Meiosis and Sexual Life Cycles

Offspring Acquire Genes from Parents by Inheriting Chromosomes

• Offspring resemble their parents more than unrelated individuals.
• Inheritance (heredity): The transmission of traits from one generation to the next.
• Sons and daughters are not identical copies of either parent or siblings; there is also variation.
• Genetics: The scientific study of heredity and inherited variation.
• Meiosis and fertilization maintain a species’ chromosome count during the sexual life cycle.
• Meiosis and fertilization contribute to genetic variation.

Comparison of Asexual and Sexual Reproduction

• Only organisms that reproduce asexually have offspring that are exact genetic copies of themselves.
• Asexual reproduction: A single individual is the sole parent and passes copies of all its genes to its offspring without the fusion of gametes.
  • Single-celled eukaryotic organisms reproduce asexually by mitotic cell division.
  • Offspring are genetically identical to the parent.
  • Clone: A group of genetically identical individuals.
  • Genetic differences occasionally arise due to mutations.
• Sexual reproduction: Two parents give rise to offspring that have unique combinations of genes inherited from the two parents.
  • Offspring vary genetically from their siblings and both parents.
  • Genetic variation is an important consequence of sexual reproduction.

Inheritance of Genes

• Parents endow their offspring with coded information in the form of genes (hereditary units).
• Genes program specific traits that emerge as we develop.
• Genetic program is written in the language of DNA (polymer of four different nucleotides).
• Inherited information is passed on in the form of each gene’s specific sequence of DNA nucleotides.
• Cells translate genes into specific enzymes and other proteins.
• The transmission of hereditary traits has its molecular basis in the replication of DNA.
• Gametes: Reproductive cells that transmit genes from one generation to the next.
• During fertilization, male and female gametes (sperm and eggs) unite, passing on genes of both parents to their offspring.
• DNA of a eukaryotic cell is packaged into chromosomes within the nucleus.
• Every species has a characteristic number of chromosomes.
  • Humans have 46 chromosomes in their somatic cells (all cells of the body except the gametes).
• Each chromosome consists of a single long DNA molecule, elaborately coiled in association with various proteins.
• One chromosome includes several hundred to a few thousand genes.
• Locus: A gene’s specific location along the length of a chromosome.
• Genome: Our genetic endowment consists of the genes and other DNA that make up the chromosomes we inherited from our parents.

Fertilization and Meiosis Alternate in Sexual Life Cycles

• Life cycle: The generation-to-generation sequence of stages in the reproductive history of an organism, from conception to production of its own offspring.

Sets of Chromosomes in Human Cells

• Human somatic cells have 46 chromosomes.
• During mitosis, chromosomes condense and are visible under a light microscope.
• Karyotype: An ordered display of the 46 human chromosomes from a single cell in mitosis, arranged in pairs.
• Homologous chromosomes (homologs): Two chromosomes of a pair with the same length, centromere position, and staining pattern.
  • Both chromosomes of each pair carry genes controlling the same inherited characters.
• Sex chromosomes: X and Y.
  • Females have a homologous pair of X chromosomes (XX).
  • Males have one X and one Y chromosome (XY).
  • Only small parts of the X and Y are homologous.
• Autosomes: The other chromosomes that are not sex chromosomes.
• We inherit one chromosome of a pair from each parent.
• The number of chromosomes in a single set is represented by $n$.
• Diploid cell: Any cell with two chromosome sets, and has a diploid number of chromosomes, abbreviated $2n$.
  • For humans, $2n = 46$ (number of chromosomes in our somatic cells).
  • In a cell in which DNA synthesis has occurred, all the chromosomes are duplicated, each consisting of two identical sister chromatids, associated closely at the centromere and along the arms.
• Haploid cells: Gametes contain a single set of chromosomes, and each has a haploid number of chromosomes ($n$).
  • For humans, $n = 23$.
  • The set of 23 consists of 22 autosomes plus a single sex chromosome.
  • An unfertilized egg contains an X chromosome; a sperm contains either an X or a Y chromosome.
• Each sexually reproducing species has a characteristic diploid and haploid number.
  • Fruit fly (Drosophila melanogaster): $2n = 8$, $n = 4$.
  • Dogs: $2n = 78$, $n = 39$.
• Chromosome number does not correlate with the size or complexity of a species’ genome. It reflects how many linear pieces of DNA make up the genome.

Behavior of Chromosome Sets in the Human Life Cycle

• The human life cycle begins when a haploid sperm from the father fuses with a haploid egg from the mother.
• Fertilization: The union of gametes, culminating in fusion of their nuclei.
• Zygote: The resulting fertilized egg, which is diploid.
• Mitosis of the zygote and its descendant cells generates all the somatic cells of the body.
• Gametes develop from specialized cells called germ cells in the gonads—ovaries in females and testes in males.
• Meiosis: A type of cell division that reduces the number of sets of chromosomes from two to one in the gametes, counterbalancing the doubling that occurs at fertilization.
  • Each human sperm and egg is haploid ($n = 23$).
• Fertilization restores the diploid condition.

The Variety of Sexual Life Cycles

• Although the alternation of meiosis and fertilization is common to all organisms that reproduce sexually, the timing of these two events in the life cycle varies.
• Three main types of life cycles:
  • Animals: Gametes are the only haploid cells.
    • Meiosis occurs in germ cells during the production of gametes, which undergo no further cell division prior to fertilization.
    • After fertilization, the diploid zygote divides by mitosis, producing a multicellular organism that is diploid.
  • Plants and some species of algae: Alternation of generations
    • Includes both diploid and haploid stages that are multicellular.
    • Sporophyte: The multicellular diploid stage. Meiosis in the sporophyte produces haploid cells called spores.
    • Spores: A haploid cell that doesn’t fuse with another cell but divides mitotically, generating a multicellular haploid stage called the gametophyte.
    • Gametophyte: Cells of the gametophyte give rise to gametes by mitosis.
    • Fusion of two haploid gametes at fertilization results in a diploid zygote, which develops into the next sporophyte generation.
  • Most fungi and some protists:
    • After gametes fuse and form a diploid zygote, meiosis occurs without a multicellular diploid offspring developing.
    • Meiosis produces not gametes but haploid cells that then divide by mitosis and give rise to either unicellular descendants or a haploid multicellular adult organism.
    • Haploid organism carries out further mitoses, producing the cells that develop into gametes.
    • The only diploid stage found in these species is the single-celled zygote.
• Either haploid or diploid cells can divide by mitosis, depending on the type of life cycle.
• Only diploid cells, however, can undergo meiosis because haploid cells have only a single set of chromosomes that cannot be further reduced.
• Regardless of life cycles genetic variation occurs.

Meiosis Reduces the Number of Chromosome Sets from Diploid to Haploid

• Meiosis, like mitosis, is preceded by the duplication of chromosomes.
• Meiosis has two consecutive cell divisions, called meiosis I and meiosis II.
• These two divisions result in four daughter cells, each with only half as many chromosomes as the parent cell—one set, rather than two.

The Stages of Meiosis

• After chromosomes duplicate in interphase, the diploid cell divides twice, yielding four haploid daughter cells.
• Sister chromatids: Two copies of one chromosome, closely associated all along their lengths, (sister chromatid cohesion).
• Together, the sister chromatids make up one duplicated chromosome.
• The two chromosomes of a homologous pair are individual chromosomes that were inherited from each parent.
• Homologs appear alike in the microscope, but may have different versions of genes at corresponding loci (alleles).
• Homologs are not associated with each other in any obvious way except during meiosis.

Exploring Meiosis in an Animal Cell

Meiosis I: Separates Homologous Chromosomes

• Prophase I:
  • Centrosome movement, spindle formation, and nuclear envelope breakdown occur as in mitosis.
  • Chromosomes condense progressively.
  • Each chromosome pairs with its homolog, aligned gene by gene, and crossing over occurs:
    • The DNA molecules of nonsister chromatids are broken (by proteins) and are rejoined to each other.
  • Each homologous pair has one or more X-shaped regions called chiasmata, where crossovers have occurred.
  • Microtubules from one pole or the other attach to the kinetochores, one at the centromere of each homolog.
    • The two kinetochores on the sister chromatids of a homolog are linked together by proteins and act as a single kinetochore).
    • Microtubules move the homologous pairs toward the metaphase plate.
• Metaphase I:
  • Pairs of homologous chromosomes are arranged at the metaphase plate, with one chromosome of each pair facing each pole.
  • Both chromatids of one homolog are attached to kinetochore microtubules from one pole; the chromatids of the other homolog are attached to microtubules from the opposite pole.
• Anaphase I:
  • Breakdown of proteins responsible for sister chromatid cohesion along chromatid arms allows homologs to separate.
  • The homologs move toward opposite poles, guided by the spindle apparatus.
  • Sister chromatid cohesion persists at the centromere, causing chromatids to move as a unit toward the same pole.
• Telophase I and Cytokinesis:
  • Each half of the cell has a complete haploid set of duplicated chromosomes.
  • Each chromosome is composed of two sister chromatids; one or both chromatids include regions of nonsister chromatid DNA.
  • Cytokinesis (division of the cytoplasm) usually occurs simultaneously with telophase I, forming two haploid daughter cells.
  • In animal cells, a cleavage furrow forms.
  • In some species, chromosomes decondense and nuclear envelopes form.
  • No chromosome duplication occurs between meiosis I and meiosis II.

Meiosis II: Separates Sister Chromatids

• Telophase II and Prophase II
  • A spindle apparatus forms.
  • Chromosomes, each still composed of two chromatids associated at the centromere, are moved by microtubules toward the metaphase II plate.
• Metaphase II:
  • The chromosomes are positioned at the metaphase plate as in mitosis.
  • Because of crossing over in meiosis I, the two sister chromatids of each chromosome are not genetically identical.
  • The kinetochores of sister chromatids are attached to microtubules extending from opposite poles.
• Anaphase II:
  • Breakdown of proteins holding the sister chromatids together at the centromere allows the chromatids to separate.
  • The chromatids move toward opposite poles as individual chromosomes.
• Telophase II and Cytokinesis
  • Nuclei form, the chromosomes begin decondensing, and cytokinesis occurs.
  • The meiotic division of one parent cell produces four daughter cells, each with a haploid set of (unduplicated) chromosomes.
  • The four daughter cells are genetically distinct from one another and from the parent cell.

Crossing Over and Synapsis During Prophase I

• Prophase I
  • After interphase, the chromosomes have been duplicated and the sister chromatids are held together by proteins called cohesins.
  • Early in prophase I, the two members of a homologous pair associate loosely along their length. Each gene on one homolog is aligned precisely with the corresponding allele of that gene on the other homolog.
  • The DNA of two nonsister chromatids—one maternal and one paternal—is broken by specific proteins at precisely matching points.
  • Formation of a zipper-like structure called the synaptonemal complex holds one homolog tightly to the other.
  • During synapsis, the DNA breaks are closed up so that each broken end is joined to the corresponding segment of the nonsister chromatid. Thus, a paternal chromatid is joined to a piece of maternal chromatid beyond the crossover point, and vice versa.
  • Points of crossing over become visible as chiasmata after the synaptonemal complex disassembles and the homologs move slightly apart from each other.
  • The homologs remain attached because sister chromatids are still held together by sister chromatid cohesion, even though some of the DNA may no longer be attached to its original chromosome.
  • At least one crossover per chromosome must occur in order for the homologous pair to stay together as it moves to the metaphase I plate.

A Comparison of Mitosis and Meiosis

• Meiosis reduces the number of chromosome sets from two (diploid) to one (haploid), whereas mitosis conserves the number of chromosome sets.
• Meiosis produces cells that differ genetically from their parent cell and from each other, whereas mitosis produces daughter cells that are genetically identical to their parent cell and to each other.
• Three events unique to meiosis occur during meiosis I:
  1. Synapsis and crossing over. During prophase I, duplicated homologs pair up and crossing over occurs. Synapsis and crossing over do not occur during prophase of mitosis.
  2. Alignment of homologous pairs at the metaphase plate. At metaphase I of meiosis, pairs of homologs are positioned at the metaphase plate, rather than individual chromosomes, as in metaphase of mitosis.
  3. Separation of homologs. At anaphase I of meiosis, the duplicated chromosomes of each homologous pair move toward opposite poles, but the sister chromatids of each duplicated chromosome remain attached. In anaphase of mitosis, sister chromatids separate.
• Sister chromatids stay together due to sister chromatid cohesion, mediated by cohesin proteins.
• In meiosis, sister chromatid cohesion is released in two steps, one at the start of anaphase I and one at anaphase II.
• The combination of crossing over and sister chromatid cohesion along the arms results in the formation of a chiasma.
• Chiasmata hold homologs together as the spindle forms for the first meiotic division.
• At the onset of anaphase I, the release of cohesion along sister chromatid arms allows homologs to separate.
• At anaphase II, the release of sister chromatid cohesion at the centromeres allows the sister chromatids to separate.
• Essentially the mechanisms for separating sister chromatids in meiosis II and mitosis are virtually identical.

Genetic Variation Produced in Sexual Life Cycles Contributes to Evolution

• Mutations are the original source of genetic diversity.
• Mutations create the different versions of genes, known as alleles.
• Reshuffling of alleles during sexual reproduction produces the variation that results in each member of a sexually reproducing population having a unique combination of traits.

Origins of Genetic Variation Among Offspring

• Three mechanisms contribute to the genetic variation arising from sexual reproduction:
  • Independent assortment of chromosomes.
  • Crossing over.
  • Random fertilization.

Independent Assortment of Chromosomes

• At metaphase I, the homologous pairs, each consisting of one maternal and one paternal chromosome, are situated at the metaphase plate.
• Each pair may orient with either its maternal or paternal homolog closer to a given pole—its orientation is as random as the flip of a coin.
• There is a 50% chance that a particular daughter cell of meiosis I will get the maternal chromosome of a certain homologous pair and a 50% chance that it will get the paternal chromosome.
• Because each pair of homologous chromosomes is positioned independently of the other pairs at metaphase I, the first meiotic division results in each pair sorting its maternal and paternal homologs into daughter cells independently of every other pair, called independent assortment.
• The number of possible combinations when chromosomes sort independently during meiosis is $2^n$, where $n$ is the haploid number of the organism.
• In humans ($n = 23$), the number of possible combinations of maternal and paternal chromosomes in the resulting gametes is $2^{23}$, or about 8.4 million.

Crossing Over

• Crossing over produces recombinant chromosomes, individual chromosomes that carry genes (DNA) from two different parents.
• In meiosis in humans, an average of one to three crossover events occurs per chromosome pair.
• At metaphase II, chromosomes that contain one or more recombinant chromatids can be oriented in two alternative, nonequivalent ways with respect to other chromosomes because their sister chromatids are no longer identical.
• Crossing over, by combining DNA inherited from two parents into a single chromosome, is an important source of genetic variation in sexual life cycles.

Random Fertilization

• In humans, each male and female gamete represents one of about 8.4 million ($2^{23}$) possible chromosome combinations due to independent assortment.
• The fusion of a male gamete with a female gamete during fertilization will produce a zygote with any of about 70 trillion ($2^{23} * 2^{23}$) diploid combinations.

The Evolutionary Significance of Genetic Variation Within Populations

• Darwin recognized that a population evolves through the differential reproductive success of its variant members.
• On average, those individuals best suited to the local environment leave the most offspring, thereby transmitting their genes.
• Natural selection results in the accumulation of genetic variations favored by the environment.
• As the environment changes, the population may survive if, in each generation, at least some of its members can cope effectively with the new conditions.
• Mutations are the original source of different alleles, which are then mixed and matched during meiosis.