HESI A2 · Biology · 6 min read
Genetics, Cell Division and Mutations
- Compare the processes, stages, and cellular outcomes of mitosis and meiosis.
- Explain Mendelian and non-Mendelian inheritance patterns, genotypes, and phenotypes using cross ratios.
- Differentiate categories of genetic mutations and explain how chromosomal abnormalities and inheritance patterns cause disorders.
The Cell Cycle and the Stages of Mitosis
Every dividing cell progresses through an orderly sequence of events known as the cell cycle. The cell cycle consists of interphase and the mitotic phase. Interphase is the preparatory period before division and is composed of the G1, S, and G2 phases. A crucial milestone during this preparation is DNA replication, which occurs specifically during the S phase of interphase to ensure that each future daughter cell will receive a complete set of genetic instructions.
Following interphase, the cell enters mitosis, which divides the replicated genetic material. The four sequential stages of mitosis are prophase, metaphase, anaphase, and telophase. During prophase, chromatin condenses into visible chromosomes. The nuclear envelope breaks down in prometaphase, which is sometimes described as late prophase. Next, during metaphase, chromosomes align along the middle of the cell at a central boundary called the metaphase plate.
The separation of genetic material takes place in the final stages of nuclear division. During anaphase, sister chromatids are pulled apart toward opposite poles of the cell. During telophase, nuclear envelopes reform around each separated set of chromosomes, and the chromosomes decondense back into an uncoiled state. Mitosis is accompanied by cytokinesis, which is the physical division of the cytoplasm into two daughter cells. In animal cells, cytokinesis involves a cleavage furrow, whereas in plant cells, a cell plate forms and develops into a new cell wall. In a diploid cell, mitosis followed by cytokinesis produces two genetically identical diploid daughter cells, barring mutations.
Common mistakes
- Confusing anaphase I and anaphase II: homologous chromosome pairs separate in anaphase I, whereas individual sister chromatids separate in anaphase II.
- Mixing up phenotypic and genotypic ratios: a monohybrid cross of two heterozygotes produces a 3:1 phenotypic ratio under complete dominance, but a 1:2:1 genotypic ratio.
- Assuming all mutations alter the final amino acid chain: silent mutations change the nucleotide sequence without altering the encoded amino acid.
- Confusing somatic mutations with germline mutations: somatic mutations happen in non-reproductive body cells and cannot be passed to offspring, whereas germline mutations occur in gametes or their precursor cells and can be inherited.
Key points
- The cell cycle consists of interphase (G1, S, and G2) and the mitotic phase, with DNA replication occurring during the S phase.
- Mitosis involves prophase, metaphase, anaphase, and telophase, followed by cytokinesis (cleavage furrow in animals, cell plate in plants) to yield two genetically identical diploid daughter cells from a diploid parent.
- Meiosis includes one round of DNA replication and two successive divisions; crossing over and independent assortment generate genetically diverse haploid products, but in human egg formation only one of the products becomes a functional egg.
- Complete dominance yields a 3:1 phenotypic ratio and a 1:2:1 genotypic ratio in a monohybrid cross, whereas incomplete dominance yields an intermediate phenotype and codominance fully expresses both alleles (such as human AB blood type).
- Point mutations include silent, missense, and nonsense mutations, while insertions or deletions that are not multiples of three cause frameshift mutations that alter downstream codons.
- Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly, causing aneuploidy such as trisomy 21 in Down syndrome.
- Genetic conditions show distinct inheritance patterns: cystic fibrosis and sickle cell anemia are autosomal recessive, Huntington's disease is autosomal dominant, and X-linked recessive disorders occur more frequently in XY males.
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