Chapter 15 Mitosis and Meiosis

Deletions and Duplications

Are changes in the total amount of genetic material in a single chromosome. When a Deletion occurs, a segment of chromosomal material is removed. The affected chromosome becomes deficient in a significant amount of genetic material. In a Duplication, a section of a chromosome occurs two or more times in a row. The effects of duplications and deletions are usually detrimental.

Meiosis 2

Meiosis 1 is followed by cytokinesis and then meiosis 2. The sorting events of meiosis 2 are similar to those of mitosis, but the starting point is different. The two cells that begin meiosis 2 each have 6 chromatids that are joined as 3 pairs of sister chromatids. Otherwise the steps that occur during prophase, prometaphase, metaphase, anaphase, and telophase of meiosis 2 are the same as a mitotic division.

Diploid-dominant species

Most animal species are diploid, and their haploid gametes are considered to be a specialized type of cell. Humans for this reason are viewed as diploid-dominant.

Importance of Mitotic cell division

1. Asexual reproduction. 2. the production and maintenance of multicellularity.

different beginning for meiosis

1. homologous pairs of sister chromatids associate with each other, lying side by side to form a bivalent, also called a tetrad. The process of forming a bivalent is termed synapsis. 2. Crossing over: involves a physical exchange between chromosome segments of the bivalent. This increases the genetic variation of sexually reproducing species.

Inversions and Translocations

Are chromosomal rearrangements. An Inversion is a change in the direction of the genetic material along a single chromosome. When a segment of one chromosome has been inverted, the order of G bands is opposite to that of a normal chromosome. A Translocation occurs when one segment of a chromosome becomes attached to a different chromosome.

Prophase

At the start of mitosis, the chromosomes have already replicated to produce 12 chromatids, joined as six pairs of sister chromatids that have condensed into highly compacted structures readily visible by light microscopy. As prophase proceeds, the nuclear envelope begins to dissociate into small vesicles. The nucleus is no longer visible.

chromosome numbering system

Eukaryotic cells contain 2 sets of chromosomes, 23 diffferent chromosomes in each set. Numbered according to size with the largest having the smallest numbers. (1, 2, and 3 are largest, which 21 and 22 are the smallest. This system does not apply to sex chromosomes.

phases of the cell cycle

G1(first gap), S(synthesis of DNA, the genetic material), G2(second gap), and M phase(mitosis and cytokinesis).

Telophase

The chromosomes have reached their respective poles and decondense. The nuclear envelope now re-forms to produce two separate nuclei containing 6 chromosomes each.

Anaphase

The connections between the pairs of sister chromatids are broken. each chromatid, now an individual chromosome, is linked to only one of the two poles by one or more kinetochore microtubule. As anaphase proceeds, the kinetochore microtubules shorten pulling the chromosomes towards the pole to which they are attached, while at the same time pushing the two poles further away from each other.

M phase

The first part of the M phase is Mitosis, where it divides one cell nucleus into two nuclei, distributing the duplicated chromosomes so each daughter cell receives the same complement of chromosomes. During mitosis, these pairs of chromatids are separated and stored so each daughter cell receives 46 chromosomes.

Prometaphase

The nuclear envelope completely fragments into small vesicles, and the mitotic spindle is fully formed. The centrosomes move apart and demarcate the two poles. As the end of prometaphase nears, the two kinetochors on each pair of sister chromatids are attached to kinetochore microtubules from opposite poles.

checkpoint proteins

They delay the cell cycle until problems are fixed or prevent cell division when problems cannot be fixed. A primary aim of checkpoint proteins is to prevent the division of a cell that may have incurred DNA damage or harbors abnormalities in chromosome number. When the functions of Checkpoint proteins are lost due to mutation, the likelihood increases that undesirable genetic changes will occur that can cause additional mutations and cancerous growth.

Checkpoints

Three critical regulatory stops are found in the cell cycle of eukaryotic cells. At these checkpoints a variety of proteins (checkpoint proteins) act as sensors to determine is a cell is in the proper condition to divide.

End of Meiosis 1

Two nuclei are produced, each with 3 pairs of sister chromatids (a reduction division). The original diploid call had its chromosomes in homologous pairs, whereas the two cells produced as a result of meiosis 1 and cytokinesis are considered haploid--they do not have pairs of homologous chromosomes.

sex chromosomes

XX = female and XY = male.

Mitotic cell division

a cell divides to produce two new cells (daughter cells) that are genetically identical to the original cell (mother cell). Mitotic cell division involves mitosis-the division of one nucleus into two nuclei- and then cytokinesis in which the mother cell divides into two daughter cells.

G1 phase

a period in a cells life when it may become committed to divide. Depending on the environmental conditions and the presence of signaling molecules, a cell in a G1 phase may accumulate molecular changes that cause it to progress through the rest of the cell cycle. cell growth typically occurs during the G1 phase.

Asexual reproduction

a process in which genetically identical offspring are produced from a single parent. certain unicellular eukaryotic organisms increase their numbers in this manner.

Sexual reproduction

a process in which two haploid gametes unite to form a diploid cell called a zygote.

Simple translocation

a single piece of chromosome is attached to another chromosome.

Kinetochore

a structure necessary for sorting each chromosome.

Gzero phase (nondividing phase)

an alternative to proceeding through G1. a cell in the Gzero phase has postponed making a decision to divide, or in the case of the terminally differentiated cells, will never divide again.

autosomes

chromosomes that are not sec chromosomes.

Mitosis in animal cells

continuum of phases known as Prophase, Prometaphase, Metaphase, Anaphase, and Telophase.

G2 phase

during the G2 phase, a cell synthesizes the proteins necessary for chromosome sorting and cell division. some cell growth may occur.

S phase

each chromosome is replicated to form a pair of sister chromatids. When the S phase is completed, a cell has twice as many chromatids as the number of chromosomes in the G1 phase.

metaphase

eventually the pairs of sister chromatids are aligned in a single row along the metaphase plate, a plane halfway between the poles. When this alignment is complete the cells are in Metaphase of mitosis--the chromatids can then be equally distributed into two daughter cells.

life cycle

for any given species, the sequence of events that produce another generation of organisms is known as a life cycle. For sexually reproducing organisms, this usually involves an alteration between haploid cells or organisms and diploid cell or organisms.

Zygote

for multicellular species such ass animals and plants, the zygote then grows and divides by mitotic cell division into a multicellular organism with many diploid cells.

interphase

in actively dividing cells, the G1, S and G2 phases are collectively known as interphase. during this phase the cell grows and copies its chromosomes in preparation for cell division.

centrosomes

in animal cells, microtubule growth and organization starts at two centrosomes- regions that are also referred to as microtubule organizing centers. A single centrosome duplicates during interphase. after they separate, each new centrosome defines a pole of the spindle apparatus, one within each of the future daughter cells.

cytokinesis

in most cases, mitosis is followed by cytokinesis, which is the division of the cytoplasm to produce two distinct daughter cells.

Chromosomal Mutation

involve the breaking and rejoining of chromosomes, are categorized as deletions, duplications, inversions, and translocations.

Meiosis

process by which haploid cell are produced from a cell that was originally diploid. The term Meiosis, which means "to make smaller" refers to the fewer chromosomes found in calls following this process. For this to occur, the chromosomes must be correctly sorted and distributed in a way that reduces the chromosome number to half its original diploid value.

Mitotic spindle

responsible for organizing and sorting the chromosomes during cell division. It is composed of microtubules--protein fibers that are components of the cytoskeleton.

karyotype

reveals the number, size, and form of chromosomes found within an actively dividing cell.

meiosis 1

separates homologous chromosomes from each other.

cytokinesis

shorting following mitosis, the two nuclei are segregated into separate daughter cells.

Metaphase 1

the bivalents are organized along the metaphase plate. unlike mitosis, in meiosis the sister chromatids are lined up in a double row rather then the single row. it is also a random alignment unlike in mitosis.

cytogenetics

the field of genetics that involves microscopic examination of chromosomes.

Prometaphase 1

the nuclear envelope is completely broken down into vesicles, and the spindle apparatus is entirely formed. The sister chromatids become attached to kinetochore microtubules. However, a key difference occurs between mitosis and meiosis 1. in mitosis, a pair of sister chromatids is attached to both poles. In meiosis 1, a pair of sister chromatids is attached to just one pole vie kinetochore microtubules.

cell division

the production of cells. is a highly regulated process that distributes and monitors the integrity of the genetic material.

Prophase 1

the replicated chromosomes condense, the homologous chromosomes form bivalents, and crossing over occurs. The nuclear envelope then starts to fragment into small vesicles.

Anaphase 1

the segregation of homologs occurs here. the connections between bivalents breaks but not the connections that hold sister chromatids together. each joined pair of chromatids migrates to one pole, and the homologous pair of chromatids moves to the opposite pole, both pulled by kinetochore microtubules.

Telophase 1

the sister chromatids have reached their respective poles and then decondense. the nuclear envelope now re-forms to produce two separate nuclei.

gametes

the sperm and egg cells, are haploid (1n) which means they contain one set of chromosomes (23).

centromere

the two sister chromatids are tightly associated at a region called the centromere. A protein called cohesion is necessary to hold the sister chromatids together. in addition, the centromere serves as an attachment site for a group of proteins that form the Kinetochore.

Cyclins and cyclin-dependent kinases (cdks)

these proteins are responsible for advancing a cell through the phases of the cell cycle. Cyclins are so named because their amount varies throughout the cell cycle. To be active, kinases controlling the cell cycle must bind to and are dependent on a cyclin.

sister chromatids

two copies are still joined to each other and are referred to as a pair of sister chromatids.

Reciprocal translocation

two different types of chromosomes exchange pieces, thereby producing two abnormal chromosomes carrying translocations.

homologous pairs

when an organism is diploid, the members of a pair of chromosomes are called homologs. homology refers to any similarity that is due to common ancestry. Homologous pairs are not usually identical because over many generations they have accumulated some genetic changes that make them distinct.

diploid

when the cells of an organism carry two sets of chromosomes (one inherited from the father and the other from the mother) then the organism is a diploid.