Amy Brown Science Stuff Why do cells divide
© Amy Brown – Science Stuff
Why do cells divide? Instead of dividing, why don’t cells just grow larger and larger?
There are two main reasons why cells divide rather than continuing to grow larger and larger: The larger a cell becomes, the ……. If the cell grows too large, it will have trouble moving enough nutrients and wastes across the cell membrane. …. more demands the cell places on its DNA.
Problem #1: Our DNA has its limits! • All of the information that a cell needs to function is stored in the DNA of the cell. • DNA is packaged into chromosomes. A chromosome consists of one very long linear DNA molecule consisting of 1000's of genes. • Each gene is the instructions for making a particular protein that the cell needs. • The cell is constantly making copies of these genes and sending the copies (in the form of RNA) out to the ribosomes. • When the cell is small, the information stored in the cell’s DNA is adequate to meet the needs of the cell. • As the cell grows too large, there is an “information crisis”. The DNA cannot keep up with the demands of running a larger cell.
Problem #2: A Growing Cell Needs More Food! A cell must take in a constant inflow of food, oxygen and water across the membrane. A larger cell will require much more food, oxygen and water. A larger cell will generate much more waste. Waste products must constantly be crossing the membrane in order to leave the cell. As the cell grows, the volume of the cell increases much more rapidly than the surface area of the cell membrane. When the cell gets too large, the membrane surface area is not adequate enough to transport the large quantities of food and water in and waste products out.
Cell division is the process by which cellular material is divided between two new daughter cells. 1 Mother Cell 2 Daughter cells. The two daughter cells will be…. …identical to each other and to the mother cell.
Each daughter is half the size of the parent cell, but immediately begins growing. A typical human cell has about 2 meters of DNA. Before the cell can divide, all of this DNA must be copied and then the two copies separated so that each daughter cell ends up with a complete set of DNA. Each species has a characteristic number of chromosomes in each cell nucleus; humans have 23 pairs or 46.
Chromosomes During Eukaryotic Cell Division copy its chromosomes Each cell must first _________ before cell division occurs. Each daughter cell gets a complete copy of that information. Cell division occurs in two main stages: Mitosis – The division of the nucleus Cytokinesis – The division of the cytoplasm
The chromosomes are not visible except during cell division. At the beginning of cell division, the chromosomes condense into compact, visible structures that are easily seen with a microscope. Replicated chromosome consisting of 2 sister chromatids. centromere Well before cell division takes place, each chromosome is replicated or copied. At the beginning of cell division, each chromosome sister chromatids consists of two identical “________”. These centromere chromatids are connected at an area called a _____.
The Cell Cycle The cell cycle is: The series of events that cells go through as they grow and divide. The cell cycle is the life of the cell from the time it is first formed from a dividing parent cell until its own division into two cells. During the cell cycle: 1. A cell grows. 2. The cell prepares for division. 3. The cell divides to form two daughter cells.
The cell cycle consists of five major phases: Two new cells are produced. mi kine tos si is s Cytokinesis is the division of the cytoplasm. to Mitosis is the division of the nucleus. G 1 cy § G 1 (first gap) § S (synthesis) § G 2 (second gap) §Mitosis §Cytokinesis S G 2
Before a cell can begin mitosis and actually divide, it must do two things: a) It must form duplicates of its chromosomes. b) It must produce a supply of organelles for the two daughter cells. interphase These preparations occur during the G 1, S, and G 2 phases of the cell cycle. These three (G 1, S, and G 2) are collectively known as interphase.
G 1 phase S phase a) The cell doubles in size. b) The enzymes, cytoplasmic organelles and other molecules double in number. Replication of DNA occurs. G 2 Phase The cell assembles the special structures needed for cell division When interphase (G 1, S, G 2) is complete, the cell is ready to begin the process of cell division.
“Let’s break mitosis down into its individual stages and see what is going on in each stage. ”
Interphase centrioles nuclear membrane nucleolus chromosomes Of the cell cycle, interphase accounts for 90% of the time. membrane well defined and bounded by the nuclear Nucleus is ________________. centrioles Their function is to: Outside of the nucleus are two _____. organize the microtubules into a spindle. They will begin to move apart as spindle microtubules grow out of them.
Interphase centrioles nuclear membrane nucleolus chromosomes G 1 phase is a period of intense biochemical activity: The cell doubles in size and the enzymes, cytoplasmic organelles and other molecules double in number. The chromosomes have duplicated during the S phase and they appear as a jumbled mass of fibers. They have not yet condensed. G 2 Phase: The cell assembles the special structures needed for cell division
Remember! Interphase includes the G 1 phase, the S phase, and the G 2 phase. It is the period of time preceding mitosis. Mitosis has 4 stages: 1. Prophase 2. Metaphase 3. Anaphase 4. Telophase
Prophase Early prophase The chromosomes coil and thicken and become distinct from one another. The chromosomes are now visible. disappears The nucleolus _____. The chromosomes are doubled throughout their length. centrioles Spindle made of microtubules Chromatids connected by a centromere. chromatid Each half of the double chromosome is a _____. centromere The chromatids are connected by a ______. centrioles separate and start moving to opposite ends of the cell. A The _____ spindle made of ______ microtubules begins to form. ____
Prophase Late prophase centrioles nuclear membrane The _________ fragments and the microtubules invade the nuclear area. The spindle is completely formed. The spindle is a structure that will help to separate the chromosomes ____________. During prophase the pairs of chromatids _____ become attached to the fibers of the spindle. Microtubules form a complete spindle. chromatids centrioles The centrioles have moved to the opposite poles, forming the spindle as they go.
Metaphase The centrioles are now at opposite sides of the cell. centrioles chromatids The spindle fibers will push and pull the centrioles chromosomes. The chromosomes line up at the center of the cell. Spindle composed of microtubules Each chromosome is connected to a spindle fiber at its centromere.
Anaphase The centromeres divide and the chromatids move to opposite sides of the cell. The microtubules begin to shorten and this pulls the chromatids apart to opposite sides of the cell. Chromatids are being pulled to opposite sides of the cell. Shortening of the microtubules By the end of anaphase, the two ends of the cell have equivalent and complete sets of chromosomes.
Telophase Nuclear membrane begins to form. Nuclear membrane is returning. cleavage furrow Nucleolus returns. The cell begins to pinch in. This is called a cleavage furrow. The end result is two cells that are exact copies of each other.
Can you name these phases? 1 - Prophase 4 - Telophase 2 - Metaphase 3 - Anaphase 5 - Interphase
Let’s practice identifying the phases! metaphase anaphase
Let’s practice identifying the phases! Metaphase Telophase Prophase
Cytokinesis two nuclei have been At the end of mitosis, _____ formed. Each nucleus has an identical set of chromosomes _______. Cytokinesis is: the division of the cytoplasm. Cytokinesis usually occurs at the same time as telophase.
In animal cells, a cleavage furrow _______ pinches the cell membrane inward until the cell is pinched into two separate cells. Each new cell contains its own nucleus, cytoplasm, and organelles.
Cytokinesis in Plant Cells not possible In plants, it is _____ for the cell to pinch inward because of the cell wall rigid _______. cell plate forms In plants, a ____ midway between the two nuclei. The cell plate continues to form across the cell until two separate cells have been formed.
M I T O S I S Takes between 30 minutes and 2 hours. One Mother Cell = Two Daughter cells. The two daughter cells are identical to the mother cell.
Results of Mitosis Importance of Mitosis • In unicellular plants and animals, it results in new offspring by asexual reproduction. • In multicellular organisms, it results in the growth and repair of the organism. • The two new cells are exact duplicates. • Insures that the new cells will be able to carry on the same functions as the mother cell.
Regulation of the Cell Cycle The frequency of cell division varies with the type of cell. Skin cells divide frequently _____ throughout our lives. Liver cells maintain the ability to _____ divide but only do so on rare occasion - say to repair a wound. The most specialized cells, such as muscle cells and nerve cells, do not divide at all.
Controls on Cell Division When cells come into contact with other cells, they respond by not growing. When an injury, like a cut in the skin occurs, the cells at the edge of the injury begin to divide rapidly. When the healing process nears completion, the rate of cell division slows down.
There are many proteins found on the inside and the outside of the cell that regulate cell division. Some of these proteins are responsible for starting and stopping cell division. Cell Cycle Regulators Other proteins seem to speed up or slow down the cell division process. These proteins send out signals that prevent excessive cell growth. This keeps the tissues of the body from disrupting one another.
Uncontrolled Cell Growth Cancer cells do not respond to the signals that regulate the growth of cells. Cancer cells divide uncontrollably and form masses of cells called tumors. If the cells in a tissue grow uncontrollably, the consequences may be severe. All cancers have one thing in common: The protein regulators that control the cell cycle have failed to do their job. These tumors can damag the surrounding healthy tissues.
Asexual Reproduction Offspring are produced by only one parent. Each parent passes ALL of its genes to the offspring.
Asexual Reproduction Advantages l It is faster l Large numbers of offspring are produced. l The parent does not have to find a mate. l All of the offspring are exactly alike. There is no variation. l The ability to adapt to a changing environment is greatly reduced.
Sexual Reproduction Sexual reproduction requires two parents. Each parent passes on HALF its genes to its offspring. Must have male and female: male to produce sperm and female to produce eggs.
Sexual Reproduction Advantages l All of the offspring l The parent must are genetically find a mate. different from l Fewer offspring each other. will be produced. Sexual Reproduction involves: Gametes: Sex cells (egg and l It takes longer. sperm) Fertilization: The union of sperm and egg. Zygote: A fertilized egg.
If an organism is the result of sexual reproduction, it will have ____ two sets of chromosomes. One set comes from the mother and one set comes from the father. These two sets are called homologous chromosomes. Homologous chromosomes are the two copies of each chromosome, one coming from the mother and one coming from the father. Homologous chromosomes carry the same genes, but they may have different expressions of that gene.
Diploid means that … …there are two of each kind of chromosome in each cell. The symbol for diploid is 2 N. is t a h W ? ? d i diplo You are a “diploid” organism. “N” is the number of different chromosomes an organism has. Humans are 2 N because we have 2 of each kind of chromosome. Diploid cells contain two complete sets of chromosomes. So in mitosis: 1 (2 N) cell -------> 2 (2 N) cells
Chromosome Number in Gametes Egg and sperm cells must have _____ half _______ the number of chromosomes so that zygote will when added together, the ______ have the proper number. Human egg cell Example: Gametes of the Human Body Egg (23) + sperm (23) zygote (46) 1 N + 1 N 2 N Gametes are said to be haploid or 1 N because they contain only one of each kind of chromosome. Human sperm cells
The cells which produce eggs and the cells which produce sperm are diploid or 2 N. So how do the egg and sperm cells get to be 1 N? ! ! s i s Meiosis is a process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes.
Phases of Meiosis Occurs in the sex cells only: the egg and sperm. Purpose is to reduce the chromosome number of the egg and sperm by half. Meiosis, like mitosis, is preceded by the replication of chromosomes. Unlike mitosis, this replication is then followed by two divisions: meiosis I and meiosis II.
el b a l st r i f s Let’ stage. h eac The stages of meiosis I and II Interphase Prophase II Telophase I (and cytokinesis) Metaphase I Anaphase I Metaphase II Anaphase II Telophase II (and cytokinesis)
The Stages of Meiosis I Interphase Prophase I Metaphase I Anaphase I The chromosomes replicate. It is similar to chromosome replication of mitosis. Two identical sister chromatids are held together by a centromere. Chromosomes shorten and thicken. Each chromosome pairs with its corresponding homologous chromosome to form a tetrad. There are 4 chromatids in a tetrad. Tetrads line up at the center of the cell. The tetrads break apart and the pairs move to opposite sides of the cell. Sister chromatids remain attached at their centromeres.
The Stages of Meiosis II Telophase I (and cytokinesis) The cell separates into two cells. Meiosis I results in 2 haploid (1 N) daughter cells Each daughter cell has half the number of chromosomes as the original cell. Prophase II Metaphase II Anaphase II Telophase II The pairs of sister chromatids start toward the center. Pairs of sister chromatids line up at the center. The pairs of sister chromatids separate and move to opposite sides of the cell. (and cytokinesis) Results in 4 new cells that are 1 N.
The Importance of Meiosis 1 (2 N) cell -------> 4 (1 N) cells The chromosome number of the egg and sperm is cut in half to insure that the zygote will have the proper number of chromosomes.
“Crossing Over” During Meiosis tetrad During prophase ____, I each pair of chromatids lines up next to its homologue _____. homologous This process is called crossing over “______” chromosomes This pairing of homologous chromosomes produces tetrads _______. A tetrad consists of 4 chromatids ______. It is possible for the chromatids within a homologous pair to twist around one another. Portions of the chromatids may break off and attach to adjacent chromatids. “Crossing over” is the exchange of genetic information (genes) between segments of homologous chromosomes during meiosis.
The result is that the offspring will receive a new combination of genetic information. This leads to variation ____ in the offspring. adaptation and change These variations will cause Variation leads to __________. better suited for their particular some of the offspring to be ______ environment. If they are better suited for their environment, it is survive to reproductive age and pass these more likely that they will _________ favorable variations on to their offspring. If the result of crossing over causes the offspring to be less suited for its environment, it may not survive. Or, if the offspring does survive, it may not be reproductively competitive. This means that it may not be able to secure a mate. These “unfavorable” genes are not likely to be passed on to the offspring. Crossing over leads to Evolution! _______.
e t e Gam ion t a m For Meiosis produces four haploid cells that are different. In males, meiosis results in 4 sperm cells. In females, 4 cells are produced, but only one will become an egg cell. All of the cytoplasm and all of the organelles are put into one egg cell. The other three cells will never be functional.
Mitosis occurs in all cells of the egg and sperm _____ except _______. body Meiosis only occurs in the egg and sperm formation of _______.
diploid cell In meiosis, each _______ divides ______ twice to produce a total of 4 cells. ___ diploid cell divides In mitosis each _______ 2 cells. once to form _____
Comparison of Mitosis and Meiosis In meiosis, each of the four cells contains half the number of chromosomes as the _____ parent cell. In mitosis, each new cell contains the same number of chromosomes as the ______ original cell.
Comparison of Mitosis and Meiosis In mitosis, the homologous pairs do not tetrads come together to form _______. In meiosis, the homologous pairs do come tetrads While the together to form _______. crossing over chromosomes are in tetrads, _______ may occur. There will be no crossing over in mitosis _______.
Comparison of Mitosis and Meiosis In meiosis, the four haploid cells different combinations of contain ____ chromosomes from each other. In mitosis, the new cells contain identical copies. ____
Created by Amy Brown – Science Stuff Copyright © May 2012 Amy Brown (aka Science Stuff) All rights reserved by author. This document is for your classroom use only. This document may not be electronically distributed or posted to a web site.
- Slides: 56