Ch.13 Meiosis and Sexual Life Cycles

clone

(1) A lineage of genetically identical individuals or cells. (2) In popular usage, an individual that is genetically identical to another individual.

mitosis vs. meiosis

(1) DNA replication. In MITOSIS, DNA replication occurs during interphase before mitosis begins In MEIOSIS, DNA replication occurs during interphase before meiosis I begins (2) Number of divisions In MITOSIS: One, including prophase, prometaphase, metaphase, anaphase, and telophase In MEIOSIS: Two, each including prophase, metaphase, anaphase, and telophase (3) Synapsis of homologous chromosomes In MITOSIS does not occur In MEIOSIS it occurs during prophase I along with crossing over between nonsister chromatids; resulting chiasmata hold pairs together due to sister chromatid cohesion (4) Number of daughter cells and genetic composition In MITOSIS, two, each diploid (2n) and genetically identical to the parent cell In MEIOSIS, four, each haploid (n), containing half as many chromosomes as the parent cell; genetically different from the parent cell and from each other (5) Role in the animal body MITOSIS enables multicellular adult to arise from zygote; produces cells for growth, repair, and, in some species, asexual reproduction. MEIOSIS produces gametes; reduces number of chromosome sets by half and introduces genetic variability among the gametes.

spores

(1) In the life cycle of a plant or alga undergoing alternation of generations, a haploid cell produced in the sporophyte by meiosis. A spore can divide by mitosis to develop into a multicellular haploid individual, the gametophyte, without fusing with another cell. (2) In fungi, a haploid cell, produced either sexually or asexually, that produces a mycelium after germination.

CHAPTER 13 KEY CONCEPTS

13.1 Offspring acquire genes from parents by inheriting chromosomes 13.2 Fertilization and meiosis alternate in sexual life cycles 13.3 Meiosis reduces the number of chromosome sets from diploid to haploid 13.4 Genetic variation produced in sexual life cycles contributes to evolution

haploid

A cell containing only one set of chromosomes (n). For humans, the haploid number is 23 (n=23). The set of 23 consists of the 22 autosomes plus a single sex chromosome. An unfertilized egg contains an X chromosome, but a sperm may contain an X or a Y chromosome.

diploid

A cell containing two sets of chromosomes (2n), one set inherited from each parent. For humans, the diploid number is 46 (2n=46), the number of chromosomes in our somatic cells.

recombinant chromosomes

A chromosome created when crossing over combines DNA from two parents into a single chromosome.

sex chromosomes

A chromosome responsible for determining the sex of an individual. The two distinct chromosomes referred to as X and Y are an important exception to the general pattern of homologous chromosomes in human somatic cells. Human females have a homologous pair of X chromosomes (XX), but males have one X and one Y chromosome (XY). Only small parts of the X and Y are homologous. Most of the genes carried on the X chromosome do not have counterparts on the tiny Y, and the Y chromosome has genes lacking on the X. Because they determine an individual's sex, the X and Y chromosomes are called sex chromosomes. The other chromosomes are called autosomes.

autosomes

A chromosome that is not directly involved in determining sex; not a sex chromosome.

gene

A discrete unit of hereditary information consisting of a specific nucleotide sequence in DNA (or RNA, in some viruses). Parents endow their offspring with coded information in the form of hereditary units called genes. The genes we inherit from our mothers and fathers are our genetic link to our parents, and they account for family resemblances such as shared eye color or freckles. Our genes program the specific traits that emerge as we develop from fertilized eggs into adults.

karyotype

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

gamete

A haploid reproductive cell, such as an egg or sperm. Gametes unite during sexual reproduction to produce a diploid zygote. In animals and plants, reproductive cells called gametes are the vehicles that transmit genes from one generation to the next. During fertilization, male and female gametes (sperm and eggs) unite, thereby passing on genes of both parents to their offspring.

alternation of generations sexual life cycle

A life cycle in which there is both a multicellular diploid form, the sporophyte, and a multicellular haploid form, the gametophyte; characteristic of plants and some algae. This type includes both diploid and haploid stages that are multicellular. The multicellular diploid stage is called the sporophyte. Meiosis in the sporophyte produces haploid cells called spores. Unlike a gamete, a haploid spore doesn't fuse with another cell but divides mitotically, generating a multicellular haploid stage called the gametophyte. Cells of the gametophyte give rise to gametes by mitosis. Fusion of two haploid gametes at fertilization results in a diploid zygote, which develops into the next sporophyte generation. Therefore, in this type of life cycle, the sporophyte generation produces a gametophyte as its offspring, and the gametophyte generation produces the next sporophyte generation.

meiosis

A modified type of cell division in sexually reproducing organisms consisting of two rounds of cell division but only one round of DNA replication. It results in cells with half the number of chromosome sets as the original cell. This type of cell division reduces the number of sets of chromosomes from two to one in the gametes, counterbalancing the doubling that occurs at fertilization. In animals, meiosis occurs only in germ cells, which are in the ovaries or testes. As a result of meiosis, each human sperm and egg is haploid (n=23). Fertilization restores the diploid condition by combining two haploid sets of chromosomes, and the human life cycle is repeated, generation after generation.

homologous chromosomes

A pair of chromosomes of the same length, centromere position, and staining pattern that possess genes for the same characters at corresponding loci. One homologous chromosome is inherited from the organism's father, the other from the mother. Also called homologs, or a homologous pair.

locus

A specific place along the length of a chromosome where a given gene is located.

fungi and protist sexual life cycle

A third type of life cycle occurs in most fungi and some protists, including some algae. After gametes fuse and form a diploid zygote, meiosis occurs without a multicellular diploid offspring developing. Meiosis produces not gametes but haploid cells that then divide by mitosis and give rise to either unicellular descendants or a haploid multicellular adult organism. Subsequently, the haploid organism carries out further mitoses, producing the cells that develop into gametes. The only diploid stage found in these species is the single-celled zygote.

sexual reproduction

A type of reproduction in which two parents give rise to offspring that have unique combinations of genes inherited from both parents via the gametes.

somatic cells

Any cell in a multicellular organism except a sperm or egg or their precursors. For example, humans have 46 chromosomes in their somatic cells—all cells of the body except the gametes and their precursors.

allele

Any of the alternative versions of a gene that may produce distinguishable phenotypic effects.

independent assortment

Because each pair of homologous chromosomes is positioned independently of the other pairs at metaphase I, the first meiotic division results in each pair sorting its maternal and paternal homologs into daughter cells independently of every other pair. In the case of humans (n=23), the number of possible combinations of maternal and paternal chromosomes in the resulting gametes is 2²³ , or about 8.4 million. Each gamete that you produce in your lifetime contains one of roughly 8.4 million possible combinations of chromosomes.

anaphase II

Breakdown of proteins holding the sister chromatids together at the centromere allows the chromatids to separate. The chromatids move toward opposite poles as individual chromosomes.

variation

Differences between members of the same species. Sons and daughters are not identical copies of either parent or of their siblings. Along with inherited similarity, there is also variation.

sister chromatid cohesion

Establishment of sister chromatid cohesion is the process by which chromatin-associated cohesin protein becomes competent to physically bind together the sister chromatids. In general, cohesion is established during S phase as DNA is replicated, and is lost when chromosomes segregate during mitosis and meiosis.

sporophyte

In organisms (plants and some algae) that have alternation of generations, the multicellular diploid form that results from the union of gametes. The sporophyte produces haploid spores by meiosis that develop into gametophytes.

gametophyte

In organisms (plants and some algae) that have alternation of generations, the multicellular haploid form that produces haploid gametes by mitosis. The haploid gametes unite and develop into sporophytes.

n

Represents the number of chromosomes in a single set

chiasmata

The X-shaped, microscopically visible region where crossing over has occurred earlier in prophase I between homologous nonsister chromatids. Chiasmata become visible after synapsis ends, with the two homologs remaining associated due to sister chromatid cohesion.

zygote

The diploid cell produced by the union of haploid gametes during fertilization; a fertilized egg.

meiosis I

The first division of a two-stage process of cell division in sexually reproducing organisms that results in cells with half the number of chromosome sets as the original cell. Meiosis I is called the reductional division because it halves the number of chromosome sets per cell—a reduction from two sets (the diploid state) to one set (the haploid state). The paired and replicated chromosomes are called bivalents or tetrads

prophase I

The first stage of mitosis, in which the chromatin condenses into discrete chromosomes visible with a light microscope, the mitotic spindle begins to form, and the nucleolus disappears but the nucleus remains intact. • Chromosomes begin to condense, and homologs loosely pair along their lengths, aligned gene by gene. • Paired homologs become physically connected to each other along their lengths by a zipper-like protein structure, the synaptonemal complex; this state is called synapsis. The paired and replicated chromosomes are called bivalents or tetrads. •Crossing over, a genetic rearrangement between nonsister chromatids involving the exchange of corresponding segments of DNA molecules, begins during pairing and synaptonemal complex formation, and is completed while homologs are in synapsis. • Synapsis has ended with the disassembly of the synaptonemal complex in mid-prophase, and the chromosomes in each pair have moved apart slightly. • Each homologous pair has one or more X-shaped regions called chiasmata (singular, chiasma). A chiasma exists at the point where a crossover has occurred. It appears as a cross because sister chromatid cohesion still holds the two original sister chromatids together, even in regions beyond the crossover point, where one chromatid is now part of the other homolog. • Centrosome movement, spindle formation, and nuclear envelope breakdown occur as in mitosis. • Microtubules from one pole or the other attach to the two kinetochores, protein structures at the centromeres of the two homologs. The homologous pairs then move toward the metaphase plate.

asexual reproduction

The generation of offspring from a single parent that occurs without the fusion of gametes (by budding, division of a single cell, or division of the entire organism into two or more parts). In most cases, the offspring are genetically identical to the parent. For example, single-celled eukaryotic organisms can reproduce asexually by mitotic cell division, in which DNA is copied and allocated equally to two daughter cells. The genomes of the offspring are virtually exact copies of the parent's genome.

life cycle

The generation-to-generation sequence of stages in the reproductive history of an organism. In the human and most other animals sexual life cycle, gametes are the only haploid cells. Meiosis occurs in germ cells during the production of gametes, which undergo no further cell division prior to fertilization. After fertilization, the diploid zygote divides by mitosis, producing a multicellular organism that is diploid.

germ cells

The only cells of the human body not produced by mitosis are the gametes, which develop from specialized cells called germ cells in the gonads—ovaries in females and testes in males.

synapsis

The pairing and physical connection of duplicated homologous chromosomes during prophase I of meiosis.

crossing over

The reciprocal exchange of genetic material between nonsister chromatids during prophase I of meiosis. In meiosis in humans, an average of one to three crossover events occur per chromosome pair, depending on the size of the chromosomes and the position of their centromeres.

genetics

The scientific study of heredity and hereditary variation.

meiosis II

The second division of a two-stage process of cell division in sexually reproducing organisms that results in cells with half the number of chromosome sets as the original cell. During the second meiotic division, meiosis II (sometimes called the equational division), the sister chromatids separate, producing haploid daughter cells. The mechanism for separating sister chromatids is virtually identical in meiosis II and mitosis.

metaphase I

The third stage of mitosis, in which the spindle is complete and the chromosomes, attached to microtubules at their kinetochores, are all aligned at the metaphase plate. • Pairs of homologous chromosomes are now arranged at the metaphase plate, with one chromosome in each pair facing each pole. • Both chromatids of one homolog are attached to kinetochore microtubules from one pole; those of the other homolog are attached to microtubules from the opposite pole.

heredity

The transmission of traits from one generation to the next.

fertilization

The union of haploid gametes to produce a diploid zygote.

bdelloid rotifers

While a few species are capable of reproducing asexually under unusual circumstances, animals that always reproduce asexually are quite rare. The best-established example, to date, is a group of microscopic animals called bdelloid rotifers (the "b" in "bdelloid" is silent).This group includes about 400 species that live in a great variety of environments around the world. They inhabit streams, lake bottoms, puddles, lichens, tree bark, and masses of decaying vegetation. Recent studies have provided convincing evidence that these animals reproduce only asexually and probably haven't engaged in sex in the 40 million years since their evolutionary origins! Does the discovery of the evolutionarily successful, asexually reproducing bdelloid rotifer cast doubt on the advantage of genetic variation arising from sexual reproduction? On the contrary, this group may be considered an exception that proves the rule. In studies of bdelloid rotifers, biologists have found mechanisms other than sexual reproduction that increase genetic diversity in these organisms. For example, they live in environments that can dry up for long periods of time, during which they can enter a state of suspended animation. In this state, their cell membranes may crack in places, allowing entry of DNA from other rotifers and even other species. Evidence suggests that this DNA can become incorporated into the genome of the rotifer, leading to increased genetic diversity. Taken as a whole, these studies support the idea that genetic variation is evolutionarily advantageous and that a different mechanism to generate genetic variation has evolved in bdelloid rotifers.

prophase II

• A spindle apparatus forms. • In late prophase II (not shown here), chromosomes, each still composed of two chromatids associated at the centromere, move toward the metaphase II plate.

telophase I and cytokinesis

• At the beginning of telophase I, each half of the cell has a complete haploid set of duplicated chromosomes. Each chromosome is composed of two sister chromatids; one or both chromatids include regions of nonsister chromatid DNA. • Cytokinesis (division of the cytoplasm) usually occurs simultaneously with telophase I, forming two haploid daughter cells. • In animal cells like these, a cleavage furrow forms. (In plant cells, a cell plate forms.) • In some species, chromosomes decondense and nuclear envelopes form. • No chromosome duplication occurs between meiosis I and meiosis II.

anaphase I

• Breakdown of proteins responsible for sister chromatid cohesion along chromatid arms allows homologs to separate. • The homologs move toward opposite poles, guided by the spindle apparatus. • Sister chromatid cohesion persists at the centromere, causing chromatids to move as a unit toward the same pole.

telophase II and cytokinesis

• Nuclei form, the chromosomes begin decondensing, and cytokinesis occurs. • The meiotic division of one parent cell produces four daughter cells, each with a haploid set of (unduplicated) chromosomes. • The four daughter cells are genetically distinct from one another and from the parent cell.

metaphase II

• The chromosomes are positioned at the metaphase plate as in mitosis. • Because of crossing over in meiosis I, the two sister chromatids of each chromosome are not genetically identical. • The kinetochores of sister chromatids are attached to microtubules extending from opposite poles.