Meiosis & Sex at the Cellular Level

I.  Asexual vs. Sexual Reproduction
     In animals, sexual reproduction involves the production of sex cells (gametes) during meiosis followed by their subsequent fusion (fertilization) to yield a zygote.  This process, meiosis followed by fertilization (= sex), allows for genetic recombination, so that offspring are genetically different than their parents.

    Asexual reproduction occurs when an organism makes a copy of itself without undergoing meiosis and fertilization.  The prefix �a� means �without�, hence, this literally translates into reproduction without sex.  Asexual reproduction is well-developed in plants.  For example, Spider plants produce "babies", strawberries send out runners, the Mother-of-thousands plant has plantlets on the leaf margins and the Piggyback plant has plantlets at the base of the leaf by the end of the petioles.  Some animals are also capable of asexual reproduction (i.e., starfish, hydra).  Asexually produced offspring are "carbon-copies", i.e. are genetically identical to the parents. 

II.  Typical Sexual Life Cycle - See diagram in class.  Some take home lessons:

A.  Part of the sexual life cycle is haploid, the other portion is diploid.  Some definitions:

1. Haploid - cell/organism that contains a single set of chromosomes; 

2. Diploid - cell/organism that contains two sets of chromosomes.  One set comes from mom and the other from dad.  Haploid structures are symbolized "n", diploid with "2n";

3. Homologous chromosomes - matching chromosomes in a diploid cell.  For every chromosome contributed by one parent, there is a matching chromosome contributed by the other.  These chromosomes are the same size, shape, and carry the same genes (genetic instructions) though the actual expression of the genes may vary.  To summarize, in diploid organisms, chromosomes come in pairs.

4. Example:  A human sperm or egg, which is haploid, contains 23 chromosomes (n=23).  After fertilization, the diploid zygote, and subsequent cells, have 46 chromosomes (2n=46).  For every chromosome in one of dad's sperm, there is a matching (homologous) chromosome in mom's egg.

B.  Gametes, the sex cells, are haploid.  Body cells, somatic cells, are diploid. 

C.  The zygote results from fertilization (fusion of the sperm and egg). 

D.  Note that there are various modifications of the sexual life cycle - not all organisms follow the same pattern as, say, the mammal pattern that is described above.  For example, in plants and yeast meiosis yields haploid spores which grow into separate structures that produce gametes mitotically.

E.  Meiosis reduces the chromosome in number in half.  In other words, a diploid cell produces haploid daughter cells after meiosis.

III.  Meiosis
     In the previous lecture we described one type of nuclear division, mitosis.  During mitosis, the nucleus divides resulting in two daughter cells each with the same chromosome number as the parent.  If a haploid cell divides mitotically, it results in haploid daughters.  If a diploid cell divides mitotically, diploid daughters result.  If a parental cell has 1000 chromosomes, or even just 1 chromosome, the daughter cells have 1000 and 1 chromosomes, respectively, after mitosis.

    Meiosis is the second type of nuclear division, which, as we said above, results in each daughter having half the number of chromosomes as the parent.  Thus if the parental cell has 46 chromosomes, each daughter has 23.  Meiosis is usually restricted in occurrence, typically to sex cells.  Its purpose is to reduce chromosome number in half for completion of the sexual life cycle.

    The basic mechanics of the meiotic division process is similar in many ways to mitosis.  The main differences between meiosis and mitosis are summarized below:

1. Homologous chromosomes pair during meiosis (not during mitosis).  This is the most important distinction because it is ultimately responsible for the other differences.

2. There are two sets of divisions in meiosis, called meiosis I and meiosis II (there is only one set of divisions during mitosis).

3. Crossing over occurs during meiosis.  Chiasmata are the observable result of this process (not during mitosis).

4. Four daughter cells result from meiosis (only two during mitosis)

5. Meiosis is restricted in occurrence; usually just sex cells.  Mitosis occurs in most cells (somatic cells), at least those capable of division (such as meristems in plants).

6. The ultimate function of meiosis is for sexual reproduction.  The function of mitosis is growth and repair.

7. Meiosis results in haploid daughter cells.  Mitosis results in daughters with the same chromosome number as the parental cells.

IV.  Mechanics of meiosis
     During interphase, the DNA replicates, just as in mitosis.  Then, at the beginning of prophase of the first meiotic division, called prophase I, the chromosomes condense and become visible.  This phase is very similar to prophase in mitosis, EXCEPT, and it's a BIG EXCEPTION, the homologous chromosomes pair up during prophase I of meiosis.  The paired chromosomes, called a tetrad, align along the cell equator during metaphase I. The homologous pairs are then separated during anaphase I, which is followed by telophase I.  Note that the function of this first meiotic division is to separate the homologous chromosomes.  The daughter cells are now haploid but each chromosome is comprised of two chromatids.

    In the second set of meiotic divisions, the chromatids are separated, resulting in four cells, each with half of the original chromosome number.  See the diagram in your text for details. 

IV.  Some Laws
    Gregor Mendel, working with peas first formulated some ideas concerning how genes work.  Although he didn't use the same terms we will, he understood the basic processes:

• Law of Segregation.  During meiosis, one of each pair of homologous chromosomes is distributed to each daughter cell.  Thus, each daughter cell gets a set of chromosomes.  This happens during meiosis I and from that point on, all cells are haploid.

• Law of Independent Assortment.  Each pair of homologous chromosomes separates into daughter cells during meiosis I independently of any other chromosomes.   Or in other words, the homologous chromosomes move into daughter cells randomly or independently of one another.

• Example.  Consider a diploid cell with 4 chromosomes (2n=4), or two homologous pairs.  We can symbolize the chromosomes as A, A', B, B'.  Thus, the letter A represents one pair of homologous chromosomes and the letter B the other pair.  The Law of Segregation states that you must have one of each pair in daughter cells.  Thus, after meiosis each daughter cell must have one A chromosome and one B chromosome.  According to the Law of Independent Assortment, it doesn�t matter which chromosomes migrate into each daughter.  Thus the following possible combinations of chromosomes are possible: AB, AB', A'B and A'B'.  The following are not possible combinations (at least not normally:  AA, AA', A'A', BB, B'B', BB'.

V.   Crossing Over
    During synapsis of the homologous chromosomes, pieces of chromosomes can exchange or swap sections with one another.  This allows for exchange of genetic information.  The ultimate result is to increase genetic variability.  This phenomonon is common.  Approximately 2-3 crossover occur per chromosome pair in humans.

VI.  Spermatogenesis (will not be discussed)
    This process occurs in the testis.  It begins at puberty and continues unabated throughout a males lifetime.  The process is summarized as follows:

spermatogonium (2n, in testis) → mitotic division to produce primary spermatocyte (2n; the function of this division is to increase sperm numbers) → divide by meiosis I to produce secondary spermatocytes (n; takes about 16 days → meiosis II (takes about 16 days) → spermatids (n, no tails, etc.) → maturation (about 16 days) → sperm

VII.  Oogenesis  (will not be discussed)
    egg production.  Occurs in ovary.  This process is summarized as follows: 

Oogonium (2n, in ovary, prenatal) → mitosis → primary oocyte (2n, approximately 400,000 in an ovary, all are formed by 3 months prior to birth, they are suspended at prophase I) → meiosis I (first occurs at puberty, then monthly thereafter) → secondary oocyte (n, released from ovary at ovulation) and a polar body (note, the egg gets all the "good stuff", this is an unequal cytoplasmic division) → meiosis II (only occurs after fertilization) → egg and polar body.     
 

VIII.  Sex determination

A.  Autosomes vs. sex chromosomes
     Sex chromosomes are those that carry important information for determining sex.  There are two sex chromosomes in mammals, X and Y.  The X chromosome, which is larger than the Y, has a centrally-positioned centromere.  All other chromosomes are termed autosomes.  Thus humans have 44 autosomes and 2 sex chromosomes.  Or, 22 homologous pairs of autosomes and 1 pair of sex chromosomes.  The sex chromosomes are homologous to one another. 

    Normal females have two X chromosomes (symbolized XX), normal males have an X and Y chromosome (XY).  Nettie Sloan first discovered this is 1900.  Note that other species have different systems for determining sex.

B.  Predicting offspring
    Thus, after meiosis, each egg will have one X chromosome, whereas, 50% of sperms will carry the X chromosome and 50% the Y chromosome (see diagram).  When sperm and egg recombine, there is a 50% chance of having a female, and 50% chance of having a male.  See punnett square.  Thus, there is 1:1 ratio of male to female offspring or 1 in 2 (1/2 or 50%) chance of having either a boy or a girl.  In practice slightly more males are conceived, possibly because the Y-chromosome carrying sperm swims faster.

    It is said that the male determines the sex of the child.  This is true in the sense that sperms come in two "flavors", X and Y, whereas eggs in only one, X.  Thus, depending upon which sperm fertilizes the egg, that will determine the sex of the offspring.

IX.  Nondisjunction
     Nondisjunction is the failure of chromosomes to properly segregate during meiosis (or mitosis)  It is responsible for unusual combinations of chromosomes.  A cell/individual with odd numbers of chromosomes is termed aneuploid.  In other words, it is a violation of the Law of Segregation.  Nondisjunction can occur at meiosis I or II.  (see diagrams in class/text).  Common aneuploids include: 

1. Trisomy 21 or Down�s syndrome, a condition in which an individual has an extra chromosome (for a total of 47).  This occurs in about 1 in 1,000 individuals.

2. XXX - Triple X syndrome, female (47 chromosomes total), occurs in about 1 in 1,000 individuals, these individuals are healthy and can�t be distinguished from XX females.

3. XXY - Kleinfelter�s syndrome, male (47), characterized by low fertility, small testicles, breast enlargement and feminine features, but otherwise normal, occurs in about 1 in 2,000;

4. XYY - Jacob�s syndrome, male (47), 1 in 1,000, these individuals tend to be taller than average, but otherwise have no distinguishable features;

5. XO - Turner's syndrome, female (45), occurs in about 1 in 5,000, normal intelligence, sterile.

    Cells can tolerate imbalances of sex chromosomes better than imbalances of autosomes.  With the exception of trisomy 21 and a couple of others (e.g., Edward's syndrome -trisomy 18 or Patau's -trisomy 13), there are few live births of autosomal aneuploids and those that do survive are severely afflicted.

    WHY? 

1. Extra X chromosomes are inactivated so they don't "mess things up".   Mary Lyon first observed that in normal females only one X chromosome remains active, any others are inactivated during development (after about 10 embryonic divisions).  The inactivated X chromosome forms a small dark staining spot in nucleus, called a Barr body.  This forms the basis for the sex test.  Thus, only one remains active, the others inactivate as Barr bodies.  Barr body number:  XX - 1 Barr body; XXX - 2 barr bodies; XXY - 1 Barr body; XYY - no Barr body; XY - no Barr body.  (Note:  the chromosome that is inactivated is random); and
    

2. Y chromosomes are small and carry comparatively little genetic information that will "mess things up".   Trisomy 21, which is probably the most common autosomal anueploid, is a trisomy of an equally tiny chromosome (number 21). 

 X.  Karyotype
    A karyotype is used to determine the chromosome composition of an individual.   White blood cells are typically used (red cells lack a nucleus), they are treated with colchicine which prevents the formation of microtubules.  Thus, cell division stops at metaphase, allowing a good view of the spread chromosomes.  The chromosomes are stained, photographed and arranged from large to small, matching the homologous chromosomes (see karyotype info). 

    Some parents want to know if the offspring they are carrying is "normal."  Amniocentesis (removal of amniotic fluid containing fetal cells is obtained) or chorionic villi biopsy (remove some of fringe around embryo) permit sampling of fetal tissue to determine karyotype and other genetic conditions.

XI.  Why Sex? 
    We started this lecture talking about asexual and sexual reproduction.  One question, for which a definitive answer is still missing, is why do organisms bother with sexual reproduction, especially when you consider that asexual reproduction seems to suit many organisms just fine and that sexual reproduction is riskier.  One answer is that sexual reproduction provides a mechanism to allow for genetic variability, which is the raw material of evolutionary change.  We have seen that genetic variability is obtained:  (1) as a result of independent assortment of homologous chromosomes during meiosis I;  (2) crossing over during meiosis I;  and (3) random fertilization.

Last updated: July 14, 2009     � Copyright by SG Saupe