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 tha 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

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