BioAP Chapter 13 Meiosis and Sexual Life Cycles

Karyotype

A display of condensed chromosomes arranged in pairs, starting with largest to shortest. This can be used to screen for abnormal numbers of chromosomes or defective chromosomes associated with certain congenital disorders, such as Down syndrome.

Locus

A genes specific location along the length of a chromosome.

Clone

A group of genetically identical individuals.

Tetrad

A paired set of homologous chromosomes, each composed of two sister chromatids. Form during prophase I of meiosis.

Asexual Reproduction

A single individual who is the sole parent and passes copies of all its genes to its offspring. Have offspring that are exact copies of itself. (ex: single-celled eukaryotic organisms, multi-cellular organisms such as hydra which are related to jellyfish)

Process of Meiosis

A special type of cell division.

How are spores unlike gametes?

A spore gives rise to a multicellular individual without fusing with another cell; divides mitotically to generate a multicellular haploid stage called the gametophyte.

Spores

Haploid cells, the product of meiosis in sporophytes. Give rise to multicelluar individuals without fusing with another cell.

Describe the process of synapsis during prophase I and explain how genetic recombination occurs.

In synapsis, a synaptonemal complex (zipper-like protein structure) forms between homologues, holding them tightly together along their lengths. Crossing over occurs, so when this complex disassembles, each chromosome pair is a tetrad.

How is inherited information passed on?

In the form of each gene's specific sequence of nucleotides

PLANTS AND SOME ALGAE (Haploid-Diploid Life Cycle/Alternation of Generations)

→ Spirogyra A) This life cycle is typical of eukaryotes. The organism spends the majority of its life with half the count of chromosomes it can have. B) Includes both diploid and haploid multicellular stages. The sporophyte undergoes meiosis producing spores; goes from diploid to hapoloid cells. Next, the spore undergoes mitotic division resulting in the gametophyte which makes gametes by mitosis; goes from haploid cells to a multicellular haploid stage. Haploid gametes undergo fertilization producing the next sporophyte generation. So, the sporphyte generation produces gametophyte offspring, and the gametopyte generation produces the next sporophyte generation. In other words, an alternation of generations is happening.

Process of Fertilization

The fusion of sperm and egg.

Life Cycle

The generation-to-generation sequence of stages in the reproductive history of an organism, from conception to production of its own offspring.

Chromosome Sets

The number of chromosomes in a single set is represented by n. There are diploid cells and haploid cells, any cell with two chromosome sets or a single set respectively.

Synapsis

The pairing of replicated homologous chromosomes during prophase I of meiosis.

Meiosis

The process by which a cell splits into gametes. A two-stage type of cell division in sexually reproducing organisms that results in cells with half the chromosome number of the original cell.

Crossing Over

The reciprocal exchange of genetic material between nonsister chromatids during prophase I of meiosis.

Zygote

The resulting fertilized egg from a haploid sperm cell from the father fusing with a haploid ovum from the mother. It is a diploid because it contains both halves.

Genetics

The scientific study of heredity and hereditary variation.

What do our genes program?

The specific traits that emerge as we develop from fertilized eggs into adults

Fertilization

The start of the human life cycle when the father's sperm and mother's ovum fuse.

Inheritance/Heredity

The transmission of traits from one generation to the next.

Homologous Chromosomes/Homologues

The two chromosomes composing a pair have the same length, centromere position and staining pattern. Both carry genes containing the same inherited characters.

Sexual Reproduction

Two parents give rise to offspring that have unique combinations of genes inherited from the two parents.

Gametophyte

When a spore undergoing the life cycle of alternate generations divides mitotically to generate a multicelluar haploid stage.

Sex Chromosomes

X and Y chromosomes which determine an individual's sex.

In the type of life cycle that occurs in humans and most other animals, are gametes the only haploid cells?

Yes.

Distinguish between the following pairs of terms: a. Heredity and variation b. Genetics and genes c. Somatic cells and gametes d. Karyotype and homologous chromosomes e. Sex chromosomes and autosomes

a. Heredity is the act of transmitting genes from one generation to the next, and variation is the resulting difference in appearance between offspring, parents, and siblings. b. Genetics is the scientific study of heredity and hereditary variation, and genes are these hereditary, or coded information. c. Somatic cells are any other cell than a gamete (reproductive cells). d. A karyotype is a display of condensed chromosomes arranged in pairs; these chromosome pairs are homologous. e. Autosomes are any other chromosome than a sex chromosome.

ANIMALS (Diploid Life Cycle)

→ Humans (prime example b/c we have 46 chromosomes) A) This life cycle occurs in humans and most other animals. B) Gametes are the only haploid cells. Meiosis occurs during the production of gametes. These gametes do not undergo further cell division before fertilization. The diploid zygote divides by mitosis, producing a diploid multicellular organism.

MOST FUNGI AND SOME PROTISTS (Haploid Life Cycle)

→ Some algae A) This life cycle occurs in plants, most fungi and some protists; or diploid organisms. B) Gametes fuse and form a diploid zygote, then meiosis goes down without this diploid offspring developing. Meiosis produces haploid cells that divide by mitosis and result in a haploid multicellular adult organism. This haploid organism carries out mitosis and produces cells that develop into gametes. The only diploid stage in these species is the single-celled zygote.

Explain the differences between homologous chromosomes, sister chromatids, nonsister chromatids, and chromosome sets.

(*Figure 13.4) - Sister chromatids are (mostly) identical, it's basically a copy of the original chromatid. - Homologous chromosomes only share the same size and shape--they may contain the same genes, but they may or may not be the same version in each chromosome of the pair. - Nonsister chromatids are any two chromatids in a pair of homologous chromosomes that are not sister chromatids. - Chromosome sets may be either diploid or haploid, any cell with two chromosome sets (2n) or a single set (n) respectively.

How are the X and Y chromosomes different?

- Only small parts of the X and Y are homologous - Most genes carried on the X chromosome do not have counterparts on the tiny Y - Y chromosome has genes lacking on the X

Describe three events that occur during meiosis I but not during mitosis.

1. Synapsis and crossing over in Prophase I: Homologous chromosomes physically connect and exchange genetic information. 2. At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes. 3. In Anaphase I, homologous chromosomes, not sister chromatids, separate.

Alternation of Generations

A cycle of life that plants and other species of algae exhibit that has both diploid and haploid multicellular stages.

List the phases of meiosis I and meiosis II and describe the events characteristic of each phase. Diagram and labeling will help.

A) INTERPHASE Duplicate genetic material B) MEIOSIS I: Separates Homologous Chromosomes i. Prophase I Duplicated chromatin condenses. Each homologous chromosome is made up of 2 pairs of sister chromatids. In crossing over, DNA molecules in nonsister chromatids break at corresponing places and then rejoin to the other's DNA. ii. Metaphase I Tetrads, the pairs of homologous chromosomes, line up at the the metaphase plate. iii. Anaphase I Homologous pairs move toward the poles; sister chromatids remain attached at the centromere. iv. Telophase I and Cytokinesis 2 daughter cells are formed, each containing only one chromosome of the homologous pair. C) MEIOSIS II: Separates Sister Chromatids i. Prophase II A spindle apparatus forms and chromosomes (each still made up of 2 chromatids) move toward the metaphase II plate. No DNA is replicated. ii. Metaphase II Chromosomes line up on the metaphase plate. Due to crossing over, the 2 sister chromatids of each chromosome are not genetically identical. iii. Anaphase II The centrosomes of each chromosome separate, as do the sister chromatids which move separately toward opposite poles. iv. Telophase II and Cytokinesis Cell division is completed resulting in 4 haploid daughter cells.

Distinguish among the three life-cycle patterns characteristic of eukaryotes, and name one organism that displays each pattern.

ANIMALS (Diploid Life Cycle): → Humans Gametes are the only haploid cells. Meiosis occurs during the production of gametes. These gametes do not undergo further cell division before fertilization. The diploid zygote divides by mitosis, producing a diploid multicellular organism. PLANTS AND SOME ALGAE (Haploid-Diploid Life Cycle/Alternation of Generations): → Spirogyra Includes both diploid and haploid multicellular stages. There is an alternation between the sporphyte (multicelluar diploid stage) and gametophyte (multicelluar haploid stage) generations. MOST FUNGI AND SOME PROTISTS (Haploid Life Cycle): → Some algae Opposite of Diploid Life Cycle in animals b/c after gametes fuse to form a diploid zygote, meiosis goes down without a multicellular diploid offspring developing first. The goal here is for meiosis to produce haploid cells which, through mitosis, will give rise to a haploid multicellular adult organism.

Chiasma

An X-shaped region in each tetrad; represents homologous chromatids that have exchanged genetic material through crossing over during meiosis.

Somatic Cell

Any cell other than a gamete.

Diploid Cell

Any cell with two chromosome sets; somatic cells. For humans, the diploid number is 46 (n = 46).

Autosomes

Any other chromosome than a sex chromosome.

Nonsister Chromatids

Any two chromatids in a pair of homologous chromosomes that are not sister chromatids.

Distinguish between asexual and sexual reproduction.

Asexual reproduction requires one parent and its offspring are clones of itself. On the flip side, sexual reproduction involves two parents which pass on their genes to produce unique offspring.

In what ways do chromosomes differ? How are they distinguished?

Because chromosomes differ in size, centromere position, and pattern of colored bands produced by certain stains, they can be distinguished from one another by microscopic examination when sufficiently condensed.

Haploid Cell

Cells with only a single chromosome set; gametes (sperm and egg cells). For humans, the haploid number is 23 (n = 23).

Homologous Chromosomes

Chromosome pairs of the same length, centromere position, and staining pattern that possess alleles of the same genes at corresponding loci. One homologous chromosome is inherited from the organism's father, the other from the mother.

Genes

Coded information in the form of hereditary units. These are segments of DNA, and the inherited information is coded into the nucleotides.

Zygotes are diploid, why?

Contains two haploid sets of chromosomes of genes representing the maternal and paternal family lines.

Explain how diploid and haploid cells differ from each other. State which cells in the human body are diploid and which are haploid.

Diploid cells contain two chromosome sets while haploid cells only contain one. Somatic cells are diploid and gametes (sperm and egg cells) are haploid.

How many chromosomes do humans have?

Every organism has a characteristic number of chromosomes. Humans have 46 chromosomes.

Why are the X and Y chromosomes an important exception to the general pattern of homologous chromosomes in human somatic cells?

Females have a homologous pair of X chromosomes (XX), but males only have one X and one Y chromosome (XY).

Explain why fertilization and meiosis must alternate in all sexual life cycles.

Fertilization and meiosis alternate in sexual life cycles, offsetting each other's effects on the chromosome number and thus perpetuating a species' chromosome count. → Meiosis maintains the normal diploid number by halving the chromosome count to create gametes (haploid cells). → The diploid number is restored through fertilization.

Explain how independent assortment, crossing over, and random fertilization contribute to genetic variation in sexually reproducing organisms.

Independent assortment- The number of possible combinations when chromosomes sort independently during meiosis is 2^n where n is the haploid number of the organism. So, in humans b/c n=23, there are 2^23 or about 8 million possible combinations of maternal and paternal chromosomes in the resulting gametes. Crossing over- Begins very early in prophase I, as homologous chromosomes pair loosely along their lengths. The DNA molecules of two nonsister chromatids (one maternal and one paternal chromatid of a homologous pair) are broken at the same place and then rejoined to each other's DNA. This allows the mixture of genetic material from each parent with new recombinant chromosomes. Random fertilization- The random nature of fertilization adds to the genetic variation, with a truly astronomical number of possibilities. In humans, the fusion of a single male gamete with a single female gamete will produce a zygote with any of over 70 trillion diploid combinations (2^23 * 2^23). You really are unique bitch berry.

Recombinant Chromosomes

Individual chromosomes that carry genes (DNA) derived from two different parents.

What do the processes of meiosis and fertilization do?

Maintain a species' chromosome count during the sexual life cycle.

Reductional Division

Meiosis I, because it halves the number of chromosome sets per cell.

Sporophyte

Multicelluar diploid stage of an organism undergoing the life cycle of alternation of generations.

Explain why heritable variation is crucial to Darwin's theory of evolution by natural selection.

New combinations of genes allow for more possibilities of offsprings' traits, and a higher chance to find the best traits for environmental success.

Explain in general terms how traits are transmitted from parents to offspring.

Offspring acquire genes from parents by inheriting chromosomes; through heredity/inheritance.

Variation

Offspring differ somewhat in appearance from parents and siblings.

What do the tens of thousands of genes we inherit from our parents constitute?

Our genome.

Sister Chromatids

Replicated forms of a chromosome joined together at the centromere and eventually separated during mitosis or meiosis II.

Gametes

Reproductive cells , which are the vehicles that transmit genes from one generation to the next in animals and plants. The only cells of the human body NOT produced by mitosis; develop in the gonads--ovaries in females and testes in males.

Independent Assortment

The act of each pair of homologous chromosomes sorting its maternal and paternal monologues into daughter cells independently of every other pair. Occurs at metaphase of meiosis I.

Genome

The combination of all of our genetic material and genes.