Chapter 2 Chromosomes and Cellular Reproduction

somatic cells

Any cells in the body other than reproductive (gamete) cells

genetic consequences of the cell cycle (mitosis)

- Producing two cells that are genetically identical to each other and with the cell that gave rise to them. - Newly formed cells contain a full complement of chromosomes. - Each newly formed cell contains approximately half the cytoplasm and organelle content of the original parental cell. Consequently, not all cells resulting from the cell cycle are identical in their cytoplasmic content.

telophase

- After the sister chromatids have separated, each is considered a separate chromosome. Telophase is marked by the arrival of the chromosomes at the spindle poles. - The nuclear membrane re-forms around each set of chromosomes, producing two separate nuclei within the cell. - The chromosomes relax and lengthen, becoming indistinguishable under the light microscope. In many cells, division of the cytoplasm (cytokinesis) is simultaneous with telophase.

prometaphase

- Disintegration of the nuclear membrane marks the start of prometaphase. - Spindle microtubules, which until now have been outside the nucleus, enter the nuclear region. Tubulin molecules are added to and removed from the microtubules, causing them to grow and shrink. When the end of a microtubule encounters a kinetochore, the microtubule becomes stabilized. Eventually, each chromosome becomes attached to microtubules from opposite spindle poles: for each chromosome, a microtubule from one of the centrosomes anchors to the kinetochore of one of the sister chromatids; a microtubule from the opposite centrosome then attaches to the other sister chromatid, anchoring the chromosome to both of the centrosomes. This arrangement is known as chromosome bi-orientation. - period of chromosome movement to equatorial plane of cell, centrioles reach poles, and spindle fibers form

oogenesis vs spermatogenesis

- In oogenesis, cytokinesis is unequal: most of the cytoplasm is allocated to one of the two haploid cells, the secondary oocyte. The smaller cell, which contains half of the chromosomes but only a small part of the cytoplasm, is called the first polar body; it may or may not divide further. The secondary oocyte completes meiosis II, and again, cytokinesis is unequal—most of the cytoplasm passes into one of the cells. The larger cell, which acquires most of the cytoplasm, is the ovum, the mature female gamete. The smaller cell is the second polar body. Only the ovum is capable of being fertilized, and the polar bodies usually disintegrate. Oogenesis, then, produces a single mature gamete from each primary oocyte. - The formation of sperm takes place continuously in a male throughout his adult reproductive life. The formation of female gametes, however, is often a discontinuous process and may take place over a period of years. Oogenesis begins before birth; at this time, oogonia initiate meiosis and give rise to primary oocytes. Meiosis is then arrested, stalled in prophase I. Thus, a female is born with primary oocytes arrested in prophase I. In humans, this period of suspended animation may last 30 or 40 years, until rising hormone levels stimulate one or more of the primary oocytes to recommence meiosis. The first division of meiosis is completed, and a secondary oocyte is ovulated from the ovary (a process called ovulation). In humans and many other species, the second division of meiosis is then delayed until contact with the sperm. When a sperm penetrates the outer layer of the secondary oocyte, the second meiotic division takes place, the second polar body is extruded from the ovum, and the nuclei of the sperm and newly formed ovum fuse, giving rise to the zygote.

sex chromosomes

- NOT homologous, differ in size and composition - however, they do have short regions of homology = pseudoautosomal (kind of like the autosomes/non-sex chromosomes in terms of homology)

Prokaryotic Cell Reproduction by Binary Fission

- Replication usually begins at a specific place on the circular chromosome, called the origin of replication. - The origins of the two newly replicated chromosomes move away from each other and toward opposite ends of the cell. - Proteins called structural maintenance of chromosomes (SMC) complexes encircle the DNA and help keep the two newly synthesized chromosomes from getting tangled as they are replicated. - Finally, a new cell wall forms between the two chromosomes, producing two cells, each with an identical copy of the chromosome. Under optimal conditions, some bacterial cells divide every 20 minutes.

homologous chromosomes

- are nearly identical in size, have the same banding pattern and centromere location, have the same genes but not necessarily the same alleles (usually less than 1% between homologs) - Human cells have 46 chromosomes: in females, there are 23 homologous pairs; in males, 22 pairs are homologous, and the last pair (X and Y chromosomes) is only partially homologous - For example, if a gene on a particular chromosome encodes a characteristic such as hair color, another copy of the gene (each copy is called an allele) at the same position on that chromosome's homolog also encodes hair color. However, these two alleles need not be identical: one might encode brown hair and the other might encode blond hair.

from an evolutionary perspective, there are three types of organisms

- bacteria - archaea - eukaryotes

metaphase

- chromosomes become arranged in a single plane, called the metaphase plate, between the two centrosomes. - The centrosomes, now at opposite ends of the cell, with microtubules radiating outward from each one and meeting in the middle of the cell, center at the spindle poles. A spindle-assembly checkpoint ensures that each chromosome is aligned on the metaphase plate and attached to spindle microtubules from opposite poles.

prophase

- chromosomes condense, becoming more compact and visible under a light microscope. A group of proteins called condensins bind to the DNA within chromosomes and bring about condensation - Because the chromosomes were duplicated in the preceding S phase, each chromosome consists of two sister chromatids attached at the centromere. - mitotic spindle forms. In animal cells, the spindle grows out from a pair of centrosomes that migrate to the opposite sides of the cell. Within each centrosome is a special organelle, the centriole, which is also composed of microtubules.

bacterial cells

- do not contain a membrane-bound nucleus - usually contain a single type of circular chromosome, found in the nucleoid - cytoplasm is enclosed by plasma membrane, which regulates nutrient uptake and waste excretion - outside membrane is a rigid cell wall for protection from breakage - flagella, sometimes outer membrane

chromosome is a relative term

- in G1 and late in the M phase, it refers to the equivalent of one chromatid - in G2 and early in the M phase, it refers to a pair of sister chromatids

eukaryotic chromosomes are inherited in sets

- most eukaryotic species are diploid - members of a pair of chromosomes are called homologs, and when two form a homologous pair

meiosis II

- separation of sister chromatids, equational division - similar to mitosis

prophase I

- synapsis: close pairing of homologous chromosomes - tetrad: closely associated four-sister chromatids of two homologous chromosomes - crossing over

MITOSIS, 2n = 4 1. how many chromosomes during metaphase? 2. how many chromosomes during anaphase?

1. 4 2. 8

# of chromosomes vs # of DNA molecules if 2n = 8: 1. metaphase I 2. metaphase II 3. after cytokineses following meiosis II

1. 8 and 16 2. 4 and 8 3. 4 and 4

# of chromosomes vs # of DNA molecules if 2n = 8: 1. G1 2. G2 3. metaphase of mitosis 4. anaphase of mitosis 5. after cytokinesis following mitosis

1. 8 and 8 2. 8 and 16 3. 8 and 16 4. 16 and 16 5. 8 and 8

In general, the number of possible combinations of gametes is...

2^n, where n equals the haploid number or homologous pairs

assuming independent assortment, how many different gametes can be formed by an organism that is heterozygous for n genes

2n for example, assuming independent assortment, how many different gametes can be formed by an organisms that is homozygous for 3 and heterozygous for 2 genes? it would be 4, since 2 x 2 = 4.

karyotype

A display of the chromosome pairs of a cell arranged by size and shape.

G0 phase

A nondividing state (quiescent) occupied by cells that have left the cell cycle, sometimes reversibly. - Before reaching the G1/S checkpoint, cells may exit the active cell cycle in response to regulatory signals and pass into G0. - maintain constant size - Many cells never enter G0; rather, they cycle continuously. - On the other hand, many cells spend most of their life span in G0

Two types of prokaryotes

Bacteria and Archaea

metaphase I

Metaphase I is initiated when homologous pairs of chromosomes align along the metaphase plate - A microtubule from one spindle pole attaches to one chromosome of a homologous pair, and a microtubule from the other pole attaches to the other member of the pair.

interphase

Cell grows, performs its normal functions (DNA is being synthesized, RNA and proteins are being produced, and hundreds of biochemical reactions necessary for cellular functions) and prepares for division; consists of G1, S, and G2 phases

metaphase II

Chromosomes (sister chromatids) line up at the metaphase plate. - To distinguish from mitosis metaphase, the number of chromosomes will be HALF the original in metaphase II!

crossing over

Crossing over generates genetic variation and is essential for the proper alignment and separation of homologous chromosomes. Each location where two chromosomes cross is called a chiasma (plural, chiasmata). - After crossing over has taken place, the sister chromatids are no longer identical. - basis for intrachromosomal recombination, the creation of new combinations of alleles on a chromatid.

T or F: similar chromosome number is indicative of similar organismal complexity or close evolutionary relationship

FALSE

nucleolus

Found inside the nucleus and produces ribosomes

A functional chromosome has three essential elements:

a centromere, a pair of telomeres, and origins of replication.

histone proteins

Histone proteins help regulate the accessibility of DNA to enzymes and other proteins that copy and read the DNA and they also enable the DNA to fit into the nucleus of a eukaryotic cell

centromere

It serves as the attachment point for spindle microtubules—the filaments responsible for moving chromosomes in cell division. - Before cell division, a multiprotein complex called the kinetochore assembles on the centromere; later, spindle microtubules attach to the kinetochore. - Chromosomes lacking a centromere cannot be drawn into the newly formed nuclei; such chromosomes are lost, often with catastrophic consequences for the cell (resulting daughter cells missing a chromosome or incorrect number of chromosomes) - classified into four types: metacentric, submetacentric, acrocentric, and telocentric

mitosis vs meiosis

Mitosis: one division forming 2 genetically identical cells (clones) - cells produced by mitosis are diploid Meiosis: two divisions forming 4 genetically different cells - meiosis II differs from mitosis in that chromosome number has already been halved in meiosis I, so the cell does not begin with the same number of chromosomes as it does in mitosis - cells produced by meiosis are haploid

meiosis in plants

Most multicellular plants and algae have a complex life cycle that includes two distinct structures (generations): a multicellular diploid sporophyte and a multicellular haploid gametophyte. - These two generations alternate; the sporophyte produces haploid spores through meiosis, and the gametophyte produces haploid gametes through mitosis. This type of life cycle is sometimes called alternation of generations. In this cycle, the immediate products of meiosis are called spores, not gametes; the spores undergo one or more mitotic divisions to produce gametes. Although the terms used to describe the plant life cycle are somewhat different from those commonly used for animals (and from some of those employed so far in this chapter), the processes in plants and animals are basically the same: in both, meiosis leads to a reduction in chromosome number, producing haploid cells.

interkinesis

Period of time between meiosis I and meiosis II - the nuclear membrane re-forms around the chromosomes clustered at each pole, the spindle breaks down, and the chromosomes relax. - In interkinesis in some types of cells, the chromosomes remain condensed and the spindle does not break down. These cells move directly from cytokinesis into metaphase II

shugoshin

Protein that protects cohesin from being degraded by separase

germ cells

Reproductive (gamete) cells that give rise to sperm and ovum

telomeres

Telomeres are repeated DNA sequences and associated proteins located at the tips of whole linear chromosomes - protect and stabilize the chromosome ends. If a chromosome breaks, producing new ends, the chromosome is degraded at the newly broken ends. - Telomeres provide chromosome stability. Research shows that telomeres also participate in limiting cell division and may play important roles in aging and cancer

what is required for the cell to pass through the spindle assembly checkpoint?

Tension is generated at the kinetochore as the two conjoined chromatids are pulled in opposite directions by the spindle microtubules. This tension is required for the cell to pass through the spindle-assembly checkpoint. If one chromatid is attached to a microtubule but the other is not, no tension is generated, and the cell is unable to progress to the next stage of mitosis.

G1 phase

The first gap, or growth phase, of the cell cycle, consisting of the portion of interphase before DNA synthesis begins. - typically lasts several hours, longest phase - near the end: G1/S checkpoint - holds the cell in G1 until the cell has all the enzymes and proteins necessary for the replication of DNA. After this checkpoint has been passed, the cell is committed to divide.

G2 phase

The second growth phase of the cell cycle, consisting of the portion of interphase after DNA synthesis occurs. - G2/M checkpoint is passed only if the cell's DNA is completely replicated and undamaged. Unreplicated or damaged DNA can inhibit the activation of some proteins that are necessary for mitosis to take place. After the G2/M checkpoint has been passed, the cell is ready to divide and enters the M phase. - Although the length of interphase varies from cell type to cell type, a typical dividing mammalian cell spends about 10 hours in G1, 9 hours in S, and 4 hours in G2

S phase

The synthesis phase of the cell cycle; the portion of interphase during which DNA is replicated. - number of DNA molecules/chromatids doubles, but number of chromosomes stay the same - DNA replication in the S phase produces identical sister chromatids (one chromatid to two chromatids, with allele in identical spot)

Mendel's principle of segregation

The two members of a gene pair (alleles) segregate (separate) from each other in the formation of gametes. Half the gametes carry one allele, and the other half carry the other allele. - how the chromosomes separate in anaphase I

virus

Viruses are neither prokaryotic nor eukaryotic because they do not possess the structure of a cell. Viruses are actually simple structures composed of an outer protein coat surrounding nucleic acid - All known viruses can reproduce only within host cells, and their evolutionary relationship to cellular organisms is uncertain. - Their simplicity and small genomes make viruses useful for studying some molecular processes and for some types of genetic analyses. - No ribosomes

telophase I

a haploid set of chromosomes consisting of one homolog from each bivalent is located near each pole of the spindle - the spindle breaks down, the chromosomes enter the second meiotic division after only a limited uncoiling - chromosome replication never takes place between the two division

cohesin

a protein that holds chromatids together, is key to the behavior of chromosomes in mitosis and meiosis. - another type of structural maintenance of chromosomes (SMC) complex, related to condensins and SMCs that help bring about chromosome condensation in eukaryotes and chromosome segregation in bacteria. - ring-like structure that encircles and holds together the sister chromatids. It is loaded onto the chromosome in the S phase and persists through G2 and early mitosis. - In anaphase of mitosis, cohesin along the entire length of the chromosome is broken down by an enzyme called separase, allowing the sister chromatids to separate

to determine the number of chromosomes, count the number of ________

centromeres

anaphase

chromosomes move toward opposite spindle poles. - Chromosome movement is due to the disassembly of tubulin molecules at both the kinetochore end (called the + end) and the spindle-pole end (called the − end) of the spindle microtubule. Special proteins called molecular motors disassemble tubulin molecules and generate forces that pull the chromosome toward the spindle pole.

diploid vs haploid

diploid is two sets of chromosomes and haploid is one set of chromosomes

prokaryotic vs eukaryotic

eukaryotes: - contains nucleus and other membrane-bound organelles like mitochondria and chloroplast - genes are located on multiple, usually linear DNA molecules (multiple chromosomes) - DNA complexed with histones in order to create chromosomes - lots of DNA present prokaryotes: - does not contain nucleus or membrane-bound organelles, in which DNA is in contact with other cellular components thus affecting the regulation of gene expression - genes are generally located on a single circular molecule of double-stranded DNA—the chromosome of a prokaryotic cell. - DNA not complexed with histones, though histones are present in some archea

prophase II

events of interkinesis are reversed: the chromosomes recondense, the spindle re-forms, and the nuclear membrane once again breaks down.

anaphase I

marked by the separation of homologous chromosomes. - The two chromosomes of a homologous pair are pulled toward opposite poles. Although the homologous chromosomes separate, the sister chromatids remain attached and travel together

The female part of the flower, the ovary, contains diploid cells called _______, each of which undergoes meiosis to produce four haploid _______, only one of which survives. The nucleus of the surviving megaspore divides mitotically three times, producing a total of eight haploid nuclei that make up the female gametophyte, otherwise known as the embryo sac. Division of the cytoplasm then produces separate cells, one of which becomes the egg.

megasporocytes megaspores

The male part of the flower, the stamen, contains diploid reproductive cells called ________, each of which undergoes meiosis to produce four haploid ________. Each microspore divides mitotically, producing an immature pollen grain consisting of two haploid nuclei. One of these nuclei, called the tube nucleus, directs the growth of a pollen tube. The other, termed the generative nucleus, divides mitotically to produce two sperm cells. The pollen grain, with its two haploid nuclei, is the male gametophyte.

microsporocytes microspores

The production of gametes in a female animal, a process called _______, begins much as spermatogenesis does. Within the ovaries, diploid primordial germ cells divide mitotically to produce _______. Like spermatogonia, oogonia can undergo repeated rounds of mitosis or they can enter meiosis. When they enter prophase I, these still-diploid cells are called _______. Each primary oocyte completes meiosis I and divides.

oogenesis oogonia primary oocytes

2n = 4

original mother cell is diploid (2n) with a total of 4 chromosomes originally

In summary, crossing over shuffles alleles on the ________ chromosome into new combinations, whereas the random distribution of maternal and paternal chromosomes shuffles alleles on _______ chromosomes into new combinations. Together, these two processes are capable of producing tremendous amounts of genetic variation among the cells resulting from meiosis

same different

anaphase II

same as anaphase I: Sister chromatids separate and move as individual chromosomes toward opposite spindle poles.

Suppose a diploid European wild raspberry is completely heterozygous at all 14 of its chromosomes (2n = 14). How many different combinations of gametes can be produced by this European wild raspberry, assuming no homologous recombination between chromosomes?

since 2n = 14, n = 7 2^n = number of different combinations 2^7 = 128

The production of gametes in a male animal, a process called ________, takes place in the testes. There, diploid primordial germ cells divide mitotically to produce diploid cells called ________ . Each spermatogonium can undergo repeated rounds of mitosis, giving rise to numerous additional spermatogonia. Alternatively, a spermatogonium can initiate meiosis and enter prophase I. Now called a ________, the cell is still diploid because the homologous chromosomes have not yet separated. Each primary spermatocyte completes meiosis I, giving rise to two haploid ________ that then undergo meiosis II, with each producing two haploid ________. Thus, each primary spermatocyte produces a total of four haploid spermatids, which mature and develop into sperm.

spermatogenesis spermatogonia primary spermatocyte secondary spermatocytes spermatids

tetrad

structure containing 4 chromatids that forms during meiosis

meiosis I

termed the reduction division because the number of chromosomes per cell is reduced by half - separation of homologous chromosome pairs, and reduction of the chromosome number by half - includes independent assortment, segregation, crossing over

Why do homologs separate in anaphase I of meiosis, whereas chromatids separate in anaphase of mitosis and anaphase II of meiosis?

the forms of cohesin used in mitosis and meiosis differ. - At the beginning of meiosis, the meiosis-specific cohesin is found along the entire length of a chromosome's arms. The cohesin also acts on the chromosome arms of homologs at the chiasmata—the crossover points between homologous chromosomes—tethering two homologs together at their ends. - In anaphase I, cohesin along the chromosome arms is broken, allowing the two homologs to separate. However, cohesin at the centromere is protected by a protein called shugoshin, which means "guardian spirit" in Japanese. Because of the protective action of shugoshin, the centromeric cohesin remains intact and prevents the separation of the two sister chromatids during anaphase I. Shugoshin is subsequently degraded. At the end of metaphase II, the centromeric cohesin—no longer protected by shugoshin—breaks down, allowing the sister chromatids to separate in anaphase II, just as they do in mitosis

locus

the physical location of a gene on a chromosome

The second process of meiosis that contributes to genetic variation is...

the random distribution of chromosomes in anaphase I after their random alignment in metaphase I. - There are four ways in which the chromosomes in a diploid cell with three homologous pairs can migrate, producing a total of eight different combinations of chromosomes in the gametes

independent assortment

the random distribution of the pairs of genes on different chromosomes to the gametes - how the chromosomes align in metaphase I

to determine the number of DNA molecules, determine whether sister chromatids exist. - If sister chromatids ARE present, the number of DNA molecules is _________ the number of chromosomes. - If the chromosomes are unreplicated (DON'T contain sister chromatids), the number of DNA molecules equals the number of _________.

two times chromosomes