Ch 13 Meiosis and Sexual Life Cycles

How many times does the cell divide in meiosis?

2

How many chromosomes are in each cell at the end of the first meiotic division? Are the resultant daughter cells haploid, or diploid?

3; haploid

How many daughter cells are formed?

4

independent assortment of chromosomes

At metaphase I, the homologous pairs, each consisting of one maternal and one paternal chromosome, are situated at the metaphase plate. Each pair may orient with either its maternal or paternal homolog closer to a given pole—its orientation is as random as the flip of a coin. Thus, there is a 50% chance that a particular daughter cell of meiosis I will get the maternal chromosome of a certain homologous pair and a 50% chance that it will get the paternal chromosome. 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. This is called independent assortment. Each daughter cell represents one outcome of all possible combinations of maternal and paternal chromosomes.

crossing over

Crossing over begins very early in prophase I as homologous chromosomes pair loosely along their lengths. Each gene on one homolog is aligned precisely with the corresponding gene on the other homolog. In a single crossover event, the DNA of two nonsister chromatids—one maternal and one paternal chromatid of a homologous pair—is broken by specific proteins at precisely corresponding points, and the two segments beyond the crossover point are each joined to the other chromatid. Thus, a paternal chromatid is joined to a piece of maternal chromatid beyond the crossover point, and vice versa. In this way, crossing over produces chromosomes with new combinations of maternal and paternal alleles.

Explain the physical events of crossing over.

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. A chiasma exists at the point where a crossover has occurred.

Figure 13.9, mitosis vs meiosis

See page 256 in text for the labeled figure.

What are three things that can be determined from karyotype?

Size of the chromosome, position of the centromere, and pattern of the stained bands

Where are the gametes of an animal produced?

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.

random fertilization

The random nature of fertilization adds to the genetic variation arising from meiosis. In humans, each male and female gamete represents one of about 8.4 million possible chromosome combinations due to independent assortment. The fusion of a male gamete with a female gamete during fertilization will produce a zygote with any of about 70 trillion diploid combinations.

How does a somatic cell compare to a gamete in terms of chromosome number?

Unlike somatic cells, gametes contain a single set of chromosomes. Such cells are called haploid cells, and each has a haploid number of chromosomes (n). For humans, the haploid number is 23.

Define chromosome. How many are in human cells?

a cellular structure carrying genetic material, found in the nucleus of eukaryotic cells, and each chromosome consists of one very long DNA molecule and associated proteins; 46

gene

a discrete unit of hereditary info consisting of a specific nucleotide sequence in DNA (or RNA, in some viruses)

gamete

a haploid reproductive cell, such as an egg or sperm; unite during sexual reproduction to produce a diploid zygote

alternation of generations

a life cycle in which there is both a multicellular diploid form; characteristic of plants and some algae

What is the purpose of 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, producing gametes, and introducing genetic variability.

homologous chromosomes

a pair of chromosomes of the same length, centromere position, and staining pattern

What are homologs (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

locus

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

What is a somatic cell?

any cell in a multicellular organism except a sperm or egg or their precursors

What are alleles?

any of the alternative versions of a gene that may produce distinguishable phenotypic effects (allele for freckles)

What type of reproduction will result in genetically identical offspring?

asexual reproduction

liver cell

diploid

skin cell

diploid

somatic cell

diploid

zygote

diploid

However, meiosis always begins with cells that are _______ , and as a result of meiosis, daughter cells are formed that are always _______. These cells can be gametes (in animals) or spores (in plants).

diploid; haploid

How is the arrangement of chromosomes different from metaphase of mitosis?

during mitosis, individual chromosomes line up at the metaphase plate; during meiosis, pairs of homologous chromosomes line up at the metaphase plate

How is a karyotype prepared?

from isolated somatic cells, which are treated with a drug to stimulate mitosis and then grown in culture for several days. Cells arrested in metaphase, when chromosomes are most highly condensed, are stained and then viewed with a microscope equipped with a digital camera. A photograph of the chromosomes is displayed on a computer monitor, and the images of the chromosomes are arranged into pairs according to their appearance.

egg cell

haploid

gamete.

haploid

sex cell

haploid

sperm

haploid

karyotype

images of the chromosomes that are arranged in pairs, starting with the longest chromosomes

There are two divisions in meiosis. What will separate in the first division in meiosis I?

in meiosis I, homologous chromosomes seperate

What are the three ways that sexually reproducing organisms bring about new alleles by changes in DNA?

independent assortment of chromosomes, crossing over, random fertilization

By what process are gametes produced?

meiosis

During which division is the chromosome number reduced?

meiosis I

number of DNA replications

mitosis: 1 meiosis: 1; occurs during interphase before meiosis I begins

number of divisions

mitosis: 1 meiosis: 2

number of daughter cells

mitosis: 2 meiosis: 4

chromosome number of daughter cells

mitosis: 2n meiosis: n

role in the animal body

mitosis: enables multicellular adult to arise from zygote; produce cells for growth, repair, and in some species, asexual reproduction meiosis: produces gametes; reduces number of chromosomes by half and introduces genetic variability among the gametes

What is the chromosome number?

n

How many times do the chromosomes duplicate?

none

autosome

not directly involved in determining sex; not a sex chromosome; 22 in humans

Synapsis and crossing over are unique to meiosis. During what specific phase do these occur?

prophase I

sex chromosome

responsible for determine the sex of an individual; 1 in humans

During meiosis I, homologous chromosomes separate. What separates during meiosis II?

sister chromatids separate during this process

What are the events of prophase I?

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 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.

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

What occurs in synapsis?

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

What is crossing over?

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

What happens to chromosome number in meiosis?

they are reduced by half

What is the purpose of meiosis??

to reduce the number of sets of chromosomes from two to one in gametes

T or F: In meiosis, the DNA is replicated during interphase, followed by two divisions. The first division is meiosis I.

true

sexual reproduction

two parents give rise to offspring that have unique combinations of genes inherited from both parents via the gametes