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Chapter 11: Meiosis and sexual reproduction Sexual vs. Asexual Reproduction Sexual: •2 parents •Offspring are genetically different from parent – mix of traits •Union of egg & sperm Asexual: •1 parent •Offspring are identical to parent •Binary fission in bacteria •Regeneration in worms •In sexual reproduction, each parent produces a reproductive cell called a gamete •Male gamete – sperm •Female gamete – egg •Zygote – formed when sperm fertilizes egg •Fertilization – process in which sperm and egg fuse together Examples of asexual reproduction: •Fragmentation – body breaks into several pieces, each of which regrows into an adult •Budding – new individuals split off from existing ones •Parthenogenesis – a female makes a viable egg that grows into an adult without being fertilized by a male •Binary fission – a type of cell division used by prokaryotic cells Advantage of sexual reproduction: •Produces offspring that are different from their parents so they have a better chance of surviving changes in the environment, diseases, etc. Advantages of asexual reproduction: •Simple and efficient •No need to find a mate •May produce many offspring in a relatively short period of time Somatic cells – body cells such as skin, hair, muscle, bones, or organs that do no participate in sexual reproduction Germ cells – cells that are specialized for sexual reproduction, such as eggs or sperm •Each species has a set number of chromosomes: •Humans – 46 •Crayfish – 200 •Corn – 20 •Adder’s tongue fern – 1262 •Chimpanzee - 48 •Sand dollar – 52 •Dog – 78 •Cat - 32 Viewing the Chromosomes • Karyotype - a photograph of the chromosomes in a dividing cell • Shows the gender of the child • Shows abnormalities in chromosome number or structure. • Humans have 23 pairs of homologous chromosomes (chromosomes with the same genes; similar in size and shape), or 46 chromosomes total. • You get 1 of each chromosome from mom, and 1 from dad. •Of the 23 pairs of chromosomes: •1 pair are sex chromosomes – determine your gender; may determine other traits as well •Females are XX •Males are XY •22 pairs are autosomes – all other pairs of chromosomes; do NOT determine gender • • • • How is karyotyping done? Cells are taken from the fetus of a pregnant woman Chemicals added to the cells make them divide Another chemical stops division at the midpoint The stained cells are photographed and can be paired based on cross-bands, size, and shape. Human karyotype preparation Normal male karyotype Down syndrome karyotype •Body cells contain the diploid (2n) number of chromosomes – contain 2 chromosomes of each kind (1 from each parent) •Sex cells (eggs and sperm) contain only 1 chromosome of each kind – haploid (1n) number of chromosomes Meiosis – form of cell division that produces daughter cells with half the number of chromosomes that are in the parent • Produces sex cells (gametes) • Female gamete – egg • Male gamete - sperm • Reduces the chromosome number • Egg or sperm cells each have only one of each kind of chromosome • Diploid cells (2n) haploid cells (1n) Overview of meiosis http://www.cellsalive.com/meiosis.htm • Meiosis involves two cell divisions (meiosis I and II) • Produces four haploid cells. Meiosis I •Original cell produces 2 new cells •Splits homologous chromosomes so that each new cell has 1 chromosome from each homologous pair •Diploid cells (2n) create haploid cells (1n) •Crossing over (chromosomes exchange genetic material) can occur Meiosis II •Each of the 2 cells made in meiosis I splits •2 haploid cells divide to make 4 haploid cells •Chromatids of chromosomes are separated •Errors in meiosis can result in missing or extra chromosomes Changes in Chromosome Number • Occurs when: – In meiosis I, homologous pair both go into the same daughter cell or – In meiosis II, the sister chromatids both go into the same gamete. • The result is either: – Trisomy (3 copies of a single chromosome) – Monosomy (1 copy of a single chromosome) Nondisjunction in meiosis I Nondisjunction in meiosis II Types of chromosomal mutations: •Deletion – a piece of a chromosome is lost •Duplication – a chromosome with a repeated section attached •Inversion –a section of chromosome is flipped in the reverse direction •Translocation – a chromosome piece ends up on another non-homologous chromosome •Gene rearrangement – an entire gene moves to a different location on the same chromosome • Ex. of deletion: Williams syndrome • Chromosome 7 loses an end piece • Children have a pixie look (turned-up noses, wide mouth, small chin, large ears) • Poor academic skills, good verbal and musical abilities • Skin ages prematurely from lack of the gene that controls the production of elastin (also affects cardiovascular health). • Ex of duplication: An inverted duplication in chromosome 15 causes inv dup 15 syndrome • Poor muscle tone, mental retardation, seizures, curved spine, and autistic characteristics Duplication • Ex of translocation: Alagille syndrome results from a deletion of chromosome 20 or a translocation that disrupts an allele on chromosome 20. • The symptoms for Alagille syndrome range from mild to severe, so people may not be aware they have the syndrome. • Distinctive face, abnormalities of eyes & internal organs, and severe itching. Translocation Meiosis in males vs. females: •Males: •Occurs in testes •Spermatogenesis •Makes 4 haploid sperm •Females: •Occurs in ovaries •Oogenesis •Makes 1 large egg & 3 polar bodies (that die) – cells divide unevenly Spermatogenesis Oogenesis Meiosis vs. Mitosis Mitosis •Body cells •Diploid cells make diploid cells •End result 2 cells •No variation in cells produced Meiosis •Sex cells •Diploid cells make haploid cells •End result up to 4 cells •Cells made are different from parent Meiosis compared to mitosis • • • • • The Human Life Cycle Requires both mitosis and meiosis. In males, meiosis occurs as spermatogenesis and produces 4 haploid sperm. In females, meiosis occurs as oogenesis and produces 1 egg cell. Mitosis is involved in the growth of a child and repair of tissues during life. Cell differentiation occurs at many times during development as cells take on specific roles Life cycle of humans