Sexual Reproduction and Meiosis

Reproduction• Organisms can reproduce asexually -

mitosis, fission, & budding

• Organisms can also reproduce sexually

– Two sex cells (usually egg and sperm) join together

– Males produce sperm, females produce eggs

• Sex cells do not have pairs of chromosomes

• They are called haploid (single- not paired up: sometimes represented as the letter N).

• They only have half the number of chromosomes as body cells.

• 23 total chromosomes

• All other body cells are diploid (paired up: 2N).

– Humans have 23 pairs of chromosomes

– 46 chromosomes total

• When an egg and sperm join together, it is called fertilization.

• The resulting cell is called a zygote

• The zygote will begin to undergo cell division

http://www.exploratorium.edu/imaging-station/gallery.php?Asset=Zebrafish%20development&Group=&Category=Development&Section=Introduction

• The zygote formed following fertilization has a diploid set of chromosomes (46 total).

• Half (23) from mom, half (23) from dad.

• Sexual reproduction occurs when the genes of two parents are combined to form an offspring.

• The offspring is a unique genetic combination of its two parents.

So how do we get specialized sex cells with only half the

chromosomes?

Meiosis

• Meiosis is a process that produces haploid sex cells.

Why does meiosis happen?

• Meiosis allows for the production of sex cells so sexual reproduction can occur.

• In meiosis I, the nucleus divides and produces two new cells with one duplicated chromosome

• Exactly like mitosis.

• In meiosis II, the division happens again and the sister chromatids separate, producing four cells with half (haploid) the number of chromosomes of the original nucleus.

• The end result of meiosis:– A cell undergoes

division twice.– This produces four

sex cells, with half the number (haploid) of chromosomes of the original cell

Mitosis vs. Meiosis

http://www.youtube.com/watch?v=Ba9LXKH2ztU

Asexual vs. Sexual reproduction• Asexually

produced organisms are genetically identical to their parent.

• Sexually produced organisms are genetically different from their parent. – This produces unique organisms who may be

more (or less) resistant to disease, more (or less) intelligent, more (or less) physical abilities, adaptable to environment, etc.

– There is variety among the offspring.– http://www.youtube.com/watch?v=toWK0fIyFlY

Review

• What is it called when a sperm joins with an egg?

• Fertilization

• After the egg is fertilized by the sperm, what is the new cell called?

• A zygote

• How many chromosomes are in a human sex cell?

• 23

• How many chromosomes are in a regular human body cell (skin, cheek, liver, muscle, etc)

• 46

• What are the names of the two sex cells produced by humans?

• Egg and sperm

• How many times do cells divide in meiosis?

• twice

• How many new cells are produced from a single cell in meiosis?

• four

• How many cells are produced from a single cell in mitosis?

• Two

• Are sex cells haploid or diploid?

• Haploid

Name each item in the

picture:

A

E

C

D

B

G

A- Chromosome

B- Nucleus

C- Cell

D- DNA

E- Gene

F- Centromere

• Why does meiosis happen?

• To produce sex cells

• Why does mitosis happen?

• GRR– Grow- an organism can get bigger– Replace- replaces damaged or old cells– Reproduce- used by some organisms to

reproduce asexually

Name each phase of mitosis