St. 6- Cell Cycle Cell Replication and Genetic Variation Biology 1 2nd semester 2015

Objectives Will understand that one parent cell of a somatic cell creates two identical daughter cells Will understand the advantages and disadvantages of sexual and asexual reproduction Will understand that each somatic cell contains the same amount of chromosomes as the original cell Will understand that meiosis reduces the amount of chromosomes by half when gametes are formed Will understand that in meiosis, four non-identical daughter cells are made Will understand the components of the cell cycle

Video: Cell Division and the Cell Cycle!

How do you think your body grows or repairs cells that are wounded or damaged? Body cells go through a process called MITOSIS. Body cells are called somatic cells- these are all body cells in an organism (except for sperm and egg cells) Examples of somatic cells might be liver cells, heart cells, brain cells, plant root cells, or leaf cells Somatic cells are diploid, which means they have TWO sets of chromosomes…

So what are chromosomes? Every cell has chromosomes that carry our DNA one molecule of DNA is wrapped up tightly to make a chromosome Every human cell has 46 chromosomes Don’t forget- DNA is the blueprint for controlling characteristics of organisms

Chromosome Structure Structure moving from largest to smallest DNA and proteins Coil within chromosome Chromosome (made of 2 chromatids) Further coiling within larger coil DNA double helix Structure moving from largest to smallest

What do you think? How would cells like muscle cells, liver cells, skin cells or stomach cells make an identical copy?

What do you think? How would cells like muscle cells, liver cells, skin cells or stomach cells make an identical copy? First of all, it’s good to know that cells go through a cycle. Most of a cells’ life is spent in interphase. This is where the cell grows and carries out normal cell functions. DNA replication also occurs during Interphase- the cell is getting ready to divide

After going through interphase, when a cell gets too big, or too old, or is damaged it has to make another copy. To do this, the cell divides one time, in phases. -The first part is how the nucleus divides. -Then the cytoplasm divides until there are TWO, identical daughter cells.

What is the Cell Cycle? Remember I-PMAT-C I- INTERPHASE (Which includes G1, S, and G2) P- PROPHASE M- METAPHASE A- ANAPHASE T- TELOPHASE C- CYTOKINESIS (Which is when the cytoplasm divides) MITOSIS (Which is when the nucleus divides)

Cell Cycle: Interphase, Mitosis (PMAT), and Cytokinesis

Interphase: G1-Normal Cell growth/ S-DNA replicates/ G2- cell prepares to divide

Mitosis: Nuclear Division (nucleus divides)

Mitosis: Nuclear Division (nucleus divides)

Cytokinesis: Cytoplasm Divides

The result of mitosis (and cytokinesis) is two identical cells Each cell has the same amount of DNA After cytokinesis is complete, the cell goes back into Interphase.

Asexual Reproduction This process is actually how prokaryotic cells and simple eukaryotic organisms reproduce. This can occur very quickly- bacteria can reproduce about every 15 minutes… ewwww. Asexual reproduction: one individual produces offspring that are genetically identical to itself. (They do not need another organism to mate with in order to reproduce) Many organisms reproduce this way, like bacteria, yeast, sea stars and sea anemeones, bees, wasps, some reptiles, There are a few different kinds of asexual reproduction- budding, fragmentation, regeneration and parthenogenisis. Budding is when organisms grow out of the body of the parent (hydras, sea sponges) Fragmentation is when the parent breaks into multiple distinct pieces that give rise to identical offspring. Regeneration is a piece of the parent organism breaks off and grows an identical organism. Parthenogenisis is when an egg that has not been fertilized develops into an individual organism (bees, ants, fish, reptiles…)

Examples of Asexual Reproduction

What is so great about Asexual Reproduction? Advantages: Animals that remain in one particular place and are unable to look for mates would need to reproduce asexually. numerous offspring can be produced quickly or without "costing" the parent a great amount of energy or time. Environments that are stable and experience very little change are the best places for organisms that reproduce asexually. Disadvantages: Lack of genetic variation. All of the organisms are genetically identical and therefore share the same weaknesses. If the stable environment changes, the consequences could be deadly to all of the individuals. Genetic variation provides populations with a variety of genes for certain traits, some of which may be a bigger advantage to particular environments than others. This ensures that some of the population will survive and be able to pass on their genes to their offspring.

Quick Check How many daughter cells are made from one parent cell? True or false: Daughter cells are not identical to their parent cell. If a parent cell has 32 chromosomes, how many chromosomes are in the daughter cells? What are some reasons that cells undergo mitosis?

What is an example of a somatic cell? What occurs to cells during INTERPHASE? What are the four phases of MITOSIS? Explain asexual reproduction. Give an example of this. What is an advantage of asexual reproduction? A disadvantage?

But what about Sex Cells? Don’t you know that sex sells? ;) The opposite of somatic cells (body cells) are gamete cells= sex cells These would be egg and sperm cells The formation of making sex cells is called MEIOSIS. Females: each egg cell undergoes meiosis from one germ cell (which is from the ovaries) Males: each sperm cell undergoes meiosis from one germ cell (which is from the testes) When an egg cell and sperm cell fuse together, this is called fertilization– a fertilized egg is called a zygote.

Why do sperm and egg cells have to go through meiosis instead of mitosis? If sex cells went through the same process as somatic cell division, when fertilization occurred the resulting cells would have FOUR SETS of chromosomes. That is WAY too many… so instead gametes are made from the process of meiosis, where it splits or halves the number of chromosomes. A gamete cell that has undergone meiosis is haploid. (Gamete cells are haploid (half)= they have one set of chromosomes)

Example Time! Example: Humans have 46 chromosomes in their somatic cells. That means a fertilized egg (which will develop into a baby) should start out with 46 chromosomes. Soooo, that means the sex cells need to have half of the amount of chromosomes. Sperm have 23 chromosomes and the egg has 23 chromosomes. Sperm (23) + egg (23) = zygote (46)

Meiosis goes through TWO divisions (remember, mitosis only divided once.) Interphase occurs, so the DNA has replicated before meiosis begins. The first division splits the pairs of chromosomes. The second division splits the chromsomes in HALF. The result is FOUR daughter cells, each with half the number of chromosomes Gamete cells are haploid (half)= they have one set of chromosomes

Meiosis A large amount of animals and plants reproduce sexually, which means two individuals combine their genes to produce offspring with unique gene combinations. For example, humans reproduce sexually- no, we can’t clone ourselves!!

But, if diploid cells (cells with 2 sets of chromosomes) fused during fertilization, the offsprings’ cells would have too many chromosomes! So how do sex cells (gametes) go from diploid to haploid? This process is called MEIOSIS

Meiosis goes through TWO divisions Cells that go through mitosis just divide once, but cells that undergo meiosis divide two times: Meiosis I and Meiosis II Before meiosis occurs, the DNA replicates, so each of the chromosomes has a copy.

Meiosis I During meiosis 1, the cell divides but the copied “sister” chromosomes (chromosomes with 2 copies) stick together. The matching “homologous” chromosomes pull apart when the cell divides, so the two daughter cells are haploid.

Meiosis II During the second division of meiosis, the sister chromosomes pull apart The result of this division is FOUR daughter cells, each with one set of chromosomes- they are haploid and are NOT identical.

So when a human egg cell (23 chromosomes) and sperm cell (also 23 chromosomes) fuse during fertilization, the result is one fertilized, diploid egg cell with 46 chromosomes (two pairs) That’s why sex cells need to go through meiosis! The result is 4: non-identical, haploid sex cells!

Meiosis 1 and Meiosis 2

Genetic Variation in Sexual Reproduction Genetic variation occurs in meiosis in three different ways: Crossing over occurs in Prophase I- homologous chromosomes “switch” genes When sister chromosomes are pulled apart in Anaphase I, they are randomly distributed- this is Independent Assortment Random Fertilization occurs when a random sperm cell fuses with the egg cell

What types of cells and organisms undergo mitosis and meiosis? What type of cells are gametes? What type of cells are somatic cells? What would organisms use mitosis for? Why would organisms use meiosis? What are some ways genetic variation can occur?

What would happen to an organism if meiosis did not occur? If meiosis stopped, the cells and organisms would not reproduce, ending the geneological line. They would not pass down their genes to any offspring.