Sexual Reproduction

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Slide 1: Questions

1. What is sexual reproduction?
What is fertilization? Why is sexual reproduction important?

2. What is meiosis?
Compare meiosis to mitosis ("normal" cell division).
Why is meiosis important?

Slide 2: What is Sexual Reproduction?

Sexual reproduction is the most common form of reproduction in eukaryotes (organisms whose cells have nuclei). It occurs in most animals (and plants!). Not just people!

Sexual reproduction results in the production of an offspring whose genetic material comes from both of its parent organisms.

This genetic material comes from two different cells (one from each parent).

Slide 3: Where does the genetic material come from?

  • Half of the genetic material comes from an egg cell.
    • The egg cell contains half of the genetic material from the mother.

  • The other half of the genetic material comes from a sperm cell.
    • The sperm cell contains half of the genetic material from the father.

Fertilization occurs when the sperm cell fuses (combines) with the egg cell.

The resulting cell (with all of the required genetic material) is called a zygote. The cells in the zygote divide by mitosis as the zygote develops into an embryo and eventually becomes a new individual organism.

Slide 4: Sexual Reproduction Vocabulary

Sperm are male reproductive cells. Both animals (and plants!) use sperm to reproduce.

Eggs (or "ova", singular "ovum") are female reproductive cells. Both animals (and plants!) use ova to reproduce.

A general word for sperm cells or egg cells is gamete.

A gamete is a cell that
  • contains half of the genetic material from a parent (one chromosome from each pair).
  • is able to combine with another gamete (of the "opposite sex") to form a zygote.

A zygote is the newly formed cell containing complete genetic information from both parents. A zygote develops into a new organism through mitosis (normal cell division).

Slide 5: Why is Sexual Reproduction Important?

Sexual reproduction is important because the offspring receive genetic material from two parents (via two different cells).

This allows for genetic diversity (reduces the chance of disease from inbreeding.)

Slide 6: What is Genetic Diversity?

Diversity means variety, differences.

Genetic diversity refers to the diversity of genes and characteristics within a species.

In the process of inheritance, genetic information is shuffled and recombined to form new combinations that are different from the parents. Every individual is a unique combination of genetic material from its parents.

Genetic diversity is important because it helps maintain the health of a population, by including genes that may be valuable in resisting diseases, pests, and other stresses. Genetic diversity gives a population a buffer against change, providing the flexibility to adapt.

Slide 7: What is Meiosis?

  • Meiosis is a type of cell division that results in four "daughter cells" each containing half the number of chromosomes from the parent cell.
  • Meiosis only happens in cells used for sexual reproduction.

Compare to Mitosis

  • Mitosis results in two daughter cells that each have a full number of chromosomes, in pairs.
  • Mitosis occurs in all cells in a growing and developing organism.

Slide 8: Meiosis Diagram

The first part of meiosis looks a lot like mitosis!


Slide 9: Why is Meiosis Important?

Meiosis is important because it
  • ensures that offspring inherit the correct number of chromosomes - one half from each parent. Without meiosis, the chromosome number would double with each generation!

  • provides genetic diversity by mixing up the possibilities and combinations of chromosomes from each parent.

Each new organism is not identical to
  • either of its parents.
  • its siblings

Slide 10: Meiosis Vocabulary

A cell containing half of the genetic information (one chromosome from each pair) is a haploid cell. The word haploid comes from a Greek word meaning "single". Gametes are haploid cells.

A cell containing pairs of chromosomes (one from each parent) is a diploid cell. The word diploid comes from a Greek word meaning "double". Zygotes (and all other cells in an organism) are diploid cells.

Slide 11: In Humans...

Human cells contain 23 pairs of chromosomes. One chromosome in each pair comes from the egg cell. The other chromosome comes from the sperm cell.

A diploid human cell contains 23 pairs of chromosomes (46 chromosomes total), half from each parent.
A haploid human cell contains 23 single chromosomes.

Most of the cells in an organism are diploid.

Mitosis results in two diploid cells, These cells are identical to each other and to the original cell, before division.

Meiosis results in four haploid cells. None of these cells are identical to each other or to the original cell that divided.

Slide 12: Meiosis Combinations

How many possible chromosome combinations are there for the gametes of an organism?

If the haploid number is 'n', then there are 2n combinations possible for a gamete.

For an organism with 2 pairs of chromosomes, there are 22 possible combinations of chromosomes for a gamete. 22 = 4.

(Each meiotic division, results in two pairs of identical gametes)*.


* This is a simplification. There's actually a lot of possibilities beyond simple division of chromosomes.

Slide 13: How many possible combinations are there for a human?

For human gametes, there are 2(23) = 8,324,608 different possible combinations of chromosomes! As a result, two gametes virtually never have exactly the same combination of chromosomes. Also each chromosome contains dozens to thousands of different genes. Only the genes in one half of each pair of chromosomes will "turn on".

Slide 14: How many possible combinations in a zygote?

A human ovum has 8 million possible chromosome combinations. A human sperm cell has the same number.

8 million x 8 million = 64 trillion possible combinations for a zygote!

No two people are exactly alike, even siblings!

Extra Credit Thought questions:
  • How do we get twins?
  • Are even twins completely identical?

Slide 15: Important Vocabulary Notes

Scientists refer to parent cells, Try not to confuse this term with parent organisms. A parent cell divides to form two new (identical) cells.
  • The original "parent" cell disappears in the process.
  • A "parent" cell is neither male nor female!

Scientists also refer to daughter cells and sister chromatids. These words do not mean that the cells are female or that they come from (or produce) a female organism!

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Topic revision: r7 - 12 Apr 2018, VickiBrown

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