Meiosis and Sexual Life Cycles Chapter 8

Genetics Heredity- is the transmission of traits from one generation to the next. Variation- shows that offspring differ somewhat in appearance from parents and siblings.

Genetics Study of heredity and hereditary variation Offspring acquire genes from parents by inheriting chromosome Genes: Are the units of heredity Are segment of DNA We inherit one set of chromosomes from our mother and one from our father

Overview of meiosis Meiosis is the type of nuclear division that reduces the chromosome number from the diploid number (2n) to the haploid number (n) Diploid (2n)- refers to the total number of chromosomes, which insist in 2 sets Haploid (n)- is half the diploid number, or a single set of chromosomes

Chromosomes In humans Each somatic cell has 46 chromosomes, made up of two sets 2n=46 During meiosis haploid (n) cells are created (sperm and egg) Gametes Gamete + Gamete = zygote

Chromosomes In diploid body cells, chromosomes occur in pairs Humans have 23 different types of chromosomes homologous chromosomes They have the same length Their centromeres are positioned in the same place One came from the father (the paternal homolog) the other from the mother (the maternal homolog) Genes for the traits located in same position Different sequences=alleles

Homologous chromosomes Karyotype- a map showing the pairs of chromosomes 22 pairs of autosomes 1 pair of sex chromosomes

Meiosis Reduces the number of chromosome sets from diploid to haploid Takes place in two set of divisions, meiosis I and meiosis II Meiosis I Reduces the number of chromosomes from diploid to haploid Meiosis II Produces four haploid daughter cells

Overview Meiosis I Meiosis II Overview of Meiosis Chromosomes are replicated prior to meiosis I Each chromosome consists of two identical sister chromatids Homologous chromosomes pair up – synapsis Homologous pairs align themselves against each other side by side at the metaphase plate The two members of a homologous pair separate Each daughter cell receives one duplicated chromosome from each pair Meiosis II DNA is not replicated between meiosis I and meiosis II Sister chromatids separate and move to opposite poles The four daughter cells contain one daughter chromosome from each pair Each daughter chromosome consists of a single chromatid The daughter cells are haploid

Meiosis Compared to Mitosis Biology, 9th ed,Sylvia Mader Chapter 10 Meiosis Compared to Mitosis Meiosis Requires two nuclear divisions Chromosomes synapse and cross over Centromeres survive Anaphase I Halves chromosome number Produces four daughter nuclei Produces daughter cells genetically different from parent and each other Used only for sexual reproduction Mitosis Requires one nuclear division Chromosomes do not synapse nor cross over Centromeres dissolve in mitotic anaphase Preserves chromosome number Produces two daughter nuclei Produces daughter cells genetically identical to parent and to each other Used for asexual reproduction and growth Meiosis & Sexual Reproduction

Genetic Variation Significance of genetic variation: Asexual reproduction produces genetically identical clones Sexual reproduction causes genetic recombinations among members of a population Asexual reproduction is advantageous when the environment is stable However, if the environment changes, genetic variability introduced by sexual reproduction may be advantageous Some offspring may have a better chance of survival

Crossing Over Exchange of genetic material between non-sister chromatids during meiosis I At synapsis, a nucleoprotein lattice appears between homologues Then homologues separate and are distributed to different daughter cells

Crossing Over Occurs During Meiosis I Biology, 9th ed,Sylvia Mader Chapter 10 Crossing Over Occurs During Meiosis I nucleoprotein lattice sister chromatids of a chromosome sister chromatids of its homologue A A a a A a B B b b B b chiasmata of nonsister chromatids 1 and 3 c C C c C c D D d d D d 1 2 3 4 1 2 3 4 1 2 3 4 Bivalent forms Crossing-over has occurred Daughter chromosomes a. b. c. d. Meiosis & Sexual Reproduction

Independent assortment When homologous chromosome pairs align at the metaphase plate: They separate in a random manner Causes random mixing of blocks of alleles into gametes

Problems Nondisjunction- failure to separate chromosomes into daughter cells Results in some new cells with 2n + 1 (3) or 2n – 1 (1)

Nondisjunction Most common is Down Syndrome Recognized by these characteristics: short stature eyelid fold flat face stubby fingers wide gap between first and second toes

Nondisjunction in sex chromosomes Results from inheriting too many or too few X or Y chromosomes Nondisjunction during oogenesis or spermatogenesis Turner syndrome (XO) Female with a single X chromosome Short, with broad chest and widely spaced nipples Can be of normal intelligence and function with hormone therapy Klinefelter syndrome (XXY) Male with underdeveloped testes and prostate; some breast overdevelopment Long arms and legs; large hands Near normal intelligence unless XXXY, XXXXY, etc. No matter how many X chromosomes are present, the presence of a chromosome Y renders the individual male

Other changes Deletion One or both ends of a chromosome breaks off Two simultaneous breaks lead to loss of an internal segment Duplication Presence of a chromosomal segment more than once in the same chromosome Translocation A segment from one chromosome moves to a non-homologous chromosome Follows breakage of two non-homologous chromosomes and improper re-assembly Inversion Occurs as a result of two breaks in a chromosome The internal segment is reversed before re-insertion Genes occur in reverse order in the inverted segment

Chromosome mutations

Differences in the life cycle In plants, haploid multicellular “individuals” alternate with diploid multicellular “individuals” (alternation of generations) The haploid individual: Is called the gametophyte The diploid individual: Is called the sporophyte Mosses are haploid for most of their life cycle In fungi and most algae, only the zygote is diploid Ferns & higher plants are diploid for most of their life cycles In plants, algae, and fungi gametes are produced by haploid individuals

Plant Life Cycle