Mitosis & Meiosis

If a cell is 2n=16, how many tetrads are present during prophase 1?

32 chromatids divided by 4, 8 tetrads.

Centriole

An organelle in many animal cells that appears to be involved in the formation of the spindle during mitosis.

Nondisjunction

Failure of disjunction or separation of homolo- gous chromosomes in mitosis or meiosis, resulting in too many chromosomes in some daughter cells and too few in others. Examples: In meiosis, both members of a pair of chromosomes go to one pole so that the other pole does not receive either of them; in mitosis, both sister chromatids go to the same pole.

In animals, which cells are haploid and which are diploid?

Germline (sex?) cells are haploids and somatic cells are diploids.

Mitosis vs. Meiosis

Meiosis has two rounds of genetic separation and cellular division while mitosis only has one of each. In meiosis homologous chromosomes separate leading to daughter cells that are not genetically identical. In mitosis the daughter cells are identical to the parent as well as to each other.

homology search

Sister chromatids search for and pair up with their homolog. Begins in leptonema stage

gametophyte

That phase of the plant life cycle that bears the gametes; the cells have n chromosomes.

Metaphase

That stage of cell division in which the chromosomes are most discrete and arranged in an equatorial plate; stage follow- ing prophase and preceding anaphase.

G2 checkpoint

The cell checks to make sure the DNA is copied correctly

What visible characteristics of chromosomes indicate that they have undergone crossing over during meiosis?

The chiasmata, which are visible late in prophase I of meiosis, indicate that chromosomes have crossed over.

Tetrad

The four cells arising from the second meiotic division in plants (pollen tetrads) or fungi (ascospores). The term is also used to identify the quadruple group of chromatids that is formed by the association of duplicated homologous chromosomes during meiosis.

Telophase

The last stage in each mitotic or meiotic division in which the chromosomes are assembled at the poles of the division spindle

Interphase

The stage in the cell cycle when the cell is not dividing; the metabolic stage during which DNA replication occurs; the stage following telophase of one division and extending to the beginning of prophase in the next division.

Monad

a single chromatid within a dyad

cell cycle

series of events in which a cell grows, prepares for division, and divides to form two daughter cells

spindle apparatus

structure made of spindle fibers, centrioles, and aster fibers that is involved in moving and organizing chromosomes before the cell divides

Diplotena

synaptonemal complex dissociates

cleavage furrow

the first sign of cytokinesis during cell division in an animal cell; a shallow groove in the cell surface near the old metaphase plate

synapsis

the pairing of homologous chromosomes during meiotic prophase

Match the stages of mitosis with the events they encom- pass: Stages: (1) anaphase, (2) metaphase, (3) prophase (4) telophase. Events: (a) re-formation of the nucleolus (b) disappearance of the nuclear membrane, (c) conden- sation of the chromosomes, (d) formation of the mitotic spindle, (e) movement of chromosomes to the equato- rial plane, (f) movement of chromosomes to the poles, (g) decondensation of the chromosomes, (h) splitting of the centromere, (i) attachment of microtubules to the kinetochore.

(1) Anaphase: (f), (h); (2) metaphase: (e), (i); (3) prophase: (b), (c), (d); (4) telophase: (a), (g)

If a cell contains 64 pairs of chromosomes immediately after completion of mitotic telophase, how many chromatids were present in metaphase?

64 x 4 = 256 chromatids

binary fission

A form of asexual reproduction in prokaryote organisms by which one cell divides into two cells of the same size

kinetochore

A proteinaceous structure associated with the centro- mere of a chromosome during eukaryotic cell division; the point at which microtubules attach to move the chromosome through the division process.

synaptonemal complex

A ribbonlike structure formed between synapsed homologues at the end of the first meiotic prophase, binding the chromatids along their length and facilitating chro- matid exchange

Diakinesis

A stage of meiosis just before metaphase I in which the bivalents are shortened and thickened

Chiasma

A visible change of partners in two of a group of four chromatids during the first meiotic prophase. In the diplotene stage of meiosis, the four chromatids of a bivalent are associated in pairs, but in such a way that one part of two chroma- tids is exchanged. This point of "change of partner" is the chiasma.

From the information given in Table 2.1 in this chapter, is there a relationship between genome size (measured in base pairs of DNA) and gene number? Explain.

Among eukaryotes, there does not seem to be a clear relationship between genome size and gene number. For example, humans, with 3.2 billion base pairs of genomic DNA, have about 20,500 genes, and Arabidopsis plants, with about 150 million base pairs of genomic DNA, have roughly the same number of genes as humans. However, among prokaryotes, gene number is rather tightly cor- related with genome size, probably because there is so little nongenic DNA.

Mitosis (I~P~M~A~T)

Chromosome complement, complete set of chromosomes, ensures that two daughter cells receive a diploid complement identical to the diploid complement of the parent cell (new somatic cells- sexual reproduction)

During meiosis, when does chromosome disjunction occur? When does chromatid disjunction occur?

Chromosome disjunction occurs during anaphase I. Chromatid disjunction occurs during anaphase II.

The normal diploid chromosome # for sugar cane is 108, state the chromatid or chromosome #.

Chromosome number is 216.

Prophase

Chromosomes become visable, nuclear envelop dissolves, spindle forms

Homologous

Chromosomes that occur in pairs and are generally similar in size and shape, one having come from the male parent and the other from the female parent. Such chromo- somes contain the same array of genes

Does crossing over occur before or after chromosome duplication in cells going through meiosis?

Crossing over occurs after chromosomes have duplicated in cells going through meiosis.

Cytokinesis

Cytoplasmic division and other changes exclusive of nuclear division that are a part of mitosis or meiosis

With a focus on the chromosomes, what are the key events during interphase and M phase in the eukaryotic cell cycle?

During interphase, the chromosomes duplicate. During M phase (mitosis), the duplicated chromosomes, each consisting of two identical sister chromatids, condense (a feature of prophase), migrate to the equatorial plane of the cell (a feature of metaphase), and then split so that their constituent sister chromatids are separated into different daughter cells (a feature of anaphase); this last process is called sister chromatid disjunction.

Meiosis (I1-P1(5 steps)-M1-A1-T1-P2-M2-A2-T2-C)

Ensures that 4 gametes cells receive a haploid complement unique to one another and the diploid complement of the parent cell (creating gametes)

Microtubules

Hollow filaments in the cytoplasm making up a part of the locomotor apparatus of a motile cell; component of the mitotic spindle.

How is independent assortment of genes on different chromosomes related to physically to the process of chromosome alignment on the metaphase plate in meiosis I?

Homologous pairs of chromosomes orient randomly at metaphase I of meiosis

In Arabidopsis, is leaf tissue haploid or diploid? How many nuclei are present in the female gametophyte? How many are present in the male gametophyte? Are these nuclei haploid or diploid?

In flowering plants sporophytic tissue is diploid. Eight nuclei are present in the female gametophyte; three are present in the male gametophyte. All the gametophytic nuclei are haploid, although in the female gametophyte, two of these nuclei fuse to form a diploid endosperm nucleus.

Distinguish between the haploid and diploid states. What types of cells are haploid? What types of cells are diploid?

In the haploid state, each chromosome is represented once; in the diploid state, each chromosome is represented twice. Among multicellular eukaryotes, gametes are haploid and somatic cells are diploid.

What is the significance of crossing over during prophase 1?

It makes each cell formed unique. Genetic diversity.

equational division (meiosis II)

Mitotic-type division that is usually the second division in the meiotic sequence; somatic mitosis and the non- reductional division of meiosis

n=12

N means they are haploid gametes. They are produced by meiosis.

independent assortment

One of Mendel's principles that states that genes for different traits can segregate independently during the formation of gametes

Reductional Division (Meiosis I)

Phase of meiosis in which the maternal and paternal chromosomes of the bivalent separate

If children obtain half their genes from one parent and half from the other parent, why aren't siblings identical?

Random distribution of chromosomes in anaphase I after their random alignment in metaphase I and the 'crossing over' in prophase I contribute towards the genetic diversity. Because the 'half' inherited is random, the chance of receiving exactly the same half is very small.

Chromomeres

Small bodies that are identified by their characteris- tic size and linear arrangement along a chromosome

Centromere

Spindle-fiber attachment region of a chromosome

Zygotena

Stage in meiosis during which synapsis occurs; after the leptotene stage and before the pachytene stage in the meiotic prophase

Leptonema (leptotene stage)

Stage in meiosis immediately preced- ing synapsis in which the chromosomes appear as single, fine, threadlike structures (but they are really double because DNA replication has already taken place)

Pachytena

Start to pull apart, chiasma forms, CROSSOVER POINT

What are the stages of the cell cycle and how are they controlled?

The progression of phases is denoted G1~S~G2~M. In this progression, S is the period in which the chromosomes are duplicated—an event that requires DNA synthesis, to which the label "S" refers. The M phase in the cell cycle is the time when the mother cell actually divides. This phase usually has two components: (1) mitosis, which is the process that distributes the dupli- cated chromosomes equally and exactly to the daughter cells, and (2) cytokinesis, which is the process that physically separates the two daughter cells from each other. The label "M" refers to the term mitosis, which is derived from a Greek word for thread; during mitosis, the chromosomes appear as threadlike bodies inside cells. The G1 and G2 phases are "gaps" between the S and M phases.

Anaphase

The stage of mitosis or meiosis during which the daugh- ter chromosomes pass from the equatorial plate to opposite poles of the cell (toward the ends of the spindle). Anaphase follows metaphase and precedes telophase.

bivalent

The structure formed by the pair of homologous chromosomes during crossing over. Also called a tetrad because it consists of four chromatids.

A sperm cell from the fruit fly Drosophila melanogaster contains four chromosomes. How many chromosomes would be present in a spermatogonial cell about to enter meiosis? How many chromatids would be present in a spermatogo- nial cell at metaphase I of meiosis? How many would be present at metaphase II?

There are 8 chromosomes in a Drosophila spermatogonial cell about to enter meiosis. There are 16 chromatids in a Drosophilia spermatogonial cell at metaphase I of meiosis. There are 8 chromatids in a Drosophilia cell at metaphase II of meiosis.

model organism characteristics

amenability to genetic manipulation rapid rate of reproduction ability to grow under controlled conditions

Centrosome

barrel-shaped organelle associated with the mitotic spindle in animal cells

Arrange the following events in the correct temporal sequence during eukaryotic cell division, starting with the earliest: (a) condensation of the chromosomes, (b) movement of chromosomes to the poles, (c) duplication of the chromo- somes, (d) formation of the nuclear membrane, (e) attach- ment of microtubules to the kinetochores, (f) migration of centrosomes to positions on opposite sides of the nucleus.

c. f. a. e. b. d.

M checkpoint

cell monitors spindle formation and attachment to kinetochores

G1 checkpoint

checks for cell size, nutrients, growth factors and DNA damage

cell plate

cytokinesis is accomplished by the formation of a membranous cell plate between the daughter cells; eventually, walls composed of cellulose are built on either side of the cell plate.

Karyokinesis

division of a cell nucleus during mitosis.

Dyad

group of two

five stages of prophase 1

leptotene, zygotene, pachytene, diplotene, diakinesis