prior to meiosis, the chromosomes have replicated or made a
copy of themselves.
 Two cell divisions occur, resulting in four haploid (n) sex cells called gametes
 During fertilization, the haploid egg & sperm join to restore the normal number of chromosomes found
in body cells (2n).
 Crossing over (the exchange of pieces of homologous chromosomes) and recombination lead to genetic
variation - new groupings of genes in the egg and sperm. This increases genetic variation in the
offspring and increases species survival in a changing environment.
* Therefore: As a result of meiosis, diploid (2n) cells divide and form haploid (n) cells which mature into
specialized reproductive cells.
Each daughter cell contains half the number of chromosomes of the original cell.
(IV) Sexual Reproduction in Animals
(A) Reproductive System
Gonads- specialized organs that produce gametes.
a) ovaries - female gonad
- produce ova (eggs)
b) testes - male gonad
- produce sperm
(B)
Hermaphrodite - Contain both male and female reproductive structures.
(C) Gametogenesis - Process by which gametes are produced in the gonads.
Two types:
Spermatogenesis – formation of sperm
Oogenesis – formation of eggs
Comparison of Ova and Sperm
Egg cells contain stored food in the form of yolk.
Egg cells are larger than sperm.
Sperm are motile (able to move), egg cells are not.
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Fertilization – Union (joining) of a haploid sperm nucleus with a haploid egg nucleus.
This results in a diploid zygote with the full number of chromosomes.
Two types of Fertilization
External Fertilization
Eggs are fertilized outside the body of the female.
Large number of eggs are required.
Takes place in an aquatic environment.
Ex: fish and frogs
Internal Fertilization
Takes place inside the body of the female.
Less eggs are required.
Ex: mammals and birds
(V) Embryonic Development
In the early stages of development, the organism is called an embryo.
The process of embryonic development includes:
1. Cleavage
2. Gastrulation
3. Growth and Differentiation
(A) Cleavage
Cleavage is a series of mitotic divisions where the cell increases in cell number but NOT in cell size.
Cleavage converts a single fertilized egg (zygote) into many cells.
Stages of Cleavage
As cleavage continues, the cells form a hollow ball-type structure filled with fluid. At this point, the
embryo is called a blastula.
(B) Gastrulation
When the blastula reaches several hundred cells, gastrulation occurs. In this stage, the cells on one side
of the blastula push in and form a two-layered embryo called the gastrula.
Gastrula - The gastrula consists of 3 layers:
a) ectoderm - outer layer
b) mesoderm –middle layer
c) endoderm – inner layer
It is the three germ layers that give rise to all the tissues and organs of an animal.
Ectoderm
Nervous system, skin,
sweat glands, hair, nails
Mesoderm
Bones and muscles; blood and
blood vessels; reproductive and
excretory system
Endoderm
Digestive system; respiratory
system; endocrine system
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(C) Growth and Differentiation
Differentiation- series of changes that transform the unspecialized embryonic cells (stem cells) into
specialized cells, tissues and organs.
Growth- not only are the number of cells increasing, but the size of the cells are increasing as well, and
the embryo as a whole starts to develop.
(VI) Development
(A) External Development
Development occurs outside the female body
a) In water:
1. Nourishment for the developing embryo is supplied by the yolk stored in the egg.
2. No parental care
3. Examples are fish and frogs
b) On land:
1. Production of egg and shell (protection)
2. SOME parental care
3. Developing embryo’s source of food is the yolk.
4. Examples are birds and reptiles
Structure of Egg
Consists of 4 membranes outside of the embryo.
•
chorion- outer-most membrane, aids in gas exchange.
•
allantois- exchange of O2 and CO2. Also a storage site for wastes.
•
amnion- sac containing amniotic fluid which surround the embryo, provides cushion
•
yolk sac- source of food for the embryo.
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Human Development
(B) Internal Development - Development occurs inside the female.
Placental Mammals
These are animals, like humans, that contain a structure called the uterus where the embryo develops.
Within the uterus, a specialized structure called the placenta forms.
Placenta - exchange of nutrients, wastes, and respiratory gases between the embryo and the mother
takes place.
The umbilical cord, which contains blood vessels, attaches the embryo to the placenta.
Marsupials
Are mammals that do not have a placenta.
The embryo is born at a relatively premature stage, and completes its development externally in a pouch
that contain mammary glands.
Ex) kangaroos and opossum
Reproduction and Development in Humans
(VII) Male Reproductive System
Testes - Paired oval structures, located in an outpocketing of the body wall, called the scrotum.
Temperature in scrotum is 2-4 degrees lower than body temperature.
Produce sperm. Secrete testosterone
Vas Deferens - Sperm duct of humans. Carries sperm and fluid in the form of semen. Empties
into the urethra.
Semen - A mixture of sperm and fluid from seminal vesicles and prostate gland
Urethra - Carries semen from vas deferens to outside through the penis.
Also carries urine from urinary bladder.
Penis - External male genitalia.
Adaptation for internal fertilization
Release of semen during ejaculation contains about 200-350 million sperm in ~ 3.5 ml
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(VIII) Female Reproductive System
Ovaries
Paired walnut-sized structures located in the lower part of the abdomen.
They are partially surrounded by funnel-like openings of the oviduct.
The oviducts are called the fallopian tubes.
The ovaries also produce sex hormones known as estrogen and progesterone.
Produce Eggs
a) estrogen- used for the development of secondary sex characteristics like mammary glands
(breasts).
b) progesterone- maintains uterus for pregnancy.
Oviducts (Fallopian Tubes) - Fertilization occurs here
Uterus (womb) - Where embryo develops if fertilization has taken place
Cervix - Muscular ring between uterus and vagina
Vagina - Birth canal; Receives semen during mating
CONCEPTION
(IX) Fertilization and Implantation
1. Fertilization occurs in the fallopian tubules (oviduct).
2. If the egg is not fertilized within 24 hours after ovulation, it will deteriorate.
3. After fertilization, the zygote undergoes cleavage and becomes an embryo.
4. The zygote arrives in the uterus 5-10 days after fertilization.
5. The embryo implants into the uterine wall and establishes pregnancy.
6. Placenta- exchange of nutrients, gases, and wastes between mother and embryo
7. Umbilical cord- attaches the embryo to the placenta.
8. Amnion- embryo develops inside the amniotic sac which contains amniotic fluid that serves as a shock
absorber.
9. Gestation Period- length of pregnancy (approximately nine months).
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(X) Multiple Births
1. Identical twins- Develops from one zygote separating into two during cleavage. One sperm and one egg is
involved and therefore the babies will look identical and must be of same sex.
2. Fraternal twins- Develops from two eggs, each fertilized by separate sperm cells. Therefore, the babies
may not look identical and could be of different sex.
Conjoined Twins - The developing embryo begins to split into identical twins but then stops part way leaving
the partially separated egg to mature into a conjoined fetus.
(XI) Menstrual Cycle
Mature egg develops and is released.
Begins at puberty and ends at menopause.
Hormones involved:
FSH
estrogen
LH
progesterone
Stages involved in order:
follicle stage  ovulation  corpus luteum  menstruation
1. Follicle Stage
10-14 days
FSH is secreted by the pituitary gland which stimulates maturation of egg in the follicle.
As the follicle develops, it releases estrogen which stimulates the uterine lining to thicken with blood.
2.
Ovulation
Secretion of LH (lutenizing hormone) from the pituitary causes a mature egg to be released from the
follicle and into the fallopian tubules (oviduct).
3. Corpus Luteum Stage
10-12 days
The burst follicle fills with cells forming the corpus luteum, which produces progesterone.
Progesterone stimulates further thickening of the uterine lining.
4. Menstruation
3-5 days
If the egg is not fertilized, progesterone secretion decreases and the lining of the uterus breaks down.
Tissue and blood are discharged from the body through the vagina as a result (period).
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