BIOL-101 Lecture 8: Reproduction

Reproduction

The ability to produce viable offspring

Asexual reproduction

A single parent produces genetically identical offspring

NO genetic variation

Requires little time and little enrgy

Four types: binary fission, budding, fragmentation, parthenogenesis

Binary fission

Division of a single parent cell equally into two daughter cells

Daughter cells- identical genetically and in size

Occurs in bacteria and in some unicellular protists such as amoebas

Budding

The unequal splitting of a parent cell/organism into two unequally sized offspring

Occurs in yeast and hydra

Fragmentation

A piece of an organism breaks away and is capable of becoming another genetically identical individual (intended or accidental)

Occurs in sponges, sea stars and some worms

Regeneration- replacement of lost body parts that is performed by the parent once the fragment breaks away

Parthenogenesis

Development of unfertilized egg into new individual; "virgin birth"

Occurs in insects, roundworms, sharks, amphibians, and reptile species

Sexual reproduction

Two parents contribute genetics to produce offspring that are genetically unique from each other; promotes genetic variation, requires time and energy

Requires fertilization and meiosis

Fertilization

Sperm and egg unite and produce zygote

Zygote

Possess diploid numbers of chromosomes (2n: 46 in total)

Gametes

Sex cells; possess haploid numbers of chromosomes (n: 23 in total)

Meiosis

Cell division that occurs in gonads of animals

Two step cell division designed to cut chromosome number in half

Diploid -> haploid, produces four cells from one

[mitosis is 1-2 cells: diploid -> diploid]

Homologous chromosomes

Identical size, shape, and traits

Humans have 22 pairs of homologous autosomes and one pair of sex chromosomes (influence gender)

Female XX homologous; male XY non-homologous

Gonads

Animal's sex organs (testes, ovaries)

Interphase

Preparation period for cell division

G1 phase- no cell growth occurs (not needed to produce gametes)

S phase- chromosomes duplicate -> two sister chromatids are held together by a centromere

G2 phase- no mass production of cell structures (not needed for gametes)

Meiosis 1

Reduction stage: homologous chromosomes separate

Yields a haploid cell with replicated chromosomes

Prophase 1

Chromosomes become thick and visible

Nucleolus disappears

Meiotic spindles form

Nuclear envelope breaks down

Tetrad formation occurs (pair of duplicated chromosomes [four chromatids])

Crossing over can occur

Crossing over

The exchange of pieces of genetic material between two chromatids within a tetrad

The point of cross over is called a chiasma

Metaphase 1

Lining up of tetrads along the middle of the cell

Spindle fibers move the tetrads to the equator of the cell

Anaphase 1

Separation and pulling apart of tetrads into dyads (duplicated chromosomes)

Telophase 1 and Cytokinesis

Formation of two haploid nuclei each with duplicated chromosomes

Meiotic spindles break down

Two haploid cells form

Interkinesis

Brief period between meiosis 1 and 2 that interphase

NOTHING HAPPENS!

No cell growth, structure production, or duplication

Meiosis 2

Division stage: sister chromatids separate

Yields four haploid cells with unreplicated chromosomes

Prophase 2

Typical events of prophase

Chromosomes thicken as dyads

Metaphase 2

Lining up of dyads along the middle of the cell

Anaphase 2

Separation and pulling apart of dyads into individual chromatids

Centromere bonds break

Telophase 2 and Cytokinesis

Formation of four haploid nuclei each with individual chromatids within four separate cells

Meiotic spindles break down

Spermatogenesis

Production of sperm in testes

One testis cell undergoing complete meiosis yields four genetically different sperm

Oogenesis

Production of eggs (ova) in the ovaries

One ovary cell undergoing complete meiosis yields one ovum and three nonviable polar bodies (much smaller than ova)

Independent assortment

Tetrads line up independently of one another during Metaphase 1 and are separated independently from one another during anaphase 1

#genetic variation