2 Meiosis Flashcards
A child closely resembles both parents but is not identical to them. How does genetic inheritance explain this phenomenon, and what roles do recombination and independent assortment play in generating genetic diversity?
Humans resemble their parents due to the inheritance of genes from both parents, but they are not identical because of genetic recombination and independent assortment. Recombination during meiosis exchanges genetic material between homologous chromosomes, creating new allele combinations. Independent assortment of chromosomes during meiosis results in different combinations of chromosomes in gametes, further contributing to genetic diversity.
Describe a human karyotype and explain what information can be obtained from examining a karyotype.
A human karyotype is a complete set of chromosomes in a cell, arranged in pairs according to size, shape, and banding pattern. A normal human karyotype consists of 46 chromosomes: 22 pairs of autosomes and one pair of sex chromosomes (XX or XY). Examining a karyotype can reveal chromosomal abnormalities such as aneuploidy (e.g., Down syndrome) or structural abnormalities (e.g., deletions or duplications).
Outline the karyotyping process and explain how chromosomes are prepared and recognized.
Karyotyping involves obtaining a sample of cells (e.g., from blood or amniotic fluid), treating them with a drug to halt cell division at metaphase, staining the chromosomes to produce banding patterns, and then photographing the chromosomes under a microscope. Chromosomes are recognized and arranged based on their size, banding pattern, and centromere position to identify abnormalities.
Define ploidy and describe the different levels of ploidy observed in human cells.
Ploidy refers to the number of sets of chromosomes in a cell. Humans are diploid (2n), meaning they have two sets of chromosomes: one from each parent. Gametes are haploid (n), containing one set of chromosomes. Ploidy levels can vary in other organisms or under specific conditions (e.g., polyploidy in plants).
Explain how chromosome number changes throughout the human life cycle, from fertilization to gamete formation.
Human somatic cells are diploid (46 chromosomes) throughout most of the life cycle. During gamete formation (meiosis), the chromosome number is reduced to haploid (23 chromosomes) to ensure that fertilization restores the diploid number. After fertilization, the zygote develops into a diploid organism with 46 chromosomes.
Compare sister chromatids and homologous chromosomes in terms of their roles during cell division.
Sister chromatids are identical copies of a chromosome connected by a centromere, formed during DNA replication. They are separated during mitosis. Homologous chromosomes are pairs of chromosomes, one from each parent, that have similar genes but may have different alleles. They are separated during meiosis I, ensuring genetic diversity in gametes.
Describe the events characteristic of each phase of meiosis and how they contribute to genetic variation.
Meiosis I:
Prophase I: Chromosomes condense, homologous chromosomes pair up (synapsis) and exchange genetic material (crossing over).
Metaphase I: Homologous chromosome pairs line up along the metaphase plate.
Anaphase I: Homologous chromosomes are pulled to opposite poles.
Telophase I: Chromosomes decondense, and the cell divides into two haploid cells.
Meiosis II:
Prophase II: Chromosomes condense again in each haploid cell.
Metaphase II: Chromosomes line up along the metaphase plate.
Anaphase II: Sister chromatids are separated.
Telophase II: Chromosomes decondense, and the cells divide, resulting in four unique haploid gametes. These phases contribute to genetic variation through independent assortment and crossing over.
A biologist is examining micrographs of dividing cells. They observe cells with pairs of chromosomes aligned along the metaphase plate and others where chromatids are separating. How can these observations be used to identify the phases of meiosis?
Observations of chromosomes aligned along the metaphase plate indicate metaphase I or II. If homologous pairs are present, it is metaphase I. If individual chromosomes are aligned, it is metaphase II. Observations of chromatids separating indicate anaphase II. Identifying these features helps in determining the phase of meiosis.
Define synapsis, crossing over, tetrad, and chiasmata, and indicate during which phase of meiosis these occur.
Synapsis: The pairing of homologous chromosomes during prophase I of meiosis.
Crossing Over: The exchange of genetic material between homologous chromosomes during prophase I.
Tetrad: The structure formed by the synapsis of homologous chromosomes, consisting of four chromatids (two pairs of sister chromatids) during prophase I.
Chiasmata: The physical points where crossing over occurs, visible during prophase I.
Compare mitosis and meiosis in terms of their purpose, number of divisions, and genetic outcomes.
Mitosis results in two identical diploid daughter cells and is used for growth, repair, and asexual reproduction. It involves one division. Meiosis results in four genetically diverse haploid gametes and is used for sexual reproduction. It involves two divisions (meiosis I and II), leading to genetic variation through independent assortment and crossing over.
Compare meiosis in female and male mammals regarding timing and outcomes.
In males, meiosis occurs continuously from puberty, producing four viable sperm per cycle. In females, meiosis begins before birth and is paused in prophase I until puberty, when one egg completes meiosis I each menstrual cycle. Only one viable egg is produced from each meiosis cycle, while the rest are polar bodies that degenerate.
How do independent assortment, crossing over, and random fertilization contribute to genetic variation in sexually reproducing organisms?
Independent assortment during meiosis I randomly distributes maternal and paternal chromosomes into gametes, leading to various combinations. Crossing over during prophase I exchanges genetic material between homologous chromosomes, creating new allele combinations. Random fertilization combines different gametes, further increasing genetic variation.
Calculate the number of possible different gametes from an organism with 2n = 8 chromosomes.
For an organism with 2n = 8 chromosomes, the number of possible different gametes is
2𝑛, where 𝑛 is the number of chromosome pairs. Therefore, 2^4 =16 possible gametes.
Explain the evolutionary significance of genetic variation in populations.
Genetic variation is crucial for evolution because it provides a pool of diverse traits upon which natural selection can act. Populations with high genetic variation are more adaptable to changing environments and have a higher likelihood of survival. Variation allows for the emergence of new traits that can be advantageous in different conditions, driving evolutionary processes.
Explain the chromosomal basis of trisomy 13 and describe how and when this chromosomal abnormality could occur.
Trisomy 13, or Patau syndrome, occurs when an individual has three copies of chromosome 13 instead of the usual two. This typically results from nondisjunction during meiosis, where chromosomes fail to separate properly. The abnormality can occur in either the egg or sperm and is present in all cells of the affected individual. It often results in severe developmental and physical abnormalities and a shortened lifespan.
