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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 - PowerPoint PPT Presentation
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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