Meiosis and Sexual Life Cycles

Offspring Acquire Genes from Parents

• Parents pass on hereditary units called genes to their offspring, which program specific traits.
• Genes are coded in DNA, a polymer of four different nucleotides.
• DNA replication ensures that copies of genes are passed from parents to offspring.
• In animals and plants, gametes (sperm and eggs) transmit genes from one generation to the next during fertilization.
• Eukaryotic cell DNA is packaged into chromosomes within the nucleus; each species has a characteristic number of chromosomes.
• Humans have 46 chromosomes in their somatic cells.
• A gene's specific location along a chromosome is its locus.
• The genome comprises the genes and other DNA that make up the chromosomes.

Asexual vs Sexual Reproduction

• Asexual reproduction produces offspring that are exact genetic copies of the parent.
• A single parent passes copies of all its genes to offspring without the fusion of gametes.
• Single-celled eukaryotic organisms reproduce asexually by mitotic cell division.
• Some multicellular organisms can also reproduce asexually, resulting in genetically identical offspring.
• A clone is an individual or group of individuals genetically identical to the parent.
• Mutations can introduce genetic differences in asexually reproducing organisms.
• Sexual reproduction involves two parents giving rise to offspring with unique combinations of genes inherited from both parents.
• Offspring of sexual reproduction vary genetically from their siblings and parents.
• Genetic variation is an important consequence of sexual reproduction.

Fertilization and Meiosis Alternate in Sexual Life Cycles

• A life cycle is the generation-to-generation sequence of stages in the reproductive history of an organism.

Sets of Chromosomes in Human Cells

• Human somatic cells have 46 chromosomes.
• Chromosomes duplicate before mitosis.
• A karyotype is an ordered display of chromosomes arranged in pairs.
• Homologous chromosomes (or homologs) have the same length, centromere position, and staining pattern, carrying genes controlling the same inherited characters.
• Human females have a homologous pair of X chromosomes (XX), while males have one X and one Y chromosome (XY).
• X and Y chromosomes are called sex chromosomes.
• Other chromosomes are called autosomes.
• The 46 chromosomes in human somatic cells are two sets of 23 chromosomes-a maternal set and a paternal set.
• The number of chromosomes in a single set is represented by $n$.
• A diploid cell has two chromosome sets and a diploid number of chromosomes, abbreviated $2n$.
• For humans, $2n = 46$. Each consists of two identical sister chromatids, associated closely at the centromere and along the arms.
• Gametes contain a single set of chromosomes and are called haploid cells, each with a haploid number of chromosomes (n).
• For humans, $n = 23$.
• An unfertilized egg contains an X chromosome; a sperm contains either an X or a Y chromosome.
• The chromosome number generally does not correlate with the size or complexity of a species' genome.

Behavior of Chromosome Sets in the Human Life Cycle

• The human life cycle begins with fertilization, where a haploid sperm fuses with a haploid egg, forming a diploid zygote.
• Mitosis of the zygote and its descendant cells generates all the somatic cells of the body.
• Gametes develop from germ cells in the gonads (ovaries in females and testes in males).
• Meiosis reduces the number of chromosome sets from two to one in the gametes, counterbalancing the doubling that occurs at fertilization.
• Each human sperm and egg is haploid ($n = 23$).

Variety of Sexual Life Cycles

• Fertilization and meiosis alternate in sexual life cycles, maintaining a constant number of chromosomes in a species.
• In animals, gametes are the only haploid cells.
• In plants and some algae, alternation of generations occurs with both diploid and haploid multicellular stages.
• The multicellular diploid stage is called the sporophyte, which produces haploid spores via meiosis. Spores divide mitotically, generating a multicellular haploid stage called the gametophyte.
• Cells of the gametophyte give rise to gametes by mitosis, which fuse to form a diploid zygote, developing into the next sporophyte generation.
• In most fungi and some protists, meiosis occurs after gamete fusion to form a diploid zygote, but without a multicellular diploid offspring.
• Meiosis produces haploid cells that divide by mitosis, leading to unicellular descendants or a haploid multicellular adult organism.
• Haploid organisms produce gametes through mitosis.
• Only diploid cells can undergo meiosis.

Meiosis Reduces Chromosome Sets from Diploid to Haploid

• Meiosis involves two consecutive cell divisions (meiosis I and meiosis II), resulting in four daughter cells with half as many chromosomes as the parent cell.

Stages of Meiosis

• After chromosome duplication in interphase, meiosis I separates homologous chromosomes, and meiosis II separates sister chromatids.
• Sister chromatids are two copies of one chromosome, associated along their lengths via sister chromatid cohesion.
• Homologous chromosomes are individual chromosomes inherited from each parent and may have different versions of genes (alleles) at corresponding loci.

Crossing Over and Synapsis During Prophase I

• During prophase I, homologous chromosomes pair up along their length, aligned allele by allele.
• DNA molecules of maternal and paternal chromatids are broken at matching points.
• A synaptonemal complex forms, attaching one homolog to the other (synapsis).
• DNA breaks are closed up, joining a paternal chromatid to a piece of maternal chromatid and vice versa (crossing over).
• At least one crossover per chromosome must occur, along with sister chromatid cohesion, to keep the homologous pair together during metaphase I.

Comparison of Mitosis and Meiosis

• Meiosis produces four cells and reduces chromosome sets from two to one, while mitosis produces two cells and conserves the number of sets.
• Meiosis produces genetically different cells, while mitosis produces genetically identical daughter cells.
• Three unique events occur during meiosis I:
  • Synapsis and crossing over: Homologs pair up, and crossing over occurs.
  • Alignment of homologous pairs: Pairs of homologs are positioned at the metaphase plate.
  • Separation of homologs: Duplicated chromosomes of each homologous pair move toward opposite poles but sister chromatids remain attached.
• Sister chromatids stay together due to sister chromatid cohesion until the end of metaphase in mitosis.
• In meiosis, sister chromatid cohesion is released in two steps: at the start of anaphase I (separating homologs) and at anaphase II (separating sister chromatids).
• Chiasmata hold homologs together as the spindle forms.
• Meiosis I reduces chromosome sets from diploid to haploid.
• Meiosis II separates sister chromatids, producing haploid daughter cells.

Genetic Variation in Sexual Life Cycles Contributes to Evolution

• Mutations are the original source of genetic diversity.
• Reshuffling of alleles during sexual reproduction results in unique combinations of traits.

Origins of Genetic Variation Among Offspring

• Three mechanisms contribute to genetic variation:
  • Independent assortment of chromosomes
  • Crossing over
  • Random fertilization

Independent Assortment of Chromosomes

• Random orientation of homologous chromosome pairs at metaphase I generates genetic variation.
• Each pair orients independently, resulting in different combinations of maternal and paternal chromosomes in daughter cells.
• The number of possible combinations is $2^n$, where n is the haploid number.
• In humans ($n = 23$), there are about 8.4 million possible combinations of chromosomes.

Crossing Over

• Crossing over produces recombinant chromosomes, carrying genes from two different parents.
• An average of one to three crossover events occurs per chromosome pair in humans.
• Crossing over creates new combinations of maternal and paternal alleles.

Random Fertilization

• The random nature of fertilization adds to genetic variation.
• The fusion of male and female gametes will produce a zygote with any of about 70 trillion diploid combinations in humans.

Evolutionary Significance of Genetic Variation

• A population evolves through the differential reproductive success of its members.
• Natural selection favors genetic variations beneficial to the environment.