REPRODUCTION LS Chapter 4

2 Types of Reproduction Sexual and Asexual “A”=without Asexual means without sex Advantages and disadvantages to each Some organisms can do either or both, depending on circumstances. Jellyfish Males release sperm, females release egg into the water They meet and fertilize and young jellyfish form on the sea floor The young jellyfish then can reproduce asexually before becoming adults

Sexual Reproduction Type of reproduction that requires two organisms DNA from each of the parents come together and form unique offspring The female sex cell is the egg The male sex cell is the sperm The process of them coming together is called fertilization The fertilized egg is called a zygote The zygote then goes through the cell cycle and becomes multicellular

Chromosomes In interphase, DNA is in the form of chromatin It winds around proteins into a tight structure called a chromatid There are 2 of them, called sister chromatids Sister chromatids attach at the centromere and form a chromosome (X-shaped) Humans have 23 pairs of chromosomes (total of 46 chromosomes) 1 from each pair come from mother, 1 from father I.E. 23 from mom, 23 from dad=23 pairs=46 chromosomes The pair of chromosomes (1 from mom, 1 from dad) are called homologous chromosomes Homologous chromosomes have the same DNA, but may have different forms of a gene Example: One may have the gene for green eyes, the other may have the gene for brown eyes (much more to come on this in the genetics section)

Meiosis Occurs in sexually reproducing organisms Splits cells into ½ the number of chromosomes Important! What if cells didn’t reduce their chromosome numbers? 46 chromosomes from mom, 46 chromosomes from dad=92 chromosomes 2x as many as you should have Very, very, very similar to mitosis… Same words are used, steps are the same Happens twice Stages: Interphase, Meiosis I, Cytokinesis, Meiosis II, another cytokinesis While mitosis produces 2 cells, meiosis produces 4 cells

Interphase Exactly the same as in mitosis Longest phase Period of growth and development Same stages G1 Phase-Growth S Phase-DNA duplicates G2 Phase-Growth and duplication of organelles

Meiosis Occurs in 2 phases Meiosis I Prophase I Metaphase I Anaphase I Telophase I Cytokinesis Meiosis II Prophase II Metaphase II Anaphase II Telophase II Cytokinesis

Prophase I Chromatin condenses into chromosomes (sister chromatids attached at the centromere) Homologous chromosomes (1 from mom and 1 from dad) attach and exchange genes The nucleus breaks down Centrioles move towards poles of cell Spindle forms

Metaphase 1 Spindle fibers attach to chromosomes at the centromere Homologous chromosomes line up in the equator of the cell

Anaphase I Homologous chromosomes are pulled apart 1 chromosome moves to each end of the cell

Telophase I 2 nuclei form 1 around each set of chromosomes Spindle breaks down Chromosomes unwind into chromatin Cell begins to split Cytokinesis occurs The new cells enter a short resting phase before Meiosis II begins

Prophase II Meiosis II happens in both cells from meiosis I at the same time Chromatin winds up into chromosomes (sister chromatids attached at the centromere) Nucleus breaks down Spindle forms

Metaphase II Spindle attaches to chromosomes Chromosomes line up in the center of the cell This time there is only 1, so the chromosomes are single file

Anaphase II Sister chromatids are pulled apart at the centromere

Telophase II 4 nuclei form Spindles break down Chromatids unwind into chromatin Cells begin to divide Another cytokinesis 4 cells are produced

Results of Meiosis Recall that in Prophase I, the homologous chromosomes exchanged genes This causes there to be 4 different cells produced from Meiosis Contrast this with the 2 identical cells resulting from mitosis Each of the cells produced from meiosis have ½ of the DNA as the parent cell In males, meiosis results in sperm cells In females, meiosis results in egg cells When the 2 cells fuse during fertilization, the DNA combines ½ + ½ = 1 total cell This cell then undergoes mitosis

Advantages and disadvantages of Sexual Reproduction Advantages: The offspring are unique due to meiosis Except for twins, no 2 sexually-reproducing organisms have the same DNA The genetic variation makes organisms more likely to adapt and evolve to changing environmental conditions Disadvantages: Slow Humans take 9 months, some elephants can be 2 years! Requires energy Need to find a mate Some organisms use a lot of energy finding a mate

Asexual Reproduction Reproducing involving only one organism The offspring is a clone of the parent It is genetically identical to the parent All offspring are also identical to one another Some asexually-reproducing organisms can exchange parts of DNA to increase genetic diversity a little 4 main types: Fission Budding Regeneration Vegetative Propagation

Fission Many prokaryotic organisms reproduce this way One organism splits into 2 Similar to mitosis

Budding Offspring grows on the parent organism Occurs in yeast, hydras, and potatoes (though may be considered to be vegetative propagation…depends who you ask)

Regeneration Offspring grows from a piece of the parent organism Happens in planarians, sea stars, and hydras, and other animals And Time Lords

Vegetative Propagation Similar to regeneration Only for plants Offspring grow from part of the parent plant Strawberries, potatoes, spider plants

Advantages and Disadvantages of Asexual Reproduction Advantages: Don’t need a mate Don’t need a lot of energy finding a mate Quick Can produce many offspring quickly Some bacteria can split every few minutes! Disadvantages: Low genetic diversity Prone to extinction in changing conditions Unable to adapt for the same reason.