Cell Cycle and Division: Mitosis and Meiosis Explained
Cell Cycle and DNA Variation
How would you explain to someone not seeing the graphics how the amount of DNA varies over time?
- G1 phase: One unit of DNA.
- S phase: DNA synthesis; DNA duplicates.
- G2 phase: Chromosomes begin to form; DNA is double the amount in G1.
- M phase: Mitosis and cytokinesis.
Mitosis
Mitosis involves nuclear division where the nuclear envelope breaks down and chromosomes condense. Then, the nucleus splits into two.
Cytokinesis
Cytokinesis is the division of the cytoplasm.
Phases of Mitosis
Anaphase: Chromosomes separate, and each chromatid moves to opposite cell poles.
Where are genes located? Where are ribosomal subunits formed? Genes are located in chromosomes. Ribosomal subunits are formed in both eukaryotic and prokaryotic cells.
Identifying Mitosis Phases
Identify the phases in the images (assuming images A-E are provided):
- A: Metaphase
- B: Prophase
- C: Telophase
- D: Anaphase
- E: Interphase
Explanation of Mitosis Phases
Interphase
The cell division period.
Prophase
Condensation of genetic material (DNA) into visible chromosomes.
Metaphase
Chromosomes align; microtubules appear.
Anaphase
Chromosomes separate; each chromatid moves to an opposite cell pole.
Telophase
Cell division occurs, forming two daughter cells with the same number of chromosomes as the parent cell.
Diploid and Haploid Cells
Can mitosis occur in diploid and haploid cells? Mitosis in a diploid cell produces two diploid daughter cells (2n chromosomes).
Chromosome Numbers
- Sperm cells: 22 autosomes
- Egg cells: 1 sex chromosome (X)
Meiosis and Sexual Reproduction
Why are meiosis and the union of gametes essential in sexual reproduction? What are the advantages of sexual reproduction? Meiosis reduces the chromosome number by half, producing gametes. Sexual reproduction allows for genetic variation and parthenogenesis (development of an unfertilized egg).
Diploid vs. Haploid Organisms
A diploid organism has two sets of chromosomes (2n), while a haploid organism has one set (n).
From Diploid to Haploid
How does a diploid cell become haploid? Meiosis is the process that reduces chromosome number, contributing to genetic variability.
The Cell Cycle
The diagram shows the cell cycle. Briefly describe two phases:
Prophase
DNA condenses into chromosomes.
Anaphase
Chromosomes separate; each chromatid moves to an opposite cell pole.
Spermatogenesis
What type of cell division occurs during spermatogenesis? How many sperm are formed from each spermatocyte? Meiosis occurs. Four sperm are formed from each spermatocyte (2n → 4n sperm).
Chromosome Numbers in Cells
How many chromosomes are in each of the three cell types? Can some sperm be heterozygous for a character? There are 23 pairs of chromosomes in each cell. No, sperm are not heterozygous.
Crossing Over
In what type of cell division does crossing over occur? Crossing over occurs during meiosis, resulting in genetic recombination.
Mitosis: Genetic Consequences
What type of cell division is mitosis? What are its genetic consequences? Mitosis preserves genetic information, producing genetically identical daughter cells. This ensures the transmission of genetic material to subsequent generations.
Chromosomes and Chromatids
What are chromosomes and their structure? What is the difference between sister chromatids and homologous chromosomes? Chromosomes are structures that organize chromatin in the cell nucleus during mitosis and meiosis. Homologous chromosomes are pairs of chromosomes in a diploid organism. Sister chromatids are identical chromosomes joined at the centromere.
Phases of the Cell Cycle
Identify and explain the main changes in each phase: G1, S, and G2 are part of interphase, where cell substances are synthesized. G1: RNA and protein synthesis. M phase: Nuclear division (mitosis) followed by cytoplasmic division (cytokinesis).
DNA Replication
Briefly explain DNA replication and its purpose. During mitosis, DNA condenses to form chromosomes, each with two chromatids joined at the centromere.
Meiosis in Sexual Reproduction
Explain the function of meiotic division in sexual reproduction. Meiosis reduces the chromosome number by half and recombines genes, resulting in genetically diverse offspring.
Phases of the Cell Cycle
What is represented in each phase? What steps are included in interphase? Where does cytokinesis occur? G1: The cell has a certain amount of DNA. S phase: DNA replication and histone synthesis. G2: Chromosomes begin to form; DNA is doubled. M phase: Mitosis and cytokinesis. Interphase includes G1, S, and G2 phases. Cytokinesis occurs in the M phase.
G0 Phase
Some cells enter a quiescent phase (G0). In what phase of the scheme does this occur, and why? The G0 phase occurs in the G1 phase. Cells in G0 may re-enter the cell cycle under the influence of mitotic activators.
Biochemical Processes in M and S Phases
Briefly explain the biochemical processes and structural changes in the M and S phases. M phase: Nuclear and cytoplasmic division. S phase: DNA replication.