Meiosis

What is meiosis?

How is it different from mitosis?

Stages

Genetic Variation

Reproduction

Asexual – reproduction resulting from mitosis (or a similar process) that involves only one parent; the offspring are genetically identical to the parent (a clone)

Sexual – reproduction resulting from an exchange of genetic material; in most organisms this involves the fusion of gametes (formed during meiosis)

Asexual reproduction

Binary Fission – separation of a parent into 2 or more individuals

ex. bacteria

Fragmentation - body breaks into several pieces

ex. worms

Asexual reproduction

Budding - new individuals split off from existing ones. Bud may break off or remain attached to parent

ex. jellyfish, corals, yeast

Asexual reproduction

Regeneration – renewal/re-growth of an organism from a segment of that organism

ex. starfish, planaria

Asexual reproduction

Sporulation – production and release of spores

ex. fungi, algae, protozoa

Asexual reproduction

Disadvantages: DNA varies little between organisms, which may result in organisms not being able to adapt to a changing environment Advantages: • Produce many offspring in short period of time without using energy to produce gametes or to find a mate • Offspring are perfectly adapted to current environment, therefore often used in stable environments

Asexual reproduction

Sexual Reproduction

Disadvantages: Sexual reproduction uses a lot of metabolic energy in the development and maintenance of gametes, as well as a lot of biochemical resources Advantages: • Frequent production of new combinations of genes • Flexibility in the gene pool of a population helps insure the survival of a species, especially if there is rapid or sudden change in the environment

Sexual Reproduction

23

chromosomes 23

chromosomes +

46 chromosomes (23 pair) zygote

Sexual Reproduction Life Cycle for Animals

Growth &

Development

Marriage

Mitosis vs. Meiosis

MITOSIS MEIOSIS

type of reproduction asexual sexual genetically ___ to parent identical unique

# divisions 1 2

# cells produced 2 4

cell type somatic gamete

# chromosomes diploid haploid

What is Meiosis?

• Meiosis- cell division that produces gametes (sperm and egg)

– This produces reproductive cells, which are genetically unique, through a series of cell divisions.

– Ensures that humans have the same number of chromosomes in each generation.

THINK: what would happen if sperm and egg cells were diploid?

How is Meiosis Different from

Mitosis?

• There are 2 divisions in meiosis – Meiosis I and meiosis II

• The result is 4 cells instead of 2

• In meiosis II, the DNA is not replicated again. (No interphase)

• The final number of chromosomes is 23 in humans (egg has 23 and sperm has 23)

• This is haploid (n)

From Pearson Education

Comparing Mitosis and Meiosis

Stages of Meiosis There are 2 divisions of the nucleus in meiosis

– Interphase (DNA replication)

– Prophase I – Metaphase I – Anaphase I – Telophase I

NO DNA replication

– Prophase II – Metaphase II – Anaphase II – Telophase II

Meiosis I

Meiosis 2

One parent cell produces four daughter cells. Daughter cells have half the number of chromosomes found in the original parent cell and are genetically unique.

Oogeneis

Spermatogenesis

Genetic Variation

Genetic variation is what makes organisms unique

How does genetic variation occur? Genetic crossover

Independent assortment

Ransom fertilization

Genetic Variation

Genetic Crossover

the exchange of genetic material (genes) that can occur between homologous chromosomes during Prophase I Produces genetically unique chromosomes

Genetic Variation

Independent assortment: homologous chromosomes are randomly sorted/distributed during meiosis

–Leads to several possible combinations of genes (why siblings aren’t always identical)

Independent Assortment

Genetic Variation

Random fertilization

• During meiosis, 4 gametes are produced… which one will fertilize to produce a zygote?

• The chance that a particular egg or sperm will fertilize is random

What is the evolutionary advantage of sexual reproduction?

• Sexual reproduction produces greater genetic variation than asexual reproduction.

• Provides greater opportunity for accelerated adaptation