Unit 2: Stages of Mitosis and Meiosis

Midterm 2

What is interphase in meiosis I?

The DNA in the cell is copied, resulting in two identical full sets of chromosomes. Outside of the nucleus are two centrosomes, each containing a pair of centrioles; these structures are critical for the process of cell division. During interphase, microtubules extend from these centrosomes.

What happens in prophase I in meiosis I?

The copied chromosomes condense into X-shaped structures that can be easily seen under a microscope. Each chromosome is composed of two sister chromatids containing identical genetic information. The chromosomes pair up so that both copies of chromosome 1 are together, and both copies of 2 are together, and so on. The pairs of chromosomes may exchange bits of DNA in a process called recombination or crossing over. At the end of prophase I, the membrane around the nucleus in the cell dissolves away, releasing the chromosomes. The meiotic spindle, consisting of microtubules and other proteins, extends across the cell between the centrioles. During prophase I, homologous chromosomes pair up, overlap, and exchange genetic material through crossing over, while the nuclear membrane breaks down and the meiotic spindle forms.

What happens in metaphase I of meiosis I?

The chromosome pairs line up next to each other along the centre (equator) of the cell. The centrioles are now at opposite poles of the cell with the meiotic spindles extending from them. The meiotic spindle fibres attach to one chromosome of each pair.

What happens in anaphase I of meiosis I? 

The pair of chromosomes are then pulled apart by the meiotic spindles, which pull one chromosome to one pole of the cell and the other chromosome to the opposite pole. In meiosis I the sister chromatids stay together; this is different from what happens in mitosis and meiosis II.

What happens in telophase I of meiosis I?

The chromosomes complete their move to the opposite poles. At each pole of the cell, a full set of chromosomes gathers together. A membrane forms around each set of chromosomes to create two new nuclei.

What happens in cytokinesis I of meiosis I?

The single cell then pinches in the middle to form two separate daughter cells, each containing a full set of chromosomes within a nucleus.

What happens in prophase II of meiosis II?

It has half the amount of DNA as the cell that began meiosis.

Now there are two daughter cells, each with 23 chromosomes (23 pairs of chromatids). In each of the two daughter cells, the chromosomes condense again into visible X-shaped structures that can be easily seen under a microscope. The membrane around the nucleus in each daughter cell dissolves away, releasing the chromosomes. The centrioles duplicate. The meiotic spindle forms again.

What happens in metaphase II of meiosis II?

In each of the two daughter cells, the chromosomes (pair of sister chromatids) line up end-to-end along the equator of the cell. The centrioles are now at opposite poles in each of the daughter cells. Meiotic spindle fibres at each pole of the cell attach to each of the sister chromatids.

What happens in anaphase II of meiosis II?

The sister chromatids are then pulled to opposite poles due to the action of the meiotic spindle. The separated chromatids are now individual chromosomes.  This separation ensures that each daughter cell will receive an equal number of chromosomes during cytokinesis.

What happens in telophase II of meiosis II?

The chromosomes complete their move to the opposite poles of the cell. At each pole of the cell, a full set of chromosomes gathers together. A membrane forms around each set of chromosomes to create two new cell nuclei.

What happens in cytokinesis II of meiosis II?

This is the last phase of meiosis; however, cell division is not complete without another round of cytokinesis.

Once cytokinesis is complete, there are four granddaughter cells, each with half a set of chromosomes (haploid):

• In people with XY chromosomes, these four cells are all sperm cells.

• In people with XX chromosomes, one of the cells is an egg cell, while the other three are polar bodies (small cells that do not develop into eggs).

Comparison and contrast of Meiosis to Mitosis:

Both—separate sister chromatids in meiosis, not mitosis—synapsis of chromosomes and crossing over occur in prophase I, reducing chromosome number in meiosis.

Sexual reproduction stage, reduction division stage is

Meiosis I the first stage of meiosis, where homologous chromosomes are separated, leading to the reduction of the chromosome number by half.

Chromosomal makeup of each daughter cell after telophase I meiosis

Haploid, and the chromosomes are each composed of two chromatids that are still joined at the centromere.

Independent assortment of chromosomes

doesn’t occur mitosis but occurs in meiosis I

Two plants are crossed, resulting in offspring with a 3:1 ratio for a particular trait. What does this suggest?

That the parents were both heterozygous for a single trait. explained—when two plants are crossed and produce offspring with a 3:1 ratio for a particular trait, it strongly suggests that both parents were heterozygous for that single trait. This is the classic outcome of a monohybrid cross (where one allele is completely dominant over the other). For example, if both parents have genotype Aa, their offspring would statistically show a 3:1 phenotypic ratio (3 dominant phenotypes to 1 recessive phenotype).

What is a monhybrid cross?

A genetic cross between two organisms that are both heterozygous for a single trait, typically resulting in a phenotypic ratio of 3:1 in the offspring.

What is a phenotype?

The observable traits or characteristics of an organism, resulting from the interaction of its genotype with the environment.

What is an allele?

A variant form of a gene that determines specific traits, occurring at the same locus on homologous chromosomes.

What is gene silencing?

A process by which the expression of a specific gene is inhibited or suppressed, often through mechanisms like RNA interference.

What is co-dominance?

A genetic scenario in which both alleles for a trait are fully expressed in the phenotype of a heterozygote, resulting in a mixture or combination of both traits.

What is incomplete dominance?

A genetic situation where the phenotype of a heterozygote is an intermediate between the phenotypes of the two homozygotes, blending traits.

What is a recessive trait?

A trait that is expressed only when two copies of the recessive allele are present, often masked by the presence of a dominant allele.

What is a dominant trait?

A trait that is expressed in the phenotype even when only one copy of the dominant allele is present, masking the effect of a recessive allele.

What is complete dominance?

A genetic situation where the phenotype of a heterozygote is the same as that of one of the homozygotes, with one allele fully masking the effect of the other.

Why do chromosomes coil during mitosis?

To allow the chromosomes to move without becoming entangled and breaking during cell division, ensuring proper segregation into daughter cells.

What are the differences and importance of phenotypes and genotypes?

Phenotypes refer to the observable physical traits of an organism, while genotypes denote the genetic constitution or combination of alleles that an organism possesses. Understanding both is crucial for studying inheritance patterns and the expression of traits.

What is a Punnett square and how does it work?

A Punnett square is a diagram used to predict the genotypes of offspring from a genetic cross, showing the potential combinations of alleles from two parents. It helps illustrate how traits may be passed on and the probabilities of different genotypes occurring.

What is a chii square and how does it work?

A chi-square is a statistical test used to determine if there is a significant association between categorical variables, comparing the observed frequencies in each category to the expected frequencies. It helps analyze data from experiments and surveys to assess independence and goodness of fit.

What is a regulatory gene?

A regulatory gene is a gene that controls the expression of one or more other genes, influencing the timing, location, and level of gene activity. It plays a critical role in the regulation of cellular functions and development.

What is a mutation?

A mutation is a change in the nucleotide sequence of an organism's DNA that can lead to alterations in the function or expression of genes. These changes can be caused by various factors, such as environmental influences or errors during DNA replication.

What is mendelian genetics?

Mendelian genetics is the study of how traits are inherited through generations based on the laws of segregation and independent assortment established by Gregor Mendel. It focuses on the inheritance patterns of single traits determined by alleles.