Meiosis and Sexual Life Cycles

• The transmission of traits from one generation to the next is called inheritance, or heredity
• Along with inherited similarity, there is also variation. The study of both heredity and inherited variation is called genetics.
• Parents endow their offspring with coded information in the form of hereditary units called genes.
• The genetic program is written in the language of DNA,

  the polymer of four different nucleotides.
• Inherited information is passed on in the form of each gene’s specific sequence of DNA nucleotides
• The transmission of hereditary traits has its molecular basis in the replication of DNA, which produces copies of genes that can be passed from parents to offspring.
• In animals and plants, reproductive cells called gametes are the vehicles 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.
• humans have 46 chromosomes in their somatic cells
• A gene’s specific location along the length of a chromosome is called the gene’s locus
• Only organisms that reproduce asexually have offspring that are exact genetic copies of themselves
• In asexual reproduction, a single individual (like a yeast cell or an amoeba) is the sole parent and passes copies of all its genes to its offspring without the fusion of gametes.
• An individual that reproduces asexually gives rise

  to a clone, an individual or group of individuals that are genetically identical to the parent.
• Genetic differences occasionally arise in asexually reproducing organisms as a result of changes in the DNA called mutations.
• In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents.
• Offspring of sexual reproduction vary genetically from their siblings and both parents: They are variations on a common theme of family resemblance, not exact replicas.
• A life cycle is the generation-to-generation sequence of stages in the reproductive history of an organism, from conception to production of its own offspring.
• This becomes clear when images of the chromosomes are arranged in pairs, starting with the longest chromosomes. The resulting ordered display is called a karyotype.
• The two chromosomes of a pair have the same length, centromere position, and staining pattern: These are called homologous chromosomes (or homologs). Both chromosomes of each pair carry genes controlling the same inherited characters.
• Typically, human females have a homologous pair of X chromosomes (XX), while males have one X and one Y chromosome (XY)
• The X and Y chromosomes are called sex chromosomes. The other chromosomes are called autosomes.
• We inherit one chromosome of a pair from each parent. Thus,

  the 46 chromosomes in our somatic cells are actually two sets of 23 chromosomes—a maternal set (from our mother) and a paternal set (from our father).
• The number of chromosomes in a single set is represented by n.
• Any cell with two chromosome sets is called a diploid cell (2n).
• In a cell in which DNA synthesis has occurred, all the chromosomes are duplicated, and therefore each consists of two identical sister chromatids, associated closely at the centromere and along the arms
• Unlike somatic cells, gametes contain a single set of chromosomes. Such cells are called haploid cells, and each has a haploid number of chromosomes (n).
• This union of gametes, culminating in fusion of their nuclei, is called fertilization.
• The resulting fertilized egg, or zygote, is diploid because it contains two haploid sets of chromosomes bearing genes representing the maternal and paternal family lines.
• As a human develops into a sexually mature adult, mitosis of the zygote and its descendant cells generates all the somatic cells of the body.
• The only cells of the human body not produced by mitosis

  are the gametes, which develop from specialized cells called germ cells in the gonads—ovaries in females and testes in males
• Gamete formation involves a type of cell division called meiosis. This type of cell division reduces the number of sets of chromosomes from two in the parent cell to one in each gamete.
• Fertilization restores the diploid condition by combining two sets of chromosomes, and the human life cycle is repeated, generation after generation.
• Fertilization and meiosis alternate in sexual life cycles, maintaining a constant number of chromosomes in a species from one generation to the next.
• Plants and some species of algae exhibit a second type of life cycle called alternation of generations.
• The multicellular diploid stage is called the sporophyte. Meiosis in the sporophyte produces haploid cells called spores.
• Unlike a gamete, a haploid spore doesn’t fuse with another cell but divides mitotically, generating a multicellular haploid stage called the 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.
• Either haploid or diploid cells can divide by mitosis, depending on the type of life cycle.
• Only diploid cells, however, can undergo meiosis: Haploid cells can’t because they already have a single set of chromosomes that cannot be further reduced.
• Meiosis, like mitosis, is preceded by interphase, which includes S phase (the duplication of chromosomes).