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

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. MitosisMitosis

•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