Bio Chap 13 Notes (Meiosis)

Heredity and Genes

  • Heredity is the transmission of traits from one generation to the next. [1] The study of heredity and inherited variation is called genetics. [1]

  • Genes are units of heredity made of DNA. [2] They provide instructions for the synthesis of enzymes and other proteins, which determine an organism's traits. [2] Inherited information is passed from parent to offspring in the form of DNA nucleotide sequences. [2]

Reproduction

There are two main types of reproduction:

  • Asexual Reproduction: A single individual produces genetically identical offspring, a clone, by mitosis. [3]

  • Sexual Reproduction: Two parents contribute genetic information to produce offspring with unique combinations of genes. [4]

Chromosomes and Karyotypes

  • Chromosomes are structures made of DNA and proteins that carry genetic information. [5] Each species has a characteristic number of chromosomes. [5] Humans have 46 chromosomes in their somatic cells. [6]

  • A karyotype is an ordered display of chromosomes arranged in pairs. [6] It can be used to identify chromosomal abnormalities. [6]

Homologous Chromosomes, Sex Chromosomes, and Autosomes

  • Homologous chromosomes (homologs) are pairs of chromosomes that have the same length, centromere position, and staining pattern. [6] They carry genes controlling the same inherited characteristics. [6]

  • Sex chromosomes determine an individual's sex. [7] In humans, females have two X chromosomes (XX) and males have one X and one Y chromosome (XY). [7]

  • Autosomes are all chromosomes other than the sex chromosomes. [7]

Diploid and Haploid Cells

  • Diploid cells have two sets of chromosomes (2n), one set from each parent. [8] Human somatic cells are diploid. [6, 8]

  • Haploid cells have one set of chromosomes (n). [9] Gametes, such as sperm and egg cells, are haploid. [9]

Fertilization, Zygote, and the Human Life Cycle

  • Fertilization is the fusion of haploid gametes (sperm and egg), resulting in a diploid zygote, or fertilized egg. [10]

  • The human life cycle begins with a diploid zygote, which undergoes mitosis to produce a multicellular diploid adult. [10] In the gonads, germ cells undergo meiosis to produce haploid gametes. [11] This alternation of meiosis and fertilization maintains a constant chromosome number across generations. [12]

Sexual Life Cycles

While the alternation of meiosis and fertilization is common to all sexually reproducing organisms, the timing of these events varies. [13]

  • Animals: Meiosis produces haploid gametes, the only haploid cells in the life cycle. [13] Fertilization restores the diploid state. [13]

  • Plants and Some Algae: Exhibit alternation of generations with both multicellular diploid and haploid stages. [14]

  • Sporophyte: The multicellular diploid stage produces haploid spores by meiosis. [14]

  • Spores: Haploid cells that divide mitotically to produce a multicellular haploid gametophyte. [14]

  • Gametophyte: Produces gametes by mitosis. [14]

  • Most Fungi and Some Protists: The only diploid stage is the zygote. [14] Meiosis in the zygote produces haploid cells that divide mitotically to produce a haploid multicellular organism or unicellular descendants. [14]

Meiosis

Meiosis is a type of cell division that reduces the number of chromosome sets from two (diploid) to one (haploid). [15, 16] This is accomplished through two consecutive cell divisions: meiosis I and meiosis II. [15] Prior to meiosis, chromosomes are duplicated during interphase. [15]

  • Meiosis I: Separates homologous chromosomes. [17, 18]

  • Prophase I:

  • Chromosomes condense and become visible. [19, 20]

  • Homologous chromosomes pair up in synapsis, forming tetrads. [19, 21]

  • Crossing over occurs: non-sister chromatids exchange genetic material. [19, 21] This exchange happens at chiasmata, the sites of crossing over. [18, 19, 22, 23]

  • The nuclear envelope breaks down. [19]

  • The spindle apparatus forms. [19]

  • Metaphase I:

  • Tetrads line up at the metaphase plate. [19]

  • Microtubules from opposite poles attach to the kinetochores of homologous chromosomes. [19]

  • Anaphase I:

  • Homologous chromosomes separate and move to opposite poles of the cell. [19]

  • Sister chromatids remain attached at their centromeres. [19]

  • Telophase I and Cytokinesis:

  • Chromosomes arrive at the poles of the cell. [19]

  • The nuclear envelope may reform, and the chromosomes may decondense. [19]

  • Cytokinesis occurs, producing two haploid daughter cells, each with one duplicated chromosome from each homologous pair. [19]

  • Meiosis II: Separates sister chromatids. [17, 24]

  • Prophase II:

  • Chromosomes condense again (if necessary). [25]

  • The nuclear envelope (if reformed) breaks down. [25]

  • The spindle apparatus forms. [25]

  • Metaphase II:

  • Chromosomes line up at the metaphase plate. [25]

  • Microtubules attach to the kinetochores of sister chromatids. [25]

  • Anaphase II:

  • Sister chromatids separate, becoming individual chromosomes, and move to opposite poles. [25]

  • Telophase II and Cytokinesis:

  • Chromosomes arrive at the poles. [25]

  • The nuclear envelope reforms, and chromosomes decondense. [25]

  • Cytokinesis occurs, producing a total of four haploid daughter cells, each with one chromosome from each homologous pair. [25]

Key Differences Between Meiosis and Mitosis

  • Meiosis: Reduces the chromosome number from diploid (2n) to haploid (n), producing four genetically different daughter cells. [16]

  • Mitosis: Preserves the chromosome number, producing two genetically identical daughter cells. [16]

Events Unique to Meiosis I

  • Synapsis and Crossing Over: During prophase I, homologous chromosomes physically pair up and exchange genetic material, which doesn't occur in mitosis. [26]

  • Homologous Pair Alignment: In metaphase I, homologous chromosome pairs line up at the metaphase plate, not individual chromosomes as in mitosis. [27]

  • Separation of Homologs: During anaphase I, homologous chromosomes are separated, while sister chromatids remain attached, unlike mitosis. [27]

Sister Chromatid Cohesion and Chiasmata

  • Sister chromatid cohesion keeps sister chromatids together until anaphase II. [27] Cohesin proteins are responsible for this cohesion. [27]

  • Chiasmata, the sites of crossing over, help hold homologous chromosomes together until anaphase I. [27]

Recombinant Chromosomes and Genetic Variation

  • Recombinant chromosomes are individual chromosomes that carry genes from both parents as a result of crossing over. [28]

  • Three mechanisms contribute to genetic variation in sexual reproduction:

  • Independent Assortment: Random orientation of homologous pairs during metaphase I leads to different combinations of maternal and paternal chromosomes in daughter cells. [29]

  • Crossing Over: Exchange of genetic material between non-sister chromatids creates new combinations of alleles. [28]

  • Random Fertilization: Any sperm can fertilize any egg, resulting in vast genetic diversity in offspring. [30]

Evolutionary Significance

  • Genetic variation is the raw material for evolution by natural selection. [31]

  • Sexual reproduction, despite being more energetically costly than asexual reproduction, is advantageous because it generates genetic diversity, which allows populations to adapt tochanging environments. [32]