Sexual Reproduction and Meiosis

Reproduction Organisms can reproduce asexually - mitosis, fission, & budding

Asexual reproduction Offspring can be produced quickly Only one parent is needed Offspring are a clone of the parent (same DNA)

Organisms can also reproduce sexually –Two sex cells (usually egg and sperm) join together –Males produce sperm, females produce eggs

Sexual Reproduction Takes two parents May take more time than asexual reproduction Offspring are a genetic blend of both parents

So how do we get special sex cells with half the DNA of the parent? Mitosis makes new cells with all the DNA of the parent cell. MEIOSIS makes new cells with half the DNA of the parent cell. Meiosis Mitosis

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 (mitosis) so it can grow station/gallery.php?Asset=Zebrafish%20development&Group=&Cat egory=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 does our body make 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. HwYh0

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

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.

Summary Mitosis –One division –Makes two cells (non sex cells) –New cells are diploid and identical to the parent cell –Used to make cells for GRR (growth, replacement, and reproduction (asexual) Meiosis –Two divisions –Make four cells (sex cells) –New cells are haploid, only have half the information of the parent cell –Used to make sex cells for sexual reproduction

Which one is mitosis, meiosis? miTosis = Two cells meioSis = Sex cells

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