2.5 Mitosis and Meiosis- Cell Division Study Guide

2.5 Mitosis and Meiosis- Cell Division Study Guide

 

Construct an explanation about the role of mitosis in the production, growth, and maintenance of systems within complex organisms. Construct an explanation about the role of Meiosis in the reproduction of multicell organisms. Emphasize the major events of the cell cycle including cell growth and DNA replication, separation of chromosomes, and separation of cell contents. (LS1.B)

Complex organisms are made of interacting systems, which are made up of cells. Use the information in this chapter to determine the role that cell division plays in the maintenance of systems in an organism.

Understand the difference between Mitosis, and sexual reproduction-Meiosis, and the stages of cell division in each.

 

Cell Division

You consist of a great many cells, but like all other organisms, you started life as a single cell. Human beings are made up of an average of 65 trillion cells, and many of those cells are still dividing. How did you develop from a single cell into an organism with trillions of cells? The answer is cell division. All cells come from preexisting cells through the process of cell division. After cells grow to their maximum size, they divide into two new cells. These new cells are small at first, but they grow quickly and eventually divide and produce more new cells. This process keeps repeating in a continuous cycle.

Every five days the lining of your small intestine replaces itself.  Every 27 days your skin cells are replaced, and if you cut yourself, your body immediately goes to work, replacing the cells that were cut away. 

We will refer to an existing mature cell as the PARENT CELL, and the cells that it divides into will be called DAUGHTER CELLS.

There are ground rules for cell division:

1. Parent Cells must provide their daughter cells with hereditary instructions (DNA).

2. Parent Cells must provide enough cytoplasmic machinery to start up their own operation- a sort of cell "start-up" kit. 

 

UNDERSTANDING DNA

1. Before Cell Division- the DNA molecules are stretched out like threads- each thread is called a CHROMOSOME.  

Chromosome-is an organized package of DNA found in the nucleus of the cell.

2. Humans have 46 total chromosomes in every cell, divided into 23 pairs.  A human gets 23 from his or her father and 23 from his or her mother.  the paired chromosomes are similar, and called homologues.  They a named 1 through 23. 

Homologue-one of a pair of chromosomes that resemble each other in size and shape and the genes they carry

3. DNA is duplicated before cell division, when it is still in its threadlike form. It later condenses to the "X" shape we commonly see in pictures. 

4. The new DNA molecule and the old are attached in the middle at a place called a centromere.  They are now called "Sister Chromatids" Now the cell is prepared to divide into two new cells. 

Diploid Cell- means having two of each type of chromosome.  One set from Mother, one set from Father. In Mitosis, every cell receives an exact copy of both sets. The daughter cell is a clone of the parent cell. Cell division by Mitosis results in diploid cells.

Haploid Cell- in Meiosis- daughter cells have received half of the DNA.  These cells are for sexual reproduction, and once a Mother and Fathers gametes ( reproduction cells) are joined, the resulting cells will now have a full set of 46 chromosomes ( 23 pairs).

 

There are two types of cell division for multi-cell organisms:

Mitosis-a process of cell division during which one cell gives rise to two genetically identical daughter cells.- the daughter cells are clones of the parent cell. Multi-celled organisms grow and repair themselves through Mitosis.  Mitosis ensures that the new cells have the same genetic information as the original cell. The main role of mitosis is to allow the organism to grow and to replace cells that are worn out or damaged. 

Meiosis- process where a single cell divides twice to produce four cells containing half the original amount of genetic information. These cells are our sex cells – sperm in males, eggs in females. Multi-celled organisms generally reproduce by Meiosis. Cell division occurs in special GERM CELLS that are set aside for sexual reproduction. Germ cells reside in the testis of male organisms and the ovaries of female organisms.  

 

MITOSIS

 

 

STEP ONE

This diagram is a pie chart of a cells life.  During Interphase, a cell grows, which is the g1 phase, it replicates its DNA, which is the S phase,  and it grows more cytoplasm and organelle machinery in the G2 phase.  Then the cell begins the process of Mitosis, which is cell division.  Once the parent cell is divided, each daughter cell will repeat the cycle.

STEP 1: PROPHASE

• -Chromosomes condense so you can see them under the microscope.
• -Microtubules organize and move to the poles of the cell. 
• -One end of each microtubule attaches to the the pole of the cell, one end attached to the centromere of each chromosome.
• -Nuclear envelope begins to break up

Step II: Metaphase

The microtubules push and pull the chromosomes to the "equator" of the cell.

Step III: Anaphase

The sister chromatids are separated by the microtubules pulling on them.

They are moved to opposite poles of the cell.

Step IV: Telophase

Microtubules release the chromosomes

A new nuclear envelopes forms around each set of chromosomes at opposite poles

 

Cytokinesis, the complete division of the cell membrane.  There are now two new daughter cells with the exact same amount of DNA as the parent cell.

 

REPRODUCTIVE CELL DIVISION- MEIOSIS

We will NOT be studying Asexual Reproduction- the produced offspring are clones, complete genetic copies of the parents.  

We will study Sexual Reproduction- Involves two parents, each with two genes for every single trait. Both parents pass on one gene to their offspring. The offspring of sexually reproducing organisms have new combinations of traits, sometimes similar, sometimes different than their parents. 

Allele- a slightly different molecular form of the same gene.  For instance one parent may give you a gene for brown eyes and another may give you a gene for blue eyes.  These are alleles.  They are both eye color genes, but slightly different. 

The big difference between Mitosis and Meiosis is that in Meiosis cell division happens TWICE!

Meiosis happens in Germ Cells. Germ Cells only exist in reproductive organs and structures.  They have the same 46 chromosomes as every other cell.  After Meiosis they only have 23 chromosomes and they are called GAMETES.

MEOSIS I

PROPHASE I

Homologue chromosomes are drawn together, and non-sister chromosomes break at the same places and trade a lengths, exchanging corresponding segments. 

Chromosomes then condense

Metaphase I- Chromosomes are positioned at the equator, both maternal and paternal, microtubules attach and pull one of each pair (homologues).

ANAPHASE I

The chromosomes are pulled to the poles.  It is random which of the pair gets pulled to each end.  You could get all your mothers at one end, all your fathers, or a combination of both.  With 23 pairs, there are 8,388,608 possibilities. 

TELEPHASE I

Nuclear envelope forms and the two cells separate.

 

MEIOSIS II:

PROPHASE II

The chromosomes condense

METAPHASE II

The chromosomes are lined up at the equator by the Microtubules

ANAPHASE II

The sister chromatids are pulled apart and pulled to the poles of the cell.

TELOPHASE II

The nuclear envelope forms around the chromosomes- half the number of the parent cell.  

Fertilization of a egg cell, results in a cell with 46 chromosomes:

Each Germ cell forms four gametes in males, usually called sperm.  

Each Germ cell forms four gametes in females, but three usually do not thrive and disintegrate, and one receives the majority of cytoplasm.  This cell is usually called an egg. 

 

CANCER, Pathologic cell division:

Cancer occurs when a cell can't shut off the instructions to divide.  Cells continue to divide and cause a growth in a tissue.  Often cancer is treated by removal of the cells that can't shut off their signal to divide.  Sometimes, when Cancer is discovered too late, the cells travel into new tissues, and begin to divide there as well.