Cell Division & Mitosis

Cell Division Practice Questions

• Drugs attacking spindle fibers in cell division affect mitosis.

• Colchicine, from Colchicum autumnale, is used in chemotherapy and targets spindle fiber formation. Consequently affecting prophase when centrioles form spindle fibers.

• For a species with 2n = 16, each somatic cell has eight homologous pairs, with each set containing 8 chromosomes. If there are 16 chromosomes in a diploid cell, then there are 8 homologs.

• DNA replication occurs during the S phase; it does not occur during mitosis.

• Chromosomes condense in prophase, chromatids separate in anaphase, spindle fibers form in prophase, and chromosomes align in metaphase.

• A fetus with mitosis issues and improper nuclear membrane reforming has problems in telophase.

• Telophase involves the reforming of the nuclear membrane.

• Gametes must be haploid so that when two haploid cells (gametes) combine, they create a diploid zygote. A diploid zygote has 46 chromosomes.

• In an image of chromosomes, each X represents a replicated chromosome. Each contains two chromatids, three homologous pairs, and a diploid number of 6.

• In chromosomes, the two halves of the chromosome from mom contain the same alleles and therefore both must have the brown allele because replicated chromosomes are identical.

Mitosis/Meiosis Comparison and Review Packet

• Meiosis creates variation in cells, leading to variation in new life.

• Gametes that are diploid are created from diploid gonads

• Meiosis creates haploid gametes from diploid gonads.

• Sperm and egg created at the end of meiosis from testes and ovaries, respectively.

• Each meiotic gamete is created with half the number of chromosomes and without homologous pairs.

• The S phase did not occur when cells undergo mitosis, and they do not have any replicated chromosomes, although organelles were replicated.

• The G1 phase of the cell cycle occurred, but the S phase did not.

• Mitosis: growth, repair, development, reproduction in unicellular organisms.

• Meiosis reduces the number of chromosomes in half to create gametes.

• Organisms have an even number of chromosomes due to pairs (homologous pairs) from the genetic mother and father.

• Chromosome number reduction in meiosis allows gametes to combine for reproduction.

• Crossing-over allows for recombination of genes/alleles, increasing genetic variation.

Meiosis I and II Ordering

• 7: Chromosomes line up in the center of the cell (M2).

• 5: Spindle fibers pull apart homologous chromosomes (A1).

• 10: Four haploid (n) daughter cells form (T2).

• 1: Cells undergo a round of DNA replication (S).

• 8: Sister chromatids separate from each other (A2).

• 3: Homologous chromosomes form tetrads (P1).

• 6: Two haploid (n) daughter cells form (T1).

• 2: Spindle fibers attach to the homologous chromosome pairs (P1).

• 9: Individual chromatids move to each end of the cell (A2).

• 4: Crossing over occurs (if any) (P1).

• Meiosis I produces haploid cells, while Meiosis II results in genetically dissimilar cells.

• Meiosis I has replicated chromosomes, but Meiosis II produces unreplicated chromosomes in the end products.

• A diploid cell with 28 chromosomes undergoing meiosis will result in daughter cells with 14 chromosomes each.

Comparing and Contrasting Mitosis vs Meiosis

Mitosis

• Function: Growth; regeneration of new cells
• Location in body: All body cells
• Number of daughter cells: 2
• Change in chromosome number: None
• Number of phases: Four
• Number of cell divisions: One
• Difference in DNA between parent cell and daughter cells: None (identical)

Meiosis

• Function: Creation of gametes
• Location in body: Gonads (testes/ovaries)
• Number of daughter cells: Four
• Change in chromosome number: Halved
• Number of phases: Four
• Number of cell divisions: Two
• Difference in DNA between parent cell and daughter cells: Homologous chromosomes assort independently, so each gamete has a unique combination of alleles

Comparing Mitosis vs Meiosis Processes

Mitosis

• No pairing of homologs occurs
• One division
• Associated with growth and asexual reproduction
• Two daughter cells produced
• Involves duplication of chromosomes
• Chromosome number is maintained
• Daughter cells are identical to parent cell

Meiosis

• Two divisions
• Four daughter cells produced
• Associated with sexual reproduction
• Involves duplication of chromosomes
• Chromosome number is halved
• Crossing over between homologous chromosomes may occur
• Daughter cells are not identical to parent cell
• Produces gametes
• Synapsis occurs in prophase

Male vs Female Meiosis

Male Meiosis (Spermatogenesis)

• Begins at puberty and ends at death.
• Results in 4 viable cells, gaining a flagellum for movement.

Female Meiosis (Oogenesis)

• Begins at fetal development, then puberty and ends at menopause.

• Results in 1 viable cell and 3 polar bodies, released during menstruation.

• Both spermatogenesis and oogenesis use steps of meiosis.

• Spermatogenesis creates 4 viable cells with a flagellum.

• Oogenesis creates 1 viable cell, 3 polar bodies, dividing cytoplasm unequally in cytokinesis.

Consequences of Nondisjunction

• Failure of homologous pairs to separate in oogenesis during the first meiotic division results in gametes with the incorrect chromosome number, results in gametes known as nondisjunction.

• Two cells would be n+1 and two would be n-1, but only one created cell could possibly become a zygote, resulting in zygotes would either be 2n+1 or 2n-1.

• Human's egg cells would have 24 or 22 chromosomes, leading to a zygote with 47 or 45 chromosomes.

• If nondisjunction occurred during spermatogenesis, the chromosome number would be impacted similarly, with four cells impacted instead of just one.

• A failure of chromatids to separate during second meiotic division would result in one cell at n+1 and one at n-1, with others being normal.

• In spermatogenesis, half of the cells would have the normal chromosome number, and half would have the incorrect number.

• Oogenesis results in a 50% chance of no effect, as nondisjunction may occur in polar bodies and the overall decrease in the likelihood of a zygote having an incorrect number of chromosomes.