Chapter 3

G0

Also called the resting phase. This is where the cell is neither dividing nor preparing to divide. Some cells never divide again including the nerve cells of the nervous system, spinal chord, and brain.

Independent Assortment

During gamete formation the segregation of the alleles of one allelic pair is independent of the segregation of the alleles of another allelic pair.

Endosperm

Food-storing tissue for the sperm.

Gametophyte

Haploid generation

Interphase

In actively dividing cells, the G1, S, and G2 phases. In addition, cells may remain permanently, or for long periods of time, in a phase of the cell cycle called G0. The M phase is Mitosis.

G1

In this part of interphase, the cell synthesizes mRNA and proteins in preparation for subsequent steps leading to mitosis. As the cell prepares to divide, if it reaches a restriction point it becomes committed on a pathway to cell division.

Sperm Cell

Male gametes that are relatively small and mobile. These usually have a long tail called a flagellum and a head. The head contains the haploid nucleus and acrosome. The head of the sperm cell has little cytoplasm and mainly genetic information.

Pollen Grain

Male gametophyte with a thick cell wall.

Sex in Bees

Males are produced from unfertilized haploid eggs. Females (worker bees and queen bees) are diploid produced from fertilized eggs. In this process, the females have twice the number of chromosomes.

Mitosis vs. Meiosis

Mitosis produces 2 diploid daughter cells, and meiosis produces 4 haploid daughter cells. Mitosis produces daughter cells that are genetically identical. Meiosis produces daughter cells that are genetically different: daughter cells contain only one homologous chromosome from each pair, and the number of possible outcomes when the chromosomes assort independently into gametes = 2^n. All unique events happen in Meiosis 1.

Chromatids

Once the chromosomes are copied, the two copies are known as chromatids.

Process of Karyotyping

1. A blood sample is collected and treated with drugs that stimulate cells division. Colchicine is added to disrupt spindle formation and stop cells in mitosis where the chromosomes are highly compressed (visible). The cells are then subjected to centrifugation. 2. Supernatant is discarded, and the cell pellet is suspended in a hypotonic solution that causes the cells to swell. 3.Cells are concentrated by centrifugation, suspended in a fixative, stained, and placed on a slide. 4. In a cytogenetics laboratory, the microscopes are equipped with a camera connected to a computer. Chromosomes are organized in a standard way, from largest to smallest.

Principles of the Chromosome Theory of Inheritance

1. Chromosomes contain the genetic material. 2. Chromosomes are replicated and passed from parent to offspring. 3. The nuclei of most eukaryotic cells contain chromosomes that are found in homologous pairs and are diploid. During meiosis, each homolog segregates into one of the two daughter nuclei. 4. During the formation of gametes, different types of chromosomes segregate independently. 5. Each parent contributes one set of chromosomes to its offspring.

Diploid

2 sets of chromosomes

Embryo Sac

A seven celled structure that is contained within an ovule that is the mature female gametophyte.

Monad

A single chromatid that is within a dyad.

Egg Cell

Also called an ovum, these are female gametes that are large and non-motile. These also store a large amount of nutrients in animal species. The longer the egg sits in a female's ovaries, the longer the egg is subjected to environmental pollutants and factors that make the DNA susceptible to mutation.

G2

Also called the pre-mitotic phase. This is the third and final sub-phase of Interphase in the cell cycle directly preceding mitosis. This is also the time when the cell accumulates materials needed for nuclear and cell division.

Test Cross

An individual that has a dominant phenotype and an unknown genotype is crossed with another individual with a recessive phenotype and genotype.

Karyotype

An organized representation of the chromosomes within a cell. This reveals how many chromosomes are found within an actively dividing somatic cell.

Prokaryotes

Bacteria and Archea are considered prokaryotes. This word comes from the Greek language meaning "pre-nucleus" because their chromosomes are not contained within a membrane-bound nucleus of the cell. These cells have a single type of circular chromosome in the nucleoid.

Isogamous

Common in simple eukaryotes, means that both gametes are morphologically similar.

X-linked inheritance

Confirmed the idea that genes are found on chromosomes.

Sex Chromosomes

Determines the sex and is represented by X and Y. The Y chromosome determines maleness.

Sporophyte

Diploid generation

Cell Cycle

Eukaryotic cells that are destined to divide progress through this series of phases. Cell division in eukaryotes is more complicated than simple binary fission.

Binary Fission

Following DNA replication, a prokaryote cell divides into two daughter cells. Each cell receives a copy of the chromosomal genetic material. This process does not involve genetic contributions from two different gametes.

Eukaryotes

From the Greek language meaning "true nucleus." This includes some simple species like protists and fungi as well as more complex, multicellular species such as plants and animals. The cells of eukaryotic species have internal membranes that enclose highly specialized compartments which then form membrane bound organelles. The mitochondria and plastids have their own genetic material.

Meiosis

From the Greek language meaning less. During meiosis, haploid cells are produced from a cell that was originally diploid. For this to occur, the chromosomes must be correctly sorted and distributed in a way that reduces the chromosomes number to half its original value. This is done in two successive divisions. The result of this process is daughter cells that are genetically different.

Bivalent

Homologs in pachytene and other steps of prophase of meiosis 1 that contain two pairs of sister chromatids.

Cleavage Furrow

In animals, constriction of the plasma membrane turns the cell into two daughter cells.

Genetic Variation

In meiosis, the cells that are produced are genetically different. The number of possible combinations when chromosomes assort independently into gametes is 2^n.

Cell Plate

In plants, the two daughter cells are separated by the formation of this plate. The cell plate undergoes maturation and then separates the mother cell into two daughter cells.

Sex in Insects

In some insects, males are X0 and females are XX. Other insects use the X-Y system like humans but the Y chromosome does not determines maleness. Instead it is the ratio between the X chromosomes and the number of sets of autosomes (X/A). In this process, if X/A= 0.5, the insect becomes a male, and iff X/A= 1.0, the insect becomes a female.

Crossing Over

In the pachytene stage, once synapsis is complete, there is a physical exchange of chromosome pieces. Depending on sizes of the chromosomes and the species, an average eukaryotic chromosome incurs a couple to a couple dozen crossovers. Crossing over is a random event that is unique to Meiosis 1. There are 8 million different possible combinations of genetic variation in humans. Crossing over makes the genetic possibilities even more numerous.

Metaphase

The alignment of the sister chromatids on the metaphase plate, and this is when each pair is attached to both poles by kinetochore tubules. The pairs become organized along a single row along the plate.

Diakenesis

The last stage of prophase of meiosis 1, the synaptonemal complex completely disappears and the nuclear membrane begins to fragment.

Chromosomes

The structures within living cells that contain the genetic material. They are composed of DNA (the genetic material) and proteins (maintain chromosome structure),

Homogametic Sex

This is only the females because they have two X's.

Dyad

Sister chromatids.

Gametes

Sperm and egg cells or their precursors. These are also called germ cells.

Tetrad

Tetra means four; this is another name for a bivalent because it is composed of four chromatids, that is four monads.

Why must cells divide?

The cell membrane transports food and oxygen to the parts inside. As a cell gets bigger, the membrane is unable to keep up because surface to volume ratio, S : V. Also, cell parts get old and stop working or there is damage to the cell and it goes through apoptosis. Cell division is also important for multi-cellularity.

Metaphase 1

The chromosomes align at the center of the cell by way of microtubule force. The pairs of sister chromatids are aligned in a double row, rather than a single row like in mitosis. The arrangement is random with regard to homologs. One pair of sister chromatids is linked to one of the poles and the other pair to the other pole.

Telophase 1

The chromosomes arrive at the poles of the cell and begin to recondense.

Prophase

The chromosomes have already replicated, resulting in 12 chromatids, joined as 6 pairs of sister chromatids. It is the process in which the nuclear membrane begins to dissociate into small vesicles, and the nucleolus becomes less visible. At the same time, the chromatids become condensed into compact structures and are able to be seen. The two centrosomes more apart and the mitotic spindle begins to form.

Telophase

The chromosomes reach their respective poles and decondense. The nuclear membrane now reforms to produce two separate nuclei.

Autosomes

The chromosomes that are not sex chromosomes.

Chromatin

The complex between DNA and protein. Each chromosome contains a very long segment of DNA and proteins which are bound together to provide it with an organized structure.

Anaphase

The connection between the chromatids is broken, and each chromatid becomes an individual chromosome linked to only one pole. As the microtubules shorten, the chromosomes move to opposite poles.

Cytogenetics

The field of genetics that involves the microscopic examination of chromosomes. Scientists that study this are called cytogeneticists. This allows the detection of individuals with chromosomal abnormalities as well as distinguishing between closely related species.

Lepotene

The first stage in the prophase of meiosis 1 where the replicated chromosomes begin to condense and become visible with a light microscope.

Diplotene

The fourth stage in the prophase of meiosis 1 where the synaptonemal complex has largely disappeared. The bivalent pulls apart slightly, and its easier to see the four chromatids.

Chromosome Theory of Inheritance

The inheritance patterns of traits can be explained by the transmission patterns of chromosomes during meiosis and fertilization. This theory allows us to understand the relationship between Mendel's laws and chromosome transmission.

Anaphase 1

The maternal and paternal homologs are separated on microtubules.

Prometaphase 1

The nuclear envelope breaks down allowing microtubule access to the chromosomes.

S

The part of the cell cycle in which DNA is replicated, occurring between G1 phase and G2 phase. At the end of this phase, the cell has double the number of chromatids as the number of chromosomes in the G1phase.

Chiasma

The plural name is chiasmata. The connection that results from crossing over.

The Journey of the Egg

The primary oocytes start meiosis 1, but enter into a dormant phase in prophase 1 until the female becomes sexually mature. At that point, the primary oocytes are periodically activated to progress through meiosis I. In humans, one oocyte per month is activated. The division in meiosis I is asymmetric producing two haploid cells. There is one of unequal size, which is a large secondary oocyte, and a small polar body. The secondary oocyte enters meiosis II, but is arrested at metaphase II. During ovulation, the egg is released into the oviduct. If the secondary oocyte is fertilized, then Meiosis II is completed and a haploid egg and a second polar body are produced. However, only one of the cells produced in meiosis becomes an egg.

Oogenesis

The process in female animals that produces eggs in specialized diploid cells of the ovary known as oogonia. This process takes place in embryonic development. In humans there are about one million primary oocytes that are produced before birth.

Spermatogenesis

The process in male animals that produces sperm within the glands known as testes. In humans, this is a continuous process and a mature human male produces several hundred million sperms per day.

Gametogenesis

The process that produces gametes.

Zygotene

The second stage in the prophase of meiosis 1 that involves a recognition process known as synapsis, in which the homologous chromosomes recognize each other and begin to align themselves. In most species, this involves the formation of synaptonemal complex that forms between the homologous chromosomes.

Sex in Birds and Fish

The sex chromosomes are designated Z and W. Males contain two Z chromosomes and are homogametic. Females have one Z and one W chromosome and are heterogametic.

Meiosis 2

The sorting events are similar to mitosis but start at a different points. The two cells that being meiosis 2 have 6 chromatids that are joined as 3 pairs of sister chromatids or 3 dyads. Other than that, the steps of meiosis 2 are the same as mitosis.

Pachytene

The third stage in the prophase of meiosis 1 in which the homologs have become completely aligned. The associated chromatids are known as bivalents. Once the bivalent has formed, crossing over can occur.

X-linked Genes

These are also called X-linked alleles, and they are the the genes that are physically located in the X chromosome.

Centrioles

These are at right angles to each other and within centrosomes.

Locus

These are different physical locations of a given gene and chromosome.

Oogonia

These are diploid that divide mitotically to produce diploid primary oocytes.

Spermatogonia

These are diploid that divide mitotically to produce two cells. One remains a spermatogonial cell to be a permanent source of spermatogonia, and one becomes a primary spermatocyte which progresses through meiosis to give sperms.

Microtubules

These are formed from the polymerization of tubulins.

Homologs

These are members of a pair of similar chromosomes. These have the same size, banding pattern, centromere position, and genes (not necessarily same alleles though). There is a < 1% difference in DNA sequence provides the allelic differences in genes, and sex chromosomes (X and Y) are not homologous because they are different in size and genetic composition.

Sister Chromatids

These are produced once the chromatids are joined together with a centromere. These are also called dyads.

Aster Microtubules

These microtubules emanate outward from the centrosome toward the plasma membrane and are important in the positioning of the spindle apparatus within the cell and later in the process of cell division.

Kinetichore Microtubules

These microtubules have attachments to a kinetichore.

Polar Microtubules

These microtubules project toward the region where the chromosomes will be found during mitosis. These play a role in the separation of the two poles.

Polar Bodies

These normally disintegrate and die as the primary and secondary oocytes develop. Since the division is asymmetrical, Meiosis conserves cytoplasm for the secondary oocyte to use for a potential fetus.

Acrosome

This contains digestive enzymes that enable a sperm to penetrate the egg's outer membrane.

Morgan's Experiment

This experiment tested for X-linked genes. Morgan used radiation to induce mutations in male fruit flies so they had white eyes rather than the normal red eyes. The first cross yielded NO white-eyed females in the F2 generation. These results indicated that the eye color alleles are located on the X chromosome and are therefore X-linked and controlled by sex.

Kinetichore

This is a group of proteins that are bound to a centromere. These proteins help to hold the sister chromatids together and also play a role in chromosome sorting.

Metaphase Plate

This is a plane where the sister chromatids align, usually along the equator of the cell.

Mitotic Spindle

This is also known as the mitotic spindle apparatus, it is composed of microtubules, and it is involved in the organization of chromosomes.

Nucleoid

This is an irregularly shaped region within the cell of a prokaryote that contains all of most of all the genetic material. This is the opposite of the nucleus of a eukaryote.

Nucleus

This is an organelle in eukaryotes that is bound by two membranes. These two membranes constitute the nuclear envelope. Most of the genetic material is contained in the chromosomes which are located here.

Somatic Cell

This is any cell of the body that is not a gamete or a precursor to a gamete.

Heterogametic Sex

This is only the males because they have one X and one Y.

Genetic Recombination

This is physical exchange of chromosomes that combines the DNA from different parents into a single chromosome.

Centromere

This is the point where the chromatids are joined together at a region of DNA.

Restriction Point

This is when a cell is committed on a pathway that leads to cell division.

Asexual Reproduction

This is when a preexisting cell divides to produce two new cells. This is how prokaryotes and unicellular organisms proliferate.

Decondensed

This is when the cells are in interphase. The chromosomes are unwound to become less tightly compacted.

Prometaphase

This is when the nuclear membrane is completely disrupted into vesicles allowing the spindle fibers to interact with sister chromatids. This is also how the sister chromatids become attached to the kinetochore tubules from opposite poles.

Spindle Pole

This is where each of the centrosomes are located.

Centrosomes

This is where the mitotic spindle is formed from two MTOC's.

Mitosis

This process is used to distribute the replicate chromosomes by dividing one cell nucleus into two nuclei. Each daughter cell receives the same complement chromosomes. This process also ensures genetic consistency from one cell generation to the next. Organismal growth and development depend on this.

MTOC's

This stands for microtubule organizing centers, and are structures found in the eukaryotic cells from which microtubules grow.

Prophase of Meiosis 1

This step is further subdivided into stages known as lepotene, zygotene, pachytene, diplotene, and diakinesis.

Sexual Reproduction

Two gametes fuse with each other in the process of fertilization to create a new diploid organism.

Heterogamous

Two morphologically different gametes.

Cytokenesis

Two nuclei are segregated into two daughter cells. This stage segregates cell parts as well.