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Part 1:
The Cell Cycle
The Cell Cycle
CELL GROWTH
CELL GROWTH
Do organisms' cells get larger as the organism grows?
Do organisms' cells get larger as the organism grows?
Nope, living things typically grow by producing more cells.
Nope, living things typically grow by producing more cells.
On average, the cells of an adult animal are no larger than those of a young animal; there's just more of them!
On average, the cells of an adult animal are no larger than those of a young animal; there's just more of them!
The larger a cell becomes the more demands the cell places on its DNA. In addition, the larger a cell becomes the harder it is for the cell to take in enough nutrients and release waste across the cell membrane.
DNA as we know is found in the nucleus of the cell. When the cell is small, the information stored in that DNA is able to meet all of the cell's needs. But as a cell increases in size, it usually does not make extra copies of DNA. If a cell were to grow without stopping then the cell would eventually not have enough DNA to meet its needs.
Before it becomes too large, a growing cell divides forming two "daughter" cells.
CELL DIVISION: the process by which a cell divides into two new daughter cells.
CELL DIVISION: the process by which a cell divides into two new daughter cells.
Before cell division occurs, the cell replicates, or copies. all of its DNA. This replication of DNA solves the problem of information storage because each cell gets one complete set of genetic information.
Remember This:
Remember This:
Living things grow by producing more cells. Although an adult is bigger than a child, their cells are the same size! The adult just has more of them!
Living things grow by producing more cells. Although an adult is bigger than a child, their cells are the same size! The adult just has more of them!
What do you think would happen if a cell just split in two without preparing in advance? Would each of the daughter's cells have everything it needs to survive?
What do you think would happen if a cell just split in two without preparing in advance? Would each of the daughter's cells have everything it needs to survive?
N O P E !! Every cell must first copy its own genetic information before cell division begins. Each daughter cell then gets a complete copy of that information.
N O P E !! Every cell must first copy its own genetic information before cell division begins. Each daughter cell then gets a complete copy of that information.
THE CELL CYCLE
THE CELL CYCLE
INTERPHASE: a period of the cell cycle between cell divisions; an "in-between" period of growth that occurs before the cell divides.
INTERPHASE: a period of the cell cycle between cell divisions; an "in-between" period of growth that occurs before the cell divides.
CELL CYCLE: the series of events that cells go through as they grow and divide.
CELL CYCLE: the series of events that cells go through as they grow and divide.
During the cell cycle, a cell grows, prepares for division, and divides to form two daughter cells, each of which then begins the cycle again.
THE FOUR PHASES OF THE CELL CYCLE:
THE FOUR PHASES OF THE CELL CYCLE:
The cell cycle consists of four phases.
MITOSIS and CYTOKINESIS take place during the M PHASE.
Chromosome replication aka synthesis takes place during the S PHASE.
Between the M and S phases are G1 and G2 PHASES. The "G" in the names of theses phases stands for "gap". But don't let the names fool you! Important processes still occur during the G1 and G2 phase. They're actually periods of major cell growth and activity.
During the normal cell cycle, interphase can be pretty long, but the other parts of cell division are relatively short.
That's because INTERPHASE makes up 3 of the 4 phases of the cell cycle: G1, S, and G2 all makeup interphase!
That's because INTERPHASE makes up 3 of the 4 phases of the cell cycle: G1, S, and G2 all makeup interphase!
1. G1 (Gap 1) Phase:
1. G1 (Gap 1) Phase:
During this phase, cells do most of their growing. Cells also will increase in size and synthesize (create) new proteins and organelles.
G1 Phase DINB Help!
G1 Phase DINB Help!
During G1 Phase the cell starts to grow, synthesizing new proteins and organelles, e.g. mitochondria, ribosomes, lysosomes, etc. This is important in the preparation for cell division as two daughter cells are produced during mitosis, and each one will require its own organelles.
2. S (Synthesis) Phase:
2. S (Synthesis) Phase:
During this phase, chromosomes are replicated and the synthesis of DNA molecules takes place.
S Phase DINB Help!
S Phase DINB Help!
During the S phase, the cell replicates its DNA. The DNA is unwound and replicated using special enzymes and proteins. This is important as each new cell will require its own genetic material to control the cell’s activities.
3. G2 (Gap 2) Phase:
3. G2 (Gap 2) Phase:
This is typically the shortest of the three phases of interphase, During this phase, many of the organelles required for cell division are made. Once the cell is done with this phase the cell is ready to enter the M phase and begin cell division.
G2 Phase DINB Help!
G2 Phase DINB Help!
During the G2 Phase the cell continues to grow, synthesizing the proteins that will be needed for cell division, e.g. centromeres and spindle fibers.
4. M Phase (Cell Division - Mitosis & Cytokinesis):
4. M Phase (Cell Division - Mitosis & Cytokinesis):
Actual cell division! This phase is broken into two parts: Mitosis and Cytokinesis (More on this in Level 2)
Cell Division Phase DINB Help!
Cell Division Phase DINB Help!
During mitosis, the two copies of DNA are separated, moving to opposite ends of the cell. The cell then splits into two by dividing its cytoplasm - this is known as cytokinesis. The result is two genetically identical daughter cells.
Remember This:
Remember This:
G1 and G2 are gap phases where cell growth and synthesis take place.
G1 and G2 are gap phases where cell growth and synthesis take place.
DNA synthesis takes place during the S phase.
DNA synthesis takes place during the S phase.
Cell division takes place during the M phase.
Cell division takes place during the M phase.
In prokaryotes, the process of cell division is a simple matter of just separating the contents of one cell into two parts. But in eukaryotes, things aren't that simple!
Cell division in eukaryotic cells is more complex and occurs in two main stages: MITOSIS and CYTOKINESIS.
Cell division in eukaryotic cells is more complex and occurs in two main stages: MITOSIS and CYTOKINESIS.
MITOSIS: the first stage of cell division in which the nucleus of the cell and its DNA are divided.
MITOSIS: the first stage of cell division in which the nucleus of the cell and its DNA are divided.
CYTOKINESIS: the second stage of cell division in which the cytoplasm of the cell is divided.
CYTOKINESIS: the second stage of cell division in which the cytoplasm of the cell is divided.
Reproduction that only requires mitosis is considered asexual since the cells produced by mitosis are genetically identical to the parent cell.
Cell growth & Cell Division must be very carefully controlled. WHY? Because uncontrolled cell growth can have some seriously bad consequences! For example...
Cell growth & Cell Division must be very carefully controlled. WHY? Because uncontrolled cell growth can have some seriously bad consequences! For example...
CANCER: a disorder in which some of the body's cells lose the ability to control cell growth.
CANCER: a disorder in which some of the body's cells lose the ability to control cell growth.
Cancer cells do not respond to the signals that regulate the growth of most cells. As a result, they divide uncontrollably and form masses of cells called tumors that can damage the surrounding tissues.
Check out this video on the Cell Cycle!!
Check out this video on the Cell Cycle!!
Part 2:
Part 2:
Chromosomes & Mitosis
Chromosomes & Mitosis
Remember: P.M.A.T.
Remember: P.M.A.T.
CHROMOSOMES
CHROMOSOMES
In eukaryotic cells, the genetic information that is passed on from one generation of cells to another generation of cells is carried by CHROMOSOMES.
CHROMOSOMES: thread like structures found within the nucleus that contain the genetic information (in the form of DNA and proteins) that is passed from one generation of cells to the next.
CHROMOSOMES: thread like structures found within the nucleus that contain the genetic information (in the form of DNA and proteins) that is passed from one generation of cells to the next.
The cells of every organism have a specific number of chromosomes. Fruit flies for example have 8 chromosomes & humans have 46 chromosomes.
CHROMOSOME VISIBILITY
CHROMOSOME VISIBILITY
Chromosomes are not visible in most cells except during cell division.
Why? Because the DNA and proteins that makeup chromosomes are usually spread so far apart in the nucleus that they can't be seen. But at the beginning of cell division (during prophase) the chromosomes condense (come together) and form visible structures that we can see using light microscopes.
CHROMOSOME STRUCTURE
CHROMOSOME STRUCTURE
As we mentioned earlier, before cell division can take place the genetic information must be duplicated. This means that the chromosomes must by copied before cell division. Because of this copying or duplication, each chromosome ends up consisting of two identical "sister" CHROMATIDS. When the cell divides, the sister chromatids separate from each other. One chromatid per each of the two new cells.
CHROMATIDS: one of the two identical "sister" parts of a duplicated chromosome.
CHROMATIDS: one of the two identical "sister" parts of a duplicated chromosome.
Each pair of chromatids is attached at an area called the CENTROMERE.
CENTROMERES: area typically located near the middle of two chromatids where the chromatids of a chromosome are attached.
CENTROMERES: area typically located near the middle of two chromatids where the chromatids of a chromosome are attached.
A human body cell entering cell division contains 46 chromosomes, each of which consists of 2 chromatids.
MITOSIS: The part of eukaryotic cell division where the cell nucleus divides.
MITOSIS: The part of eukaryotic cell division where the cell nucleus divides.
Biologists divide the events of mitosis into four phases: prophase, metaphase, anaphase, and telophase.
Biologists divide the events of mitosis into four phases: prophase, metaphase, anaphase, and telophase.
#1: Prophase
#1: Prophase
The first and longest phase of mitosis. During prophase, the chromosomes condense and become visible. In addition, structures are known as CENTRIOLES separate and take up positions on opposite sides of the nucleus.
The centrioles are found in an area called the CENTROSOME that helps to organize the MITOTIC SPINDLE. During prophase, the condensed chromosomes become attached to fibers in the spindle at a point near the centromere of each chromatid.
Near the end of prophase, the chromosomes coil even more tightly. Also, the nucleolus disappears, and the nuclear envelope breaks down.
CENTRIOLES: two tiny structures located in the cytoplasm near the nuclear envelope.
MITOTIC SPINDLE: fanlike microtubule structure that helps to separate the chromosomes.
Prophase DINB Help!
Prophase DINB Help!
Inside the nucleus the DNA condenses into chromosomes. The nuclear envelope starts to break down. Bundles of proteins called centrioles start moving to opposite ends of the cell. Two centrioles from a centrosome.
#2: Metaphase
#2: Metaphase
The second phase of mitosis. During metaphase the chromosomes line up across the center of the cell. Microtubules connect the centromere of each chromosome to the two poles of the spindle.
Metaphase DINB Help!
Metaphase DINB Help!
The chromosomes are now free in the cytoplasm. Each centrosome has formed spindle fibers, collectively known as the spindle. The chromosomes line up along the middle of the cell and attach to the spindle by their centromeres.
#3: Anaphase
#3: Anaphase
Anaphase is the third phase of mitosis. During anaphase the centromeres that connect the sister chromatids together split, allowing the sister chromatids to separate and become individual chromosomes.
The chromosomes continue to move until they have separated into two groups near the poles of the spindle.
Anaphase ends when the chromosomes stop moving.
Anaphase DINB Help!
Anaphase DINB Help!
The spindle fibers contract, separating the sister chromatids. Once they are separated we refer to them as chromosomes again. They are pulled by their centromeres to opposite ends of the cell, also known as "poles". As they are pulled along centromere first, they appear V-shaped.
#4: Telophase
#4: Telophase
Telophase is the fourth and final phase of mitosis. During telophase chromosomes, which were visible and condensed, begin to disperse into tangles of dense materials.
A nuclear envelope then re-forms around each cluster of chromosomes. In addition, the spindle breaks apart and a nucleolus becomes visible in each daughter nucleus.
This is the end of mitosis, but the process of cell division is not finished yet!
Telophase DINB Help!
Telophase DINB Help!
When the chromosomes reach the poels of the cell they begin to uncoil, becoming longer and thinner. A nuclear envelope will form around each group of chromosomes and the cytoplasm will start to divide in the process of cytokinesis.
Cytokinesis
Cytokinesis
(Cytokinesis is not technically a part of mitosis, but because it occurs immediately after mitosis ends it is typically grouped together with mitosis.)
As a result of mitosis, two nuclei each with a duplicate set of chromosomes are formed within the cytoplasm of one cell. All that remains in the M Phase of the cell cycle is cytokinesis. Cytokinesis typically takes place at the same time as telophase.
CYTOKINESIS: the division of the cell's cytoplasm.
Cytokinesis in Animal Cells
Cytokinesis in Animal Cells
In most animal cells the cell membrane is drawn inward until the cytoplasm is pinched into two nearly equal parts. Each part contains its own nucleus and cytoplasmic organelles.
Cytokinesis in Plant Cells
Cytokinesis in Plant Cells
In plants, a structure known as the CELL PLATE forms midway between the divided nuclei. The cell plate gradually develops into a separating membrane. A cell wall then begins to appear in the cell plate.
Daughter Cells DINB Help!
Daughter Cells DINB Help!
The chromosomes have fully decondensed and are no longer visisible. When the cytoplasm has fully divided, cytokinesis is complete. There are now two daughter cells. These are genetically identical to each other and to the original parent cell. The daughter cells are now in interphase and will prepare for the next round of mitosis.
Check out this Amoeba Sisters Video About Mitosis!
Check out this Amoeba Sisters Video About Mitosis!
Part 3:
Part 3:
Meiosis
Meiosis
HOMOLOGOUS: Term used to describe chromosomes that each have a corresponding chromosome from the opposite-sex (opposite gender) parent.
HOMOLOGOUS: Term used to describe chromosomes that each have a corresponding chromosome from the opposite-sex (opposite gender) parent.
This is Fred. Fred is an adult fruit fly *buzz buzz*.
Fred the fruit fly has 8 chromosomes. 4 of these eight chromosomes came from Fred's dad, and the other 4 came from Fred's mom.
These two sets of chromosomes that Fred the fruit fly has are HOMOLOGOUS, meaning that each of the 4 chromosomes that came from Fred's dad has a corresponding chromosome from Fred's mom.
These two sets of chromosomes that Fred the fruit fly has are HOMOLOGOUS, meaning that each of the 4 chromosomes that came from Fred's dad has a corresponding chromosome from Fred's mom.
Fred the fruit fly has a diploid (2N) number of 8 chromosomes (2N = 8).
Fred the fly has a haploid (N) number of 4 chromosomes (N = 4).
A cell that contains both sets of homologous chromosomes are DIPLOID CELLS.
DIPLOID: "two-set"; a term used to refer to a cell that contains both sets of homologous chromosomes.
DIPLOID: "two-set"; a term used to refer to a cell that contains both sets of homologous chromosomes.
The number of chromosomes in a diploid cell is sometimes represented by the symbol 2N. Diploid cells contain two complete sets of chromosomes and two complete sets of genes.
On the other hand, GAMETES that are made from sexually reproducing organisms only have a single set of chromosomes and only a single set of genes. Such cells (i.e. gametes) are called HAPLOID. The number of chromosomes in a haploid cell is sometimes represented by the symbol N.
GAMETES: specialized cell involved in sexual reproduction.
GAMETES: specialized cell involved in sexual reproduction.
HAPLOID: "one set"; a term used to refer to a cell that contains only a single set of chromosomes and only a single set of genes.
HAPLOID: "one set"; a term used to refer to a cell that contains only a single set of chromosomes and only a single set of genes.
Homologous Chromosomes
Homologous Chromosomes
MEIOSIS: A process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell.
MEIOSIS: A process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell.
So how are the haploid cell (N) gamete cells made? During MEIOSIS !!!
During meiosis, the number of chromosomes per cell is cut in half through the separation of the homologous chromosomes.
During meiosis, the number of chromosomes per cell is cut in half through the separation of the homologous chromosomes.
The result of meiosis is 4 haploid cells that are genetically different from one another and from the original cell.
The result of meiosis is 4 haploid cells that are genetically different from one another and from the original cell.
Meiosis involves two cell divisions called Meiosis I and Meiosis II.
By the end of Meiosis II the diploid cell that entered meiosis has become 4 haploid cells.
Meiosis I:
Homologous Chromosomes Separate & Crossing Over Takes Place
Meiosis II:
Sister Chromatids Separate (Like Mitosis)
DINB Help: Meiosis an Overview
DINB Help: Meiosis an Overview
At the start of Meiosis I, there is one diploid (2n) cell with two homologous pairs of chromosomes. In humans each cell would have 46 chromosomes in total.
At the end of Meiosis I the homologous pairs have separated, resulting in two haploid (n) cells, each with 23 chromosomes that are non-homologous.
At the start of Meiosis II, there are two haploid (n) cells each with 23 chromosomes formed of two sister chromatids.
At the end of Meiosis II, the sister chromatids have separated, resulting in four cells each with 23 chromosomes. These cells are haploid (n) and we call them gametes.
CROSSING OVER OCCURS DURING PROPHASE 1 of MEIOSIS!
DINB Help: Meiosis Crossing Over
DINB Help: Meiosis Crossing Over
Crossing over happens when non-sister chromatids in a homologous pair wrap around each other. This puts tension on the chromatids and parts of them break off and swap places. In this way genes, sections of DNA that code for a trait, are exchanged.
This increases genetic diversity of the offspring, which benefits the species as it may result in new beneficial characteristics.
YOUR TURN:
YOUR TURN:
Which Karotype is from a male? Which is from a female?
Human Chromosomes
Human Chromosomes
To analyze our chromosomes scientists photograph cells during mitosis since this is when chromosomes are condensed and easy to see. They then cut out chromosomes from these pictures and group them together in pairs forming a KARYOTYPE.
KARYOTYPE: photograph of chromosomes grouped together in pairs.
KARYOTYPE: photograph of chromosomes grouped together in pairs.
The number of chromosomes found in a typical human's karyotype is 46. 23 of these chromosomes coming from the father and the other 23 coming from the mother.
Two of these 46 chromosomes are known as SEX CHROMOSOMES. Females have two copies of the large X chromosome. While males have on X and one small Y chromosome.
AUTOSOMES or AUTOSOMAL CHROMOSOMES are the other 44 chromosomes that are not sex chromosomes.
So in a typical human cell, you will find both autosomes and sex chromosomes. 46, XX for females and 46, XY for males.
SEX CHROMOSOMES: one of two chromosomes (X or Y) that determine an organism's sex.
SEX CHROMOSOMES: one of two chromosomes (X or Y) that determine an organism's sex.
AUTOSOMES: chromosomes that are not sex chromosomes.
AUTOSOMES: chromosomes that are not sex chromosomes.
Mitosis vs. Meiosis
Mitosis vs. Meiosis
Mitosis results in the production of two genetically identical diploid cells (2N), whereas Meiosis produces four genetically different haploid cells (N).
Mitosis results in the production of two genetically identical diploid cells (2N), whereas Meiosis produces four genetically different haploid cells (N).
PRODUCTS OF MITOSIS
PRODUCTS OF MITOSIS
A diploid cells that divides by mitosis gives rise to two diploid (2N) daughter cells. The daughter cells have two sets of chromosomes and alleles that are identical to each other and to the original parent cell.
A diploid cells that divides by mitosis gives rise to two diploid (2N) daughter cells. The daughter cells have two sets of chromosomes and alleles that are identical to each other and to the original parent cell.
Mitosis allows an organism's body to grow and replace cells. In asexual reproduction, a new organism is produced by mitosis of the cell or cells of the parent organism.
PRODUCTS OF MEIOSIS
PRODUCTS OF MEIOSIS
Meiosis, on the other hand, begins with a diploid cell but produces four haploid (N) cells. These cells are genetically different from the diploid cell and from one another.
Meiosis, on the other hand, begins with a diploid cell but produces four haploid (N) cells. These cells are genetically different from the diploid cell and from one another.
Meiosis is how sexually reproducing organisms produce gametes. In contrast, asexual reproduction involves only mitosis.
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