NAME: ________________________ ID# ____________________

Labs 8/9

Chromosomes: Mitosis and MeiosisHuman Genetics

Introduction: Cells come into existence through the division of their parent cells, and most cells divide in turn to produce daughter cells. Division of the nucleus usually occurs by mitosis, in which the genetic material is duplicated so exactly one copy is passed on to each daughter cell. Mitosis is generally followed by cytokinesis, or cytoplasmic division, in which the rest of the cell divides in half to form two new cells. Mitosis is the type of cell division carried out by the cells of a developing embryo and by ordinary body or somatic cells.Mitosis is part of the cell cycle and is the only period during which the chromosomes are visible in the light microscope. The rest of the cycle is known as interphase. Trace one turn of the cycle in the figure below.

During G1 (first gap) of interphase, the decision to begin the process of cell division is made. DNA replication occurs in the S (synthesis of DNA) period. At this time, each chromosome doubles so that it consists of two identical sister chromatids joined in the region of their centromeres. Although the chromosomes are uncoiled and therefore invisible in the light microscope at this time, chemical techniques can be used to show that the chromosomes are replicating. The replicated chromosomes that appear at the beginning of mitosis are double-stranded; one strand will enter each of the daughter cells. The nuclei of the two daughter cells produced by mitosis are exactly equivalent to the parent cell nucleus and can be thought of as carbon copies. During the G2 (second gap) period, preparations for division are completed and the cell enters the brief M (mitosis) period when discrete chromosomes can be observed to actually divide.

1

In animals and plants that reproduce sexually, two sex cells or gametes from the two parents fuse during fertilization to form the zygote. The zygote always has twice as much genetic material as the gamete. Therefore, at some stage before the next fertilization, the genetic material must be reduced by half so that the amount of nuclear material stays the same from one generation to the next. The reduction in nuclear material is the process of meiosis. Only specialized cells, germ cells, carry out meiosis. These cells occur in the gonads of animals and in the sex organs of plants and fungi. The gametes or spores resulting from meiosis contain only one-half of the genetic material of their parent cells. A second important consequence of meiosis is that the genetic material in the gametes or spores has been shuffled, resulting the genetic recombination. The gametes, and their offspring, are genetically unique and genetically different from the parental cells that produced them.This heritable variation among offspring is the major advantage of sexual reproduction. It is very important as one source of the variation on which natural selection acts during the course of evolution. Heritable variation allows the better adapted organisms in a population to tend to survive when conditions change and to pass on their better adapted heritable traits to their offspring.Many organisms can carry out asexual reproduction as well as sexual reproduction. Asexual reproduction always involves mitosis rather than meiosis and the offspring are genetically identical to the single parent. There is little variation among the genetically identical offspring, so this type of reproduction is suited to situations in which the organisms are well adapted to a relatively constant environment.In this exercise, you will study both mitosis and meiosis. You must thoroughly understand both processes and the differences between them before undertaking the topic of genetics.Finally, there are two important terms associated with the genetic material that you need to understand from the outset:Haploid: a haploid cell contains one unique set of genetic information. It includes one chromosome of each type found in a particular organism, and it has the same set of chromosomes that would be found in a gamete or spore from that organism. The symbol for the haploid number for each species is n. In humans, n=23.Diploid: a diploid cell contains two sets of genetic information. It includes a pair (two) of chromosomes of each type found in a particular organism, and it has the same set of chromosomes that would be found in the ordinary body cells of an animal or plant. The diploid number is always 2n. In humans, 2n=46.Most organisms have either haploid or diploid cells, although other levels of ploidy (3n, 4n, etc) are possible.With these two terms, we can restate the concepts of mitosis and meiosis. In mitosis, a cell produces two daughters identical to itself in ploidy; typically, a diploid cell produces two genetically identical diploid daughters. In meiosis, a diploid cell is converted to haploid cells, which are genetically different from itself and from each other.

Mitosis in an Animal CellAn animal cell about to undergo mitosis has already replicated its DNA during the S period of interphase.How many strands does each chromosome have? __________

2

What are the strands called? _________The centrioles, pairs of organelles, found only in animal cells, have also replicated. The daughter centrioles move to the opposite ends or poles of the cell and their position indicate how the chromosomes will move during mitosis. The chromosomes always move toward the centrioles, and the cell always divides in a plane perpendicular to a line joining them.Centrioles have an interesting structure closely related to that of cilia and flagella, which they help organize. They are small cylinders made of nine triplets of microtubules, and each centriole is made up of two such cylinders at right angles. During mitosis they orient themselves at the poles of the mitotic spindle. The spindle is also made of microtubules, some extending from a centriole to the equator. More microtubules called astral rays extend out from the centrioles, as they are surrounded by dense pentricentriolar material.The centrioles, mitotic spindle, and astral rays are collectively called the mitotic apparatus; they take up much of the interior of the cell when it is in mitosis. Mitosis requires a great deal of energy, so there are many mitochondria associated with the mitotic apparatus to provide energy for synthesis and chromosome movement.

Interphase (G1 – S – G2)During interphase, as genetic material is in the form of greatly extended fibers of DNA that form a tangled mass called chromatin. The fibers are too small to be seen in the microscope so the nucleus looks homogenous. The nucleus is bounded by the double nuclear membrane and contains nucleoli, which are centres for the synthesis of RNA. They may be visible in every cell.How many nucleoli do you see in the interphase cells? __________Prophase (M)As the cell enters prophase, the chromatin starts to condense into discrete chromosomes, the nucleoli disappear, and the nuclear membrane breaks down. By the end of the prophase, the centrioles have migrated to opposite poles, the chromosomes are moving to the centre of the cell, and the mitotic spindle starts to appear.Metaphase (M)The spindle forms, and the chromosomes take up positions on the metaphase plate, an imaginary plane in the middle of the cell halfway between the two poles. They are located on the outside of the spindle, so they would be arranged in a circle if you viewed them from one of the poles. Each double-stranded chromosome is attached to a spindle microtubule at its centromeres, and its flexible arms are free to move about.Anaphase (M)Metaphase ends and anaphase begins when the centromeres of each chromosome separates. Each sister chromatid now has its own centromere and is called a chromosome, so that there are twice as many chromosomes as there were during metaphase. If a certain cell has three pairs of chromosomes (n=3) how many chromosomes does the cell have during metaphase?

3

How many chromosomes does it have during anaphase? _______________Spindle microtubules, attached to the centromeres, cause chromosomes to move, with the arms of the chromosomes trailing passively behind. A chromosome without a centromere will not move during anaphase and is randomly left behind in one of the two daughter cells.Telophase (M)The chromosomes gather at the poles in telophase and begin to form the daughter nuclei. Since each original chromosome in the cell contributed a chromosome identical to itself to each nucleus, the two daughter nuclei will be identical to the nucleus of the parent cell. The nuclear membrane begins to reform, the nucleoli reappear, and the chromosomes begin to unwind and disappear from view. At the end of telophase, the nuclei resemble interphase nuclei, and in fact have entered interphase of the next cell cycle.Cytokinesis (M-G1)Cyytokinesis, or division of the cytoplasm, usually accompanies telophase. In animal cells, the membrane constricts in a ring around the middle of the cell, and eventually pinches the cell in two. In embryos, this ring is called the cleavage furrow. The cytoplasm and all its constituents are passively divided so that each cell gets about half the materials and organelles in the cell.

EXERCISE 1:View prepared slides of animal cells undergoing mitosis, such as the cells in whitefish embryos. Using the above descriptions as a guide, complete the diagram below to show interphase, the stages of mitosis, and Cytokinesis.

4

Meiosis in a Plant CellMeiosis in plants results in formation of spores, which eventually give rise to gametes. For the sake of brevity, we will only examine meiosis in the male reproductive structure of flowering plants.

1. Interphase: During interphase, the nucleus of each diploid microsporocyte is distinct, containing granular-appearing chromatin. The cells are compactly arranged.

2. Early prophase 1: Now the chromatin has begun to condense into discrete chromosomes, which have the appearance of fine threads within the nucleus.

3. Mid-prophase 1: Additional condensation of the chromosomes has taken place. Pairing of homologous chromosomes is taking place.

4. Late prophase 1: The chromosomes have condensed into short, rather fat structures. Synapsis and crossing over are taking place. Note that the nuclear envelope has disorganised.

5. Metaphase 1: The homologous chromosomes lie in the region of the spindle equator. The spindle, composed of spindle fibres, can be discerned as fine lines running toward the poles. (Note the absence of centrioles in plant cells.)

6. Early anaphase 1: Separation of homologous chromosomes is beginning to take place.7. Later anaphase 1: Homologous chromosomes have nearly reached the opposite poles.

Reduction division has occurred.8. Telophase 1: The homologous chromosomes have aggregated at opposite poles. The spindle

remains visible.9. Cytokinesis 1: The cell plate is forming in the midplane of the cell. Spindle fibres, which are

aggregations of microtubules, are visible running perpendicularly through the cell plate. The 5

microtubules are directing the movement of Golgi vesicles, which contain the materials that form the cell plate. A nuclear envelope has re-formed about the chromosomes, resulting in a well defined nucleus in each daughter cell.

10. Interkinesis: In these plant cells, a short stage exists between meiosis 1 and 2. Distinct nuclei are apparent in the two daughter cells. A cell wall has formed across the entirety of the midplane.

11. Prophase 2: The chromosomes in each nucleus of the two daughter cells condense again into distinct, thread-like bodies. As was the case at the end of prophase 1, the nuclear envelope disorganises.

12. Metaphase 2: Chromosomes consisting of sister chromatids line up on the spindle equator in both cells.

13. Anaphase 2: The sister chromatids (now more appropriately considered unduplicated chromosomes) are being drawn to their respective poles in each cell. Before anaphase two begins, sister chromatids are attached to each other along their length. Shortening of the spindle fibres, which are attached to the chromatids at kinetochores within their centromeres, causes the chromatids to separate, beginning in the region of the centromere. This causes a v-shaped configuration of the chromosomes.

14. Telophase 2 and Cytokinesis: Nuclear envelopes are now reforming around each of the four sets of chromosomes. Cell plate formation is occurring perpendicular to the cell wall that was formed after telophase 1. After cell wall formation is complete, the four haploid cells (microspores) will separate. Subsequently, each will develop into a pollen grain inside which sperm cells will form.

EXERCISE 2:Using the above description as a guide cut out the photomicrographs given in the appendix and arrange them on the figure below to depict the meiotic events leading to microspore formation.

6

7

Heritability of Human FingerprintsDermatoglyphics is the study of the patterns formed by the epidermal ridges of the skin of the palms, fingers, soles, and toes. These ridge patterns are laid down during the first trimester of human development. Fingerprint formation is determined by polygenetic inheritance, for example, a number of genes act additively. A small amount of environmental influence on these patterns also occurs during early development. This must be true because identical twins, although they share the same set of genes and occupy the same uterine environment during development, have slightly different fingerprints due to slight difference in their environments. Besides being extremely important for identifying persons in forensic investigations, dermatoglyphics can be used to help diagnose a number of genetically determined conditions, such as Down syndrome.In this study, you will learn how to determine the Total Ridge Count (TRC) for yourself and family members. The data for the class as a whole can be used to determine the extent to which this trait is genetically determined, or its heritability (H). The overall variability in class TRC values is known as the phenotypic variance (Vp ). The variation in phenotype due to different genotypes is called the genetic variation (VG) while that due to environmental differences is called the environmental variance (V E). The heritability of the trait is that proportion of a trait that is caused by genetic variance.If the heritability of a trait is 100% (H=1.0), the environment has no effect and all the observed variation is genetic. On the other hand, if the heritability is zero (H=0.0), then all the observed variation is due to the environment. For many qualitative traits, it is difficult to assess heritability because genes and

8

environment interact. Fingerprint TRC, however, shows negligible gene-environment interaction so it is a good trait to study.

Classification of FingerprintsThe three types of fingerprints are arches, loops, and whorls. The print may have a triradius, which is a point at which three groups of ridges from three different directions intersect (see below).

EXERCISE 3:

Count the triradii in the arch in the figure below:

Count the triradii in the loop:

Count the triradii in the whorl:

An arch in which ridges come to a point is a tented arch. Arches, which have no triradii, are the simplest and least frequent fingerprint pattern. A loop has a single triradius and a core that looks like a ridge that has turned back on itself. The whorl pattern has two or more triradii, it may be a double loop or, commonly, a circular type f pattern. Since the triradii are important in analyzing the fingerprints, you must include all the triradii present in each print that you take.

Taking your Fingerprints: Use a #2 pencil to shade in the 4x4 cm square at the end of this topic, or use an ink pad, as

directed by your laboratory instructor.

9

Rub one of your fingers on the square, pressing firmly and making sure you have covered all the triradii in the print.

Carefully roll your blackened finger over a piece of wide scotch tape, so that the tape comes in contact with the entire print. Make a certain you include any triradii on the outer edges of your finger.

Peel away the tape and tape the print to the corresponding box below. Repeat this process to make a complete set of your fingerprints.

R-1 R-2 R-3 R-4 R-5

L-1 L-2 L-3 L-4 L-5

Determining Total Ridge Count (TRC) Examine each print carefully, using a hand lens. Classify each print as arch, loop, or whorl type and record print type in the table below. Determine the ridge count for each print by counting the number of ridges from the center of

the pattern to the triradius if present (see figure below) and record the results below. If no triradius is present, the ridge count is zero for that print. If more than one triradius is present, record the value for each one.

10

For each print with more than one triradius, circle the highest value and use it to calculate TRC. Sum up the counts for the ten fingers. This is your TRC value.

INDIVIDUAL FINGERPRINT DATAFINGER TYPE OF PRINT NUMBER OF RIDGES

(CIRCLE VALUE USED)L-1L-2L-3L-4L-5R-1R-2R-3R-4R-5

TOTAL RIDGE COUNT:

Human Physical Genetic TraitsMany human physical traits are genetically controlled. The genes on the chromosomes of males seem to determine that the child will become male, and other genes determine eye color, hair color, height, skin color, and many other traits that distinguish individuals from one another. Several characteristics are known to be controlled by single gene differences. Some of these are listed and illustrated below.

1. Pigmented iris: The P allele for a pigmented iris (green, hazel, brown, or black eyes) is dominant over the p allele for lack of pigment (gray or blue eyes).

2. Tongue rolling: The R allele for the ability to roll the tongue into a U shape is dominant over the r allele for lack of this ability.

3. Bent little finger: The B allele for a bent little finger is dominant over the b allele for a straight little finger.

4. Widow’s peak: The W allele for a widow’s peak is dominant over the w allele for a straight hairline.

5. Thumb crossing: The C allele for crossing the left thumb over the right thumb when you interlace your fingers is dominant over the c allele for crossing the right thumb over the left.

11

6. Attached ear lobes: The a allele for attached ear lobes is recessive to the A allele for unattached ear lobes.

7. Hitchhiker’s thumb: the h allele for the ability to bend the last joint of the thumb back at an angle of 600 or more is recessive to the H allele for the lack of ability to bend the thumb back this far.

EXERCISE 4: Determine your phenotype for each of the above characteristics and record your phenotypes in

the table below. Fill in your genotypes for each of the traits.

Determine your phenotypes and genotypes for the traits listed in the following table.Trait Dominant Allele Your Phenotype Your Genotype(s)

Pigmented iris P= pigment presentTongue roller R= ability to roll

Bent little finger B= bent little fingerWidow’s peak W= widow’s peak

presentThumb crossing C=left over right

Attached ear lobes A=unattached ear lobesHitchhiker’s thumb H=<600 angle of the

thumb

If you have the trait determined by the recessive allele, there is only one possible genotype. If your trait is determined by the dominant allele, you should list both possible genotypes, unless one of your parents is homozygous recessive. In that case, you are heterozygous.

12

APPENDIX

13