MEIOSIS and SEXUAL REPRODUCTION

TERMS

Gametes – specialized cells involved in sexual reproduction (male and female) Fertilization – fusion of the nuclei of the male and female gametes Zygote – the single cell that results from fertilization

Meiosis – cell division that results in ½ the number of chromosomes of the parent cell

Somatic cells – all cells of the body except the sex cells Homologous chromosomes – chromosomes in the cells that are arranged in pair of similar size, shape, and genetic content

Diploid cells – cells that contain homologous chromosomes Haploid cells (monoploid) – cells that contain only one of each of the homologous pairs of chromosomes (gametes)

Synapsis – the pairing of the duplicated homologous pairs joined at the centromeres Tetrad – the group of four chromatids (duplicated homologous pair)

Crossing over - the exchange of parts of chromatids that occurs during synapsis

Disjunction – the process of separation of the tetrads (during anaphase I.)

THE STAGES OF MEIOSIS

INTERPHASE the original cell has 6 chromosomes 3 chromosome pairs chromosomes duplicate

PROPHASE I chromosomes already duplicated chromosomes match up with its homologous pair four chromatids are fastened at their centromere four chromatids are called a “tetrad” crossing over and exchange of segments may occur nuclear membrane disappears spindles form tetrads move toward the equator

METAPHASE I Centromeres of the tetrads line up at the equator Spindle microtubules fasten on to the centromeres

Homologous chromosome tetrad separate “disjunction” ANAPHASE I Homologous chromosome tetrad separate “disjunction”

Cytokinesis occurs – two daughter cells produced TELOPHASE I Cytokinesis occurs – two daughter cells produced

Daughter cells form spindles Chromosomes move toward the equator PROPHASE II Daughter cells form spindles Chromosomes move toward the equator

Spindle attaches to centromeres Chromosomes line up at the equator METAPHASE II Spindle attaches to centromeres Chromosomes line up at the equator

ANAPHASE II Centromeres divide, the two chromatids separate and move to opposite poles

TELOPHASE II Both daughter cells divide Four haploid cells form Nuclear membranes form

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COMPARING MITOSIS AND MEIOSIS *Occurs in production of: gametes in animals spores in plants some simple organisms *Occurs in: growth asexual reproduction *Homologous chromosomes not paired up *No exchange in homologous chromosomes *Homologous chromosomes paired up *Exchange in homologous chromosomes *Involves two cell divisions *Involves one cell division *Results in daughter cell receiving the same # of chromosomes as parent cell Mitosis maintains chromosome number *Results in each daughter cell receiving half the number of chromosomes as the parent cell Meiosis reduces the chromosome number to one-half

SEXUAL REPRODUCTION IN SIMPLE ANIMALS Conjugation No distinctly different sexes but there are biochemical and chromosomal differences called (+) and (-) A bridge forms and there is an exchange of nuclear material Introduces variation into the species Examples are: bacteria, spirogyra, paramecium

SEXUAL REPRODUCTION IN ANIMALS Reproductive systems Gonads – special organs that produce gametes Male = testes produce sperm Females = ovaries produce ova (eggs)

HERMAPHRODITISM An organism that has both testes and ovaries Mostly slow moving animals (earthworms, snails, hydras) Rarely self-fertilize Usually exchange sperm into sperm receptacle of its partner Both animals have fertilized eggs

MEIOSIS IN FEMALES AND MALES GAMETOGENESIS MEIOSIS IN FEMALES AND MALES OOGENESIS: formation of eggs

OOGENESIS: formation of eggs

GAMETOGENESIS MEIOSIS IN FEMALES AND MALES SPERMATOGENESIS: formation of sperm

SPERMATOGENESIS: formation of sperm

GAMETOGENESIS OOGENOSIS SPERMATOGENESIS Production of eggs Only one ovum (mature egg) Continual production of sperm Meiotic divisions Meiotic division Meiotic process results in only Meiosis process results in Ova are Sperm are stops at birth through out life continual develops per month unequal equal one mature ovum (egg) four mature sperm haploid cells haploid cells ♦ Round shape ♦ Head, middle, flagellum ♦ Non-motile ♦ Motile ♦ May have food stored as yolk ♦ Mitochondria for energy ♦ Larger than sperm ♦ Microscopic

Female ova are undeveloped until the female reaches sexual maturity Female ova are undeveloped until the female reaches sexual maturity. Then only one ovum/month develops. Sperm structure Head Nucleus DNA material Enzymes for penetrating the egg Middle Mitochondria for energy to swim to the egg Flagellum For motility

FERTILIZATION AND ZYGOTE FORMATION Haploid (egg) cell + haploid (sperm) cell = diploid cell or zygote Enzymes from the head of the sperm penetrate the egg cell Only the nucleus of the sperm enters (DNA) All cytoplasm is from the egg (female) Fertilization membrane forms immediately to stop other sperm from entering the egg

TWO TYPES OF FERTILIZATION External fertilization Internal fertilization

External Fertilization – outside the females’ body (fish) Need a water medium for sperm to swim in Very hazardous – many are released Spawning – female releases eggs and then male swims over the area immediately and releases sperm Amplexus – male embraces female and eggs and sperm are released at the same time

Internal fertilization – sperm released into the female body (land animals, sharks and lobsters) Fewer eggs needed because they are well protected Greater numbers of sperm Timing must be right Some animals can store sperm for later use (bees and bats) Reproductive adaptations controlled by hormones singing color display feathers displayed

PARENTOGENESIS Development of an unfertilized egg into an adult animal (bees, wasps, aphids, ants) bees - unfertilized eggs become male drones (workers) - fertilized eggs become female workers or queens