Mitosis vs. Meiosis Asexual vs. Sexual Reproduction

Mitosis “The replication of the chromosomes and the production of two nuclei in one cell; usually followed by cytokinesis” (736). “The replication of the chromosomes and the production of two nuclei in one cell; usually followed by cytokinesis” (736). Mitosis is the process of one cell dividing into two identical copies of the cell, known as daughter cells. Mitosis is the process of one cell dividing into two identical copies of the cell, known as daughter cells. Mitosis preserves the number of chromosomes that each cell contains (diploid cells - 2N) Mitosis preserves the number of chromosomes that each cell contains (diploid cells - 2N) Mitosis is used for growth, repair or asexual reproduction. Mitosis is used for growth, repair or asexual reproduction.

Asexual Reproduction New individuals originate from a single parent New individuals originate from a single parent Either the parent divides into two (or more) individuals or new individuals arise as buds from the parent’s body Either the parent divides into two (or more) individuals or new individuals arise as buds from the parent’s body The only genetic changes arise as the result of mutations (chance events that change the DNA of the offspring) The only genetic changes arise as the result of mutations (chance events that change the DNA of the offspring) Low genetic diversity. Low genetic diversity.

Asexual reproduction is a form of regeneration. Sponges, starfish and salamanders are well known for regeneration. w-superhero-science-limb- regeneration.html w-superhero-science-limb- regeneration.html w-superhero-science-limb- regeneration.html w-superhero-science-limb- regeneration.html

Sexual Reproduction Why does sex exist? 99.9% of organisms bigger than bacteria do it. Why does sex exist? 99.9% of organisms bigger than bacteria do it. It increases diversity of the offspring and species! It increases diversity of the offspring and species! More diversity means more resistance to disease, and the ability to adapt to changing circumstances in the environment More diversity means more resistance to disease, and the ability to adapt to changing circumstances in the environment

Gamete In humans and many other animals, the gametes (sex cells) are eggs (ova) and sperm. In humans and many other animals, the gametes (sex cells) are eggs (ova) and sperm. In flowering plants, these are ovules and pollen (sperm). In flowering plants, these are ovules and pollen (sperm).

Gametes continued The gametes contain only ½ the genetic information required to make a new individual (Haploid: n) The gametes contain only ½ the genetic information required to make a new individual (Haploid: n) What would happen if gametes were created through mitosis, and had the normal number of chromosomes? What would happen if gametes were created through mitosis, and had the normal number of chromosomes? Consider a gamete like this from an organism with 10 chromosomes…. Consider a gamete like this from an organism with 10 chromosomes….

How many chromosomes? Gametes are haploid cells, containing 1 complete set of “n” chromosomes. Gametes are haploid cells, containing 1 complete set of “n” chromosomes. “n” for humans is 23 “n” for humans is 23 All other body cells are diploid, having “2n” or 2 complete sets of chromosomes. All other body cells are diploid, having “2n” or 2 complete sets of chromosomes. “Ploidy” refers to how many complete sets of chromosomes there are. “Ploidy” refers to how many complete sets of chromosomes there are.

Th ere is often a large difference in the size of male vs. female gametes.

The structure of a chromosome after replication

The chromosomes come in pairs! Homologous chromosomes are matching pairs of chromosomes, similar in shape and content. Homologous chromosomes are matching pairs of chromosomes, similar in shape and content. One chromosome of each pair came from the father (sperm), and one came from the mother (egg). One chromosome of each pair came from the father (sperm), and one came from the mother (egg).

Human chromosomes from skin cells. This is what stained chromosomes really look like This is what stained chromosomes really look like Chromosomes #1-22 are called autosomes and always make matching pairs Chromosomes #1-22 are called autosomes and always make matching pairs Chromosome #23 is the sex chromomsome and can be either “X” or “Y” Chromosome #23 is the sex chromomsome and can be either “X” or “Y”

Karyotype of the human chromosomes. Geneticists literally match up the homologous chromosomes from a photograph to make a karyotype. Geneticists literally match up the homologous chromosomes from a photograph to make a karyotype. They contain similar genes but from the two different parents. They contain similar genes but from the two different parents. Humans have 2 sets of 23 chromosomes for a grand total of 46. “n” is 23 for humans. Humans have 2 sets of 23 chromosomes for a grand total of 46. “n” is 23 for humans.

Males have an X and a Y chromosome #23, so their sperm can either carry and “X” or a “Y” Males have an X and a Y chromosome #23, so their sperm can either carry and “X” or a “Y” Females have two copies of the “X” chromosome #23, so their eggs always carry an “X” Females have two copies of the “X” chromosome #23, so their eggs always carry an “X”

The sex of the child is therefore determined by the male gamete (sperm)

Meiosis Haploid (n) cells or “gametes” are formed with meiosis Haploid (n) cells or “gametes” are formed with meiosis Meiosis also jumbles up the genetic material Meiosis also jumbles up the genetic material This only happens in your gonads (ovaries or testes) This only happens in your gonads (ovaries or testes) Meiosis happens in two phases or cell divisions Meiosis happens in two phases or cell divisions

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Crossing Over! One source of variation between gametes HUGE jump in genetic diversity! HUGE jump in genetic diversity! Chromosomes “mom” or “dad” already have, randomly mix themselves up! Chromosomes “mom” or “dad” already have, randomly mix themselves up! Then they assort randomly into different gametes. Then they assort randomly into different gametes.

Independent assortment of chromosomes- another source of variation between gametes

Variation from Sexual Reproduction Three main sources create MILLIONS of options Three main sources create MILLIONS of options Crossing over causes mixing BEFORE meiosis separates chromosomes Crossing over causes mixing BEFORE meiosis separates chromosomes Independent Assortment during Anaphase (ex: In mom’s egg cells, each of the 23 sets will separate independently…some of her mom’s chromosomes may go to an egg, and some of her dad’s chromosomes) Independent Assortment during Anaphase (ex: In mom’s egg cells, each of the 23 sets will separate independently…some of her mom’s chromosomes may go to an egg, and some of her dad’s chromosomes) Random Fertilization: it is random which egg and sperm meet during fertilization Random Fertilization: it is random which egg and sperm meet during fertilization