Bio Lecture 23

Why is it necessary for gametes to be haploid not diploid?

Because sperm and egg together form diploid. If an organism exist in a diploid state all that means all individuals need to be diploid. IF you put diploid and diploid together that is four sets of chromosomes which is no good. During sexual reproduction egg and sperm combine and return to diploid state.

Why does cancer development require mutations in multiple genes?

Cancer arises after mutations in multiple genes that contain the information for proteins important for controlling cell division. There needs to be multiple mutations because there are many mechanisms that work together in a cell to prevent uncontrolled cell growth. So this means that the system will have to break in multiple places for the cell to go out of control. (goes though mitosis more than it should)..

Describe Meiosis 2.

DNA is not replicated prior to meiosis 2. Stages are the same as mitosis except there are four haploid cells being produced. Meiosis 2 begins with two haploid cells with replicated chromosomes. The sister chromatids are separated and the product is four genetically unique haploid cells.

Describe the process of Meiosis 1?

DNA is replicated before Meiosis 1 in loose uncondensed state. After DNA replication happens, chromosomes condense, spindle fibers form and nuclear membrane breaks down and homologous chromosomes are pairing up. Paired homologous chromosomes can swap pieces of DNA called crossing over. Spindle fibers from opposite sides of the cell get attached to one of each of the homologous chromosomes. Pairs line up in the middle of the cell and each lines up independently of the other pairs. Homologous chromosomes get pulled by spindle fibers to opposite sides of cell and cytokinesis generates two cells.

What event in meiosis explains why gametes are haploid?

Homologous chromosomes are separated in meiosis I. Mastering Bio Input: In meiosis I, homologous chromosomes are separated, which means that the cells produced by meiosis I are haploid. Haploid cells have ONE allele for each gene because every chromosome has a different set of genes (there are no homologous pairs).

Explain why homologous chromosomes, which have the same genes at the same positions (loci), can have different DNA sequences.

Homologous chromosomes are similar but not identical. This allows them to have the same genes but different alleles which are the versions of a particular gene.

What process during meiosis explains why the combination of alleles carried on the same chromosome can be different in the gametes produced by meiosis than in the parent cell that divides?

Homologous chromosomes can exchange pieces of DNA (crossing over). Mastering Bio Input: When replicated homologous chromosomes pair up in meiosis I, non-sister chromatids can exchange pieces of DNA that have the same genes. If (and only if) the homologous chromosomes have different alleles, this can generate new allele combinations. For example, imagine that one homologous chromosome has a purple flower allele and a green pod allele, for the flower color and pod color genes, respectively. The other homologous chromosome has a yellow flower allele and a yellow pod allele. If the non-sister chromatids swap the piece of DNA with the flower color gene, you'll end up with a chromatid that has a purple flower allele and a yellow pod allele, and a chromatid that has a yellow flower allele and green pod allele. When sister chromatids are separated in meiosis II, the four cells will have chromosomes with the allele combinations listed below. Draw out this example (or a different one with your choice of genes) to help you visualize the effects of crossing over on gamete genotypes. Possible Gamete Allele Combinations:-yellow flower; yellow pod-yellow flower; green pod-purple flower; green pod-purple flower; yellow pod

An error occurs during meiosis of a human cell, and the pair of chromosome 11's (homologous chromosomes) fail to separate in meiosis I. At the end of meiosis II, two of the gametes each have an extra chromosome 11 (two rather than just one). Will the other two gametes have zero, one, or two chromosome 11's? Explain your answer. Hint - Draw pictures to help you visualize what happened; don't worry about drawing all the other chromosomes.

If homologous chromosomes fail to separate during meiosis I, the result is two gametes that lack that chromosome and two gametes with two copies of the chromosome.

what would happen if the sister chromatids of chromosome 7 in a cell did not separate during meiosis II during formation of a human gamete. Don't worry about including all the other chromosomes.

If sister chromatids fail to separate during meiosis II, the result is one gamete that lacks that chromosome, two normal gametes with one copy of the chromosome, and one gamete with two copies of the chromosome.

Select ALL statements that correctly describe mitosis and meiosis. In mitosis, the daughter cells are genetically identical, but in meiosis the daughter cells are genetically different. Sister chromatids are distributed to opposite sides of a cell during mitosis, but this does not occur during either part of meiosis. Two diploid cells are produced when a diploid cell divides by mitosis; division of a diploid cell by meiosis produces four haploid cells. DNA is replicated once prior to mitosis, but two rounds of DNA replication occur when a cell divides by meiosis.

In mitosis, the daughter cells are genetically identical, but in meiosis the daughter cells are genetically different. Two diploid cells are produced when a dipoid cell divides by mitosis; division of a diploid cell by meiosis produces four haploid cells. Mastering Bio Input: DNA is replicated once prior to mitosis, and is also replicated once prior to meiosis. Therefore, both processes begin with cells that have replicated chromosomes (pairs of sister chromatids). Sister chromatids are distributed to opposite sides of a dividing cell during mitosis AND meiosis (specifically meiosis II). Mitosis involves one cell division, producing two genetically identical daughter cells. Therefore division of a diploid cell by mitosis produces two dipolid cells. Meiosis, which must start with a dipolid parent cell, involves two cell divisions and results in four genetically unique haploid cells.

At the end of meiosis 1, haploid or diploid? Are there still pairs of sister chromatids?

Meiosis 1 generates two haploid cells that have replicated chromosomes. Homologous chromosomes have been separated into two different cells and each has one set=haploid. Sister chromatids have not been separated and each cell has replicated chromosome. Note: if you have a pair of homologous chromosomes the cell is diploid if you separate the homologous chromosomes then the cell is haploid. Despite there being replicated chromosomes the homologous chromosomes are separated making the cell haploid

Explain why haploid cells cannot divide by meiosis.

Meiosis divides up from sets of chromosomes meaning it goes from two sets of chromosomes (diploids( to one set of chromosomes (haploid) therefore the starting cell (parent cell) must be diploid

Explain why cancer that occurs due to mutations in the genes of body cells cannot be passed on to an individual's offspring.

Mutations in body cells cannot be passed to the next generation because it is not in the gametes. Only mutations in gametes (egg or sperm cells) can be passed from parent to offspring.

Draw a pair of replicated homologous chromosomes. Label sister chromatids and non-sister chromatids. Explain which ones must have the same DNA sequences and which ones can be different. Assume that no mutations occurred during DNA replication.

Replicated chromosomes are composed of sister chromatids. Sister chromatids must have the same DNA sequence/alleles while the non sister chromatids chromosomes might have different alleles.

Do the two cells produced in meiosis I have replicated or un-replicated chromosomes? Are they haploid or diploid?

The two cells produced in meiosis 1 are haploid cells that have replicated chromosomes

A mutation in the gene PTEN has been linked to cancer. The mutation increases the activity of the protein produced by expression of PTEN. Based only on this information, do you predict that this protein promotes or inhibits cell division? Explain your reasoning

This protein promotes cell division Mastering Bio Input: Cell division is tightly controlled. There are certain proteins that prevent a cell from dividing before it's ready (e.g. the DNA has not been replicated yet), or if there are issues that could be problematic if the cell divides (e.g. significant DNA damage). Other proteins promote cell division when a cell is ready to divide. Mutations that increase the activity or expression of proteins that promote cell division can cause cells to divide uncontrollably, leading to cancer. Mutations inhibit the function/expression of proteins that prevent cell division can also result in uncontrolled growth and cancer. Remember that cancer-promoting mutations must occur in multiple cell division regulatory genes for a cell to become cancerous.

What is independent orientation?

homologous chromosomes line up independently; another important source of genetic variation

All of the events below occur in meiosis. Which one, if any, does NOT occur in mitosis? Chromosomes condense Spindles of cytoskeleton form nuclear envelope breaks down homologous chromosomes pair up

homologous chromosomes pair up

When do homologous chromosomes pair up? mitosis and meiosis I meiosis I only meiosis I and meiosis II meiosis II only mitosis only

meiosis I only Mastering Bio Input: In meiosis I, homologous chromosomes form pairs. This is important because it allows each chromosome of a homologous pair to be distributed to a different daughter cell, resulting in haploid cells (one set of chromosomes). Homologous chromosomes do not pair up in mitosis. And homologous chromosomes cannot pair up during meiosis II because the homologous chromosomes have already been separated in meiosis I.