BIO 360 CCell 3B

What are the stages of meiosis?

1. Interphase (2n)

2. Prophase I: Crossing over occurs.

3. Metaphase I

4. Anaphase I: Homologous chromosomes pull apart

5. Telophase I followed by cytokinesis

6. Prophase II: begins with two diploid (2n)

7. Metaphase II

8. Anaphase II: Sister chromatids pull apart

9. Telophase II followed by cytokinesis. Results in four 1n cells

How many total human chromosomes are found in cells immediately after S-phase, mitosis, or immediately after meiosis?

S-phase: 46 (2n)

Mitosis: 46 (2n)

Meiosis: 23 (1n)

What is cross-over? When does cross-over occur in meiosis?

A cross-over is when homologous chromosomes exchange segments of their DNA, creating new combinations of alleles and thus increasing genetic diversity. This occurs in prophase I of meiosis.

What are sister chromatids? Are sister chromatids and homologous pairs the same thing?

Sister chromatids and homologous pairs are not the same thing. Sister chromatids are two identical copies of a single chromosome joined at the centromere while homologous pairs are two different chromosomes, one from each parent, that carry the same genes but may have different alleles.

What are the major differences between mitosis and meiosis?

Mitosis yields 2 diploid copies of exact copies of original cells. Meiosis yields 4 haploid cells of which are genetically different than the original cell. 

What are the fundamental principles of the chromosomal theory of inheritance?

The chromosome theory of inheritance describes how the transmission of chromosomes account for mendelian patterns of inheritance (law of segregation and law of independent assortment). It is based upon the following fundamental principles:

1. Chromosomes (DNA) contain the genetic material

2. Chromosomes are replicated and passed from parent to offspring.

3. The nuclei of most eukaryotic cells contain chromosomes found in homologous pairs (diploid).

4. During gamete formation, other (nonhomologous) chromosomes segregate independently.

5. Each parent contributes one set of chromosomes to its offspring.

How do Mendel’s laws of segregation and independent assortment relate to the chromosomal theory of inheritance?

Chromosomal theory of inheritance states how chromosomes are passed to offspring and how genetic diversity increases when doing so.

Law of Segregation: Two alleles separate during gamete formation, then unite (one from each parent) at fertilization.

-Relates because theory states homologous pairing and segregation of chromosomes occurs during meiosis. Thus, alleles separate but will eventually come together upon fertilization.

Law of Independent Assortment: Two different genes will randomly assort their alleles during the formation of haploid cells.

-Relates because theory states nonhomologous chromosomes randomly segregate during gamete formation and have no set “configuration.”

What are some of the ways that sex is determined in eukaryotes that is different from humans?

In bees, if an egg is unfertilized (1n) it becomes a male but if it is fertilized (2n) it becomes a female

In birds males are homogametic (ZZ) and females are heterogametic (ZW)

Regarding meiosis, what does 1n and 2n refer to?

Haploid (1n) and diploid (2n). Haploid means containing a single set of chromosomes while diploid means containing two sets of chromosomes.

What does it mean when it is said that most eukaryotes are heterogamous?

Heterogamous means that they produce gametes that are morphologically different. In human males 4 sperm are produced and in females 1 egg cell and 2 polar bodies (one haploid and one diploid) are produced.

The process of forming gametes is called [...]

gametogenesis

How does genetic reshuffling occur in eukaryotes?

Homologous eukaryotic chromosomes undergo independent assortment and genetic recombination (aka crossing over).

The egg or ovum stores a large amount of [...]

nutrients

What is the difference between the final number of gametes formed between male and female gametogenesis?

Male: 4 sperm

Female: 1 egg and two polar bodies (one diploid and one haploid)