1. REPRODUCTION BY SEXUAL MEANS
MEIOSIS
Meiosis I
I-PMAT
Meiosis II
PMAT
REPRODUCTION BY SEXUAL MEANS

2. OBJECTIVES
Upon completion of this unit students will be able to:
1. Recognize that each species has a characteristic number of chromosomes.
2. Recognize that chromosomes exist in pairs in body cells.
3. Differentiate haploid (monoploid) and diploid cells.
4. Identify homologous chromosomes.
5. Recognize that meiosis only occurs in gonads and results in the production of gametes.
6. Define the term gametogenesis.
7. Explain HOW and WHY the chromosome number is cut in half by meiosis.
8. Describe the first meiotic division using the following terms: tetrad, synapsis, crossing over,
and disjunction.
9. Explain how nondisjunction may result in Down’s Syndrome.
10. Explain how the second meiotic division is just like mitosis WITHOUT replication.
11. Explain HOW and WHY the cells formed as the result of meiosis show variation from
each other and their parent cell.
12. Compare and contrast the processes of spermatogenesis and oogenesis.
13. Compare and contrast the structure of a sperm and egg cell.
KEY WORDS
1.           crossing over
2.           diploid
3.           disjunction
4.           egg
5.           fertilization
6.           gametes
7.           gametogenesis
8.           gonad
9.           haploid
10.     homologous chromosome
11.     meiosis
12.     oogenesis
13.     ovaries
14.     polar bodies
15.     sexual reproduction
16.     somatic cells
17.     sperm
18.     spermatogenesis
19.     synapsis
20.     testes

3. INTRODUCTION
Sexual reproduction involves parents of two sexes: male and female. It is made possible by the production of separate male and female sex cells that unite during fertilization, resulting in offspring that share characteristics of both parents.
Mitosis and meiosis take place during cell division, and in some ways these two processes are similar. Chromosomes replicate before either process begins (this is interphase). However, the results of mitosis and meiosis are very different. When mitosis is completed, the chromosome number remains the same as the original parent cells. When meiosis is completed, the chromosome number is half the original number.

4. SEXUAL VS. ASEXUAL REPRODUCTION
Involves only 1 parent
Daughter cells or offspring are genetically IDENTICAL from the original cell or parent.
SEXUAL
Involves 2 parents/cells.
Daughter cells or offspring are genetically DIFFERENT from the original cell or parent.

5. Single-stranded Single-stranded 2n Double-stranded 2n Double-stranded
MALES:SPERM
FEMALES:EGG

6. SEXUAL REPRODUCTION
A. ADVANTAGES
In sexual reproduction, there is always A FUSION OF NUCLEI FROM 2 CELLS WHICH COME FROM 2 SEPARATE PARENT ORGANISMS
The offspring produced in sexual reproduction ARE NOT IDENTICAL TO EITHER PARENT and show NEW COMBINATIONS OF CHARACTERISTICS; GENETIC VARIATION
By making a population more varied, sexual reproduction helps to ENSURE THE SURVIVAL OF A SPECIES (ADAPTATIONS)

7. B. GAMETES
GAMETOGENESIS, the production of specialized sex cells, known as GAMETES is the first stage of sexual reproduction.
The sperm and egg cells, or sex cells, are called GAMETES
In the process of sexual reproduction, the NUCLEI of the gametes join together
This fusion of the nuclei is called FERTILIZATION
The resulting cell, a fertilized egg cell, is called a ZYGOTE
Each gamete has exactly ONE-HALF the normal number of chromosomes

9. MEIOSIS
A.       THE CHROMOSOMES
Each body cell (also known as SOMATIC CELLS) of an organism contains a number of chromosomes characteristic for that species (i.e., humans have 46)
This number (46) is known as the DIPLOID CHROMOSOME; the term “diploid” literally means “the 2 number” and refers to the fact that the CHROMOSOMES IN THE NUCLEUS ARE FOUND IN PAIRS WITH SIMILAR STRUCTURE.

10. These chromosome pairs with similar structure are known as HOMOLOGOUS CHROMOSOMES and CARRY GENES FOR THE SAME TRAIT (EYE COLOR, ETC.)
The symbol “2n” is used to represent the diploid chromosome number
In gametogenesis, cells located in the sex organs (GONADS) undergo a special type of
division (MEIOSIS) that
results in the formation of
sperm cells in males, or
egg cells in females.
In this process, the
number of chromosomes
is reduced by HALF   
A karyotype is prepared by arranging pictures of matching chromosomes by size and shape.

11. MEIOSIS is a special type of cell division that results in DAUGHTER CELLS WITH ONE-HALF THE CHROMOSOME NUMBER OF THE “PARENT” CELL
This reduced number of chromosomes is known as the HAPLOID NUMBER and is represented by the symbol “n”

12. B.STAGES OF MEIOSIS
In meiosis, HOMOLOGOUS CHROMOSOME PAIRS SEPARATE AND MOVE TO OPPOSITE POLES OF THE CELL BEFORE CELL DIVISION and, because the number of chromosomes is “reduced” in meiosis, this type of cell division is frequently referred to as REDUCTION DIVISION
Meiotic cell division occurs in two distinct separate divisions: THE FIRST MEIOTIC DIVISION AND THE SECOND MEIOTIC DIVISION
Before the actual phases of cell division occur, REPLICATION of the cell’s single-stranded chromosomes, forming double-stranded chromosomes, occurs. This results in CHROMATIDS THAT ARE GENETIC DUPLICATES OF THE ORIGINAL CHROMOSOME STRANDS (remember this also happens with mitosis-----it’s interphase)

13. I. The First Meiotic Cell Division (I-PMAT)
The phases of the first meiotic division, in order, are: PROPHASE I, METAPHASE I, ANAPHASE I, TELOPHASE I
1.       PROPHASE I
Each chromosome lines up with its HOMOLOGOUS CHROMOSOME, becoming attached at the CENTROMERE; this is called SYNAPSIS
This forms a group of four CHROMATIDS which are called TETRADS

14. When the strands of the tetrads twist around each other (which SOMETIMES happens), they may exchange genetic information with each other. This is called CROSSING-OVER (this is one source of genetic variation!)
As in prophase of mitosis, the nuclear membrane DISAPPEAR and spindle fibers FORM
The homologous chromosomes begin to move towards the EQUATOR

15. http://www. uic. edu/classes/bios/bios100/lecturesf04am/metaphase1m

16.  2. METAPHASE I
The centromeres of the chromosome pairs (TETRADS) LINE UP ON THE EQUATOR
The chromosomes are attached to the SPINDLE FIBERS at their CENTROMERE
In metaphase I, we get one of each HOMOLOGOUS CHROMSOME from each parent: THE MATERNAL CHROMSOME FROM MOM AND THE PATERNAL CHROMSOME FROM DAD
There is no rule as to how the homologous pairs line up on the equator. Each time meiosis occurs, the lining up can be different. This is known as INDEPENDENT ASSORTMENT:

17. 3. ANAPHASE I
Homologous chromosomes of each tetrad separate from each other and MOVE TO OPPOSITE POLES OF THE CELL
This separation of homologous pairs into two groups is called DISJUNCTION
The result of disjunction is THE FORMATION OF TWO SETS OF DOUBLE-STRANDED CHROMOSOMES; the chromosomes are still double stranded, but now they are HAPLOID

18.  4. TELOPHASE I
The cytoplasm divides which separates the SETS OF DOUBLE STRANDED CHROMOSOMES INTO EACH OF THE TWO NEW CELLS
These new cells contain the HAPLOID NUMBER OF CHROMOSOMES
Nuclear membranes may form, but usually cells continue straight to the second mitotic cell division.

19.  II. The Second Meiotic Cell Division (PMAT)
The phases of the second meiotic division, in order, are: PROPHASE II, METAPHASE II, ANAPHASE II, TELOPHASE II
Meiosis II also passes through four stages.
In meiosis II, the chromosome number is NOT BEING REDUCED but rather the CHROMATIDS from each double-stranded chromosome are BEING SEPARATED
Meiosis II in humans begins with two cells that have 23 double-stranded chromosomes each. It ends with 4 cells, each with 23 single-stranded chromosomes.
1.       PROPHASE II
Each of the daughter cells forms a spindle, and the double-stranded chromosomes move toward the middle of the spindle.

20.  2.     METAPHASE II
The chromosomes become attached to the spindle fibers at the CENTROMERES and the chromosomes line up on the EQUATOR
 3.     ANAPHASE II
The centromeres divide and the CHROMATIDS separate
The chromatids are now SINGLE-STRANDED and move towards the opposite poles
4.      TELOPHASE II
Both daughter cells divide to form 4 haploid cells
In each cell, chromosomes return to their INTERPHASE state and the nuclear membrane reforms

22. SEXUAL REPRODUCTION IN SIMPLE ORGANISMS
A.       CONJUGATION AND MATING TYPES
In simple organisms, sexual reproduction has the effect of RESTORING THE ORGANISM’S ABILITY TO GROW AND REPRODUCE. IT ALSO PERMITS A RECOMBINATION OF HEREDITARY MATERIAL TO INTRODUCE GENETIC VARIATION WITHIN THE SPECIES
Although no male or female can be DISTINGUISHED in simple organisms, there are two different MATING TYPES
These mating types are designated as PLUS OR MINUS
CONJUGATION is the type of sexual reproduction most commonly found among simple organisms (i.e., spirogyra and paramecium)
In conjugation, A CYTOPLASMIC BRIDGE FORMS BETWEEN TWO CELLS AND NUCLEAR INFORMATION IS TRANSFERRED
Conjugation occurs between TWO CELLS OF DIFFERENT MATING TYPES

23. SEXUAL REPRODUCTION IN ANIMALS
A.       REPRODUCTIVE SYSTEMS
The gametes of animals develop in specialized organs called GONADS
The gonads of males are TESTES and the gonads of females are OVARIES
B.       HERMAPHRODITES
In most animal species, sperm cells and egg cells are produced in separate (male or female) animals. In a few species, however, known as HERMAPHRODITES, both male and female gonads are found in the same organism.
Hermaphroditism is normally found among SLOW-MOVING OR SESSILE ANIMALS such as EARTHWORMS, SNAILS, AND HYDRA

24. A.       GAMETOGENESIS
The diploid (2n) cells in which the meiotic process begins are known as PRIMARY SEX CELLS. The haploid (n) cells that result from the meiotic process are the GAMETES.
SPERMATOGENESIS is carried out in the testis and produces a SPERM CELL (4)
Spermatogenesis normally results in 4 haploid nuclei which eventually mature to form MATURE SPERM CELLS
Spermatocytes, in males, do not undergo meiosis until the male reaches sexual maturity. However, the immature sperms cells (called spermatogonia, which eventually become spermatocytes) divide mitotically throughout the male life. Hence, the number of sperm is not limited, but the number of eggs is.

26. OOGENESIS is carried out in the ovaries and produces an OVUM (EGG CELL)
Oogenesis normally results in 4 haploid nuclei, but only 1 of these has the potential to mature into a functional egg cell. The remaining cells are known as POLAR BODIES
Females are born with the amount of eggs they will possess for life These oocytes do not undergo meiosis until the female reaches sexual maturity.

27. HAS YOLK TO NOURISH EMBRYO NON-MOTILE MADE OF HEAD, NECK AND TAIL
A.       EGG VS. SPERM
Eggs are ROUND AND NONMOTILE and stored in the form of YOLK (nourishment)
Eggs are usually LARGER than the sperm of the same species
A sperm is made up of A HEAD, MIDDLE PIECE, AND TAIL (FLAGELLUM)
Sperm are MOTILE
EGG
SPERM
CONTAIN 23 CHROMOSOMES
HAS YOLK TO NOURISH EMBRYO
NON-MOTILE
MADE OF HEAD, NECK AND TAIL
MOTILE

28. A.       FERTILIZATION
Fertilization is the union (fusion) of two nuclei.
Fertilization restores the species NUMBER OF CHROMOSOMES

29. I. INTERNAL FERTILIZATION Fertilization within the body of a FEMALE
It is characteristic of animals that ARE TERRESTRIAL (LAND), BUT SOME AQUATIC ANIMALS ALSO
Internal fertilization requires a SPECIALIZED SEX ORGAN TO TRANSFER SPERM FROM THE MALE TO THE FEMALE
ADVANTAGES: NO SCATTERING OF GAMETES, NO HAZARDS OF THE OUTSIDE ENVIRONMENT, FEWER EGGS ARE NEEDED BECAUSE THEY ARE WELL PROTECTED, CHANCES OF FERTILIZATION ARE GREATER
DISADVANTAGES: FERTILIZATION CAN ONLY OCCUR WITHIN CERTAIN TIME PERIODS
Occurs in LAND (TERRESTRIAL) ANIMALS AND SHARKS, LOBSTERS
·       

30. II. EXTERNAL FERTILIZATION
·      Fertilization of the egg OUTSIDE OF THE BODY OF THE FEMALE
·      Limited to AQUATIC ANIMALS
·      Fertilization occurs directly in the WATER AFTER EACH PARENT RELEASES ITS GAMETES
·      Environmental factors: PREDATORS, TEMPERATURES, OXYGEN LEVEL
·      Occurs in FISH, AMPHIBIANS AND OTHER AQUATIC ANIMALS

31. MITOSIS VS. MEIOSIS Mitosis Meiosis Before the process begins:
Mitosis
Meiosis
Before the process begins:
Associated with:
Parent cells
Daughter cells
INTERPHASE
ASEXUAL
REPRODUCTION
DIPLOID
IDENTICAL TO PARENT CELL
TWO DAUGHTER CELLS
(Before Meiosis I only)
SEXUAL REPRODUCTION
HAPLOID
SPECIALIZED FOR REPRODUCTION
FOUR DAUGHTER CELLS

32. In conclusion, meiosis has two all-important functions
In conclusion, meiosis has two all-important functions. First, meiosis maintains the normal species chromosome number by preparing haploid gametes. The two haploid gametes are able to fuse during sexual reproduction to make a diploid zygote, with the characteristic species chromosome number.
Second, meiosis increases genetic variability by recombining genes in egg and sperm. Because of meiosis, sexual reproduction results in offspring that are different from each other and from their parents. A greater variety of characteristics in offspring increases the chances that some individuals will be better suited than others to survive in a particular place and time.

33. MEIOSIS and FERTILIZATION maintain the DIPLOID number of a species from generation to generation