Chapter 13 - Meiosis and Sexual Life Cycles

Question 1

What is heredity?

Answer 1

The transmission of trains from one generation to the next; inheritance

Question 2

What is genetics?

Answer 2

The scientific study of heredity and hereditary variation

Question 3

What is variation?

Answer 3

Variation is demonstrated by the differences in appearance that offspring show from parents and siblings

Question 4

What are genes?

Answer 4

Units of hereditary code; coded information in the form of hereditary units made up of DNA; (genes) program the specific traits that emerge as we develop from fertilized eggs into adults

Question 5

What are gametes?

Answer 5

Reproductive cells that act as vehicles to transmit genes from one generation to the next; sperm and eggs

Question 6

How is DNA packaged?

Answer 6

In chromosomes

Question 7

How many chromosomes do humans have?

Answer 7

46 in their somatic cells- 2 pairs of 23, one from mom and one from dad

Question 8

What are somatic cells?

Answer 8

Non-reproductive cells; all cells except gametes and their precursors

Question 9

What is the locus?

Answer 9

A genes specific location along the length of a chromosome

Question 10

What is asexual reproduction?

Answer 10

A single individual is the sole parent and passes copies of all its genes to its offspring without the fusion of gametes; i.e. single celled eukaryotic organisms

DNA is copied and allocated equally to two daughter cells and the genomes of the offspring are virtually exact copies of the parent’s genome

*Sometimes genetic differences arise in asexually reproducing organisms as a result of mutations

Question 11

What is a clone?

Answer 11

A group of genetically identical individuals; an individual that reproduces asexually gives rise to a clone

Question 12

What is sexual reproduction?

Answer 12

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

Question 13

What is a life cycle?

Answer 13

The generation to generation sequence of stages in the reproductive history of an organism

Question 14

What is a karyotype?

Answer 14

An ordered display of the pairs of chromosomes in a cell

Question 15

What are homologous chromosomes?

Answer 15

The two chromosomes in each pair; the pair have the same length, centromere position and staining pattern; also called homologs

Homologous pairs are the same length and shape and carry genes controlling the same inherited characteristics

• Each pair of homologous chromosomes include one chromosome from each parent

Question 16

What are sex chromosomes?

Answer 16

Determine the sex of an individual; x and y chromosomes

Females - xx

Males - xy

Question 17

What are autosomes?

Answer 17

The remaining 22 pairs of chromosomes

Question 18

What is a diploid cell?

Answer 18

2n; a cell with two sets of chromosomes

For humans, the diploid number is 46 (2n = 46)

Question 19

What is a haploid cell?

Answer 19

Gametes contain a single set of chromosomes; each haploid cell has a haploid number of chromosomes (n)

For humans, the haploid number is 23 (n = 23)

The set of 23 consists of 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)

Question 20

What is fertilization?

Answer 20

The union of gametes (sperm and egg) culmination from the fusion of their nuclei

Question 21

What is a zygote?

Answer 21

A fertilized egg; has one set of chromosomes from each parent

(diploid because it contains two haploid sets of chromosomes bearing genes representing the maternal and paternal family lines)

*The zygote produces somatic cells by mitosis and develops into an adult

Question 22

When are haploid gametes produced?

Answer 22

At sexual maturity, the ovaries and testes produce haploid gametes

Gametes are the only types of human cells produces by meiosis

Question 23

What is a key to genetic variation that is common to all organisms that reproduce sexually?

Answer 23

The alternation of meiosis and gertilization

Question 24

How do the three types of sexual life cycles differ?

Answer 24

The timing of meiosis and fertilization in the life cycle varies depending on the species; these variations can be grouped into three main types of life cycles

Question 25

What is alternation of generations?

Answer 25

Plants and some species of algae exhibit this alternation of generations

• Includes both diploid and haploid stages that are multicellular; the multicellular diploid stage is called the sporophyte
• The diploid organism, called the sporophyte, makes haploid spores by meiosis
• Each spore grows by mitosis into a haploid organism called a gametophyte; a gametophyte makes haploid gametes by mitosis
• Fertilization of gametes results in a diploid sporophyte

Question 26

What is the sexual life cycle in animals?

Answer 26

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

Question 27

What is the third type of sexual life cycle that takes place in most fungi and some other protists?

Answer 27

The only diploid stage is the single celled zygote; there is no multicellular diploid stage

The zygote produces haploid cells by meiosis; each haploid cell grows by mitosis into a haploid multicellular organism

The haploid adult produces gametes by mitosis

Question 28

What type of cells can divide by mitosis?

Answer 28

Either haploid or diploid cells can divide by mitosis, depending on the type of life cycle

• However, only diploid cells can undergo meiosis
• In all three life cycles, the halving and doubling of chromosomes contributes to genetic variation in offspring

Question 29

What does meiosis reduce?

Answer 29

Meiosis reduces the number of chromosome sets from diploid to haploid

Question 30

What is meiosis?

Answer 30

Preceded by the duplication of chromosomes (S phase); this single duplication takes place in two consecutive cell divisions meiosis I and meiosis II

These two divisions result in four daughter cells, each with only half as many chromosomes as he parent cell - one set rather than two

Question 31

What are the basic stages of meiosis?

Answer 31

• Chromosomes duplicate during interphase
• The resulting sister chromatids are closely associated along their lengths; sister chromatid cohesion
• The chromatids are sorted into four haploid daughter cells

Question 32

What happens in meiosis I?

Answer 32

Homologous chromosomes separate

Question 33

What happens in meiosis II?

Answer 33

Sister chromatids separate

Question 34

What are the four stages of meiosis I and what happens at each stage?

Answer 34

Prophase I (duplicated homologous chromosomes pair and exchange segments)

• Centrosome movement, spindle formation, and nuclear envelope breakdown occur as in mitosis; chromosomes condense progressively throughout this phase
• During early prophase I, each chromosome pairs with its homolog, aligned gene by gene, and crossing over (sharing of genetic information) occurs; the DNA molecules of non sister chromatids are broken by proteins and are rejoined to each other
• Each homologous pair has one or more x-shaped regions called chiasmata, where cross overs have occurred
• Later in prophase I, microtubules from one pole or the other will attach to the two kinetochores, one at the centromere of each homolog; the homologous pairs will then move toward the metaphase plate

Metaphase I (chromosomes line up by homologous pairs)

• 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

Anaphase I (each pair of homologous chromosomes separate)

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

Telophase I and Cytokinesis (two haploid cells form; each chromosome still consists of two sister chromatids)

• When telophase I begins, each half of the cell has a complete haploid set of duplicated chromosomes; each chromosome is composed of two sister chromatids; one of both chromatids include regions of non-sister chromatid DNA
• Cytokinesis usually occurs simultaneously with telophase I, forming two haploid daughter cells
• In animal cells like these, a cleavage furrow forms (plants = cell wall)

*No chromosome duplication occurs between meiosis I and meiosis II

Question 35

What are the four stages of meiosis II and what happens at each stage?

Answer 35

The second round of cell division where the sister chromatids finally separate; four haploid daughter cells result, containing unduplicated chromosomes

Prophase II

• A spindle apparatus forms
• In late prophase II, chromosomes, each still composed of two chromatids associated at the centromere, move toward the metaphase II plate

Metaphase II

• The chromosomes are positioned at the metaphase place 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

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

Telophase II and Cytokinesis

• Nuclei form, the chromosomes being 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