Biology - Meiosis

During meiosis II, the secondary oocyte divides unevenly, with one cell (the ovum) receiving half of the chromosomes and nearly all the cytoplasm and organelles, while the other cell, the polar body, is much smaller and eventually degenerates. Propose an explanation to explain why the secondary oocyte divides in this way.

To get the correct number of chromosomes, enough cytoplasm and organelles to insure the survival of the zygote

At the end of meiosis I, two cells have been produced. How many replicated chromosomes are in each of these cells?

Two

Model 5 is a condensed version of meiosis I. Notice the two possible arrangements of chromosomes in late prophase I. Considering what you know about DNA replication and meiosis, is either arrangement equally likely during the formation of tetrads in late prophase I? Explain.

Yes - it just depends on how the chromosomes pair up.

Compare the recombinant chromatids with the original pair; Are the genes on a recombinant chromatid the same as the original chromatid?

Yes, because each chromosome in the homologous pair has the same genes in the same place.

When the chromosomes come together as homologous pairs, the arms of the sister chromatids may cross over. What are these crossover points called?

Chiasmata

Considering what you already know about mitosis in cells, what event must take place during interphase before a cell proceeds to division?

DNA replication

Cells with a full set of chromosomes are referred to as diploid o 2n, whereas cells with half the chromosomes are haploid or n. At which stage(s) of meiosis I are the cells diploid and at which stages are they haploid?

Diploid = Prophase, metaphase and anaphase Haploid = Telophase, after the cell has split

In Model 1, how many replicated chromosomes does the cell contain during prophase?

Four

Write a statement to explain the origin of the chromosomes found in the zygote. Include the term homologous pair.

Half of the homologous pairs come from one parent and the other pair comes from another parent.

What would the ploidy of the zygote be if egg and sperm were produced by mitosis rather than meiosis? How would this affect the ploidy of each successive generation?

If the sperm and egg were to be produced by mitosis, they would both be diploid and the zygote would have four sets of chromosomes. If the zygote produced sperm and egg with four sets of chromosomes then that next generation would have eight sets, and so on.

Before fertilization, what happens to the secondary oocyte?

Proceeds to meiosis II

At which stage of meiosis are the chromosomes in Model 4?

Prophase I

What phrase is used to describe the chromatids after crossing over takes place and the homologous chromosomes separate?

Recombinant

What is the name given to the cells produced at the end of meiosis I in females?

Secondary oocyte

In meiosis II, during anaphase II< which structures separated - homologous chromosomes or sister chromatids?

Sister chomatids

What two structures make up a single replicated chromosome?

Sister chromatids

Refer to Model 3; At the end of meiosis II in males, what cells are produced?

Spermatids

Once the chromosomes have formed a pair, what are they called?

Tetrads

At the end of meiosis II are four daughter cells. Are they haploid or diploid?

The cells are haploid because they only have one set of the homologous pairs of chromosomes

Which of the statements below correctly describes the relationship between the cells at the end of meiosis II and the original cell?

The new cells have one copy of half of the genetic information in the original cell

Which of the statements below correctly describes the relationship between the cells at the end of telophase I and the original cell?

The new cells have two copies of half the genetic information in the original cell

At which stage in meiosis I do the pairs of homologous chromosomes come together?

Late prophase I

Refer to Model 3; What do these cells (from the previous question) eventually become?

Mature sperm

During fertilization which two cells come together? Be specific in your answer.

Mature sperm and secondary oocyte

According to Model 2, where did each of the cells come from that started meiosis II?

Meiosis I

Are the alleles on a recombinant chromatid the same as the original chromatid?

No, because the chromosomes in the homologous pair can have different alleles for each gene.

Considering the genetic makeup of the homologous pairs, will the cells at the end of telophase I be genetically identical to each other?

No, because they carry the same genes but different types

According to Model 3, what is the name given to the cells produced at the end of meiosis I in males?

Spermocyte

What is the ploidy of the zygote produced by fertilization - haploid or diploid?

Diploid

In meiosis I, during anaphase I, which structures separated - homologous chromosomes or sister chromatids?

Homologous chromosomes

If there were three sets of homologous chromosomes in the cell in Model 5, how many possible arrangements would there be for the tetrads in late prophase I?

2x2x2 = 8 possible arrangements.

Meiosis and sexual reproduction each lead to variation in the genetic make-up of every person. Explain how meiotic events, as well as the random fertilization of eggs and sperm, together lead to this genetic variation.

1) Independent assortment causes a mixing/scrambling of the homologous pairs 2) Crossing over causes exchanges of alleles on the homologous pairs 3) Random fertilization unites different combinations of maternal and paternal chromosomes

When homologous chromosome pairs align on the spindle during metaphase I the orientation of one pair is independent of the orientation of any other pair. This is known as independent assortment. Humans have 46 chromosomes, arranged as 23 pairs. During metaphase I each pair lines up independently, which results in 2 (23) possible combinations. Calculate the number of possible genetic combinations due to independent assortment.

2 (23) = 8,388,608

According in Model 1, in what type of organs are the cells that enter meiosis I found?

The sex cells

Describe what happens to the chromatids during crossover.

They exchange pieces of chromosome (DNA)