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The life cycle of an eukaryotic cell can be separated into two phases.
Interphase and Mitosis
Interphase:
-90% of the cell’s life cycle
-Cell grows and performs normal cell functions
Mitosis:
-relatively short period
-part of the cell cycle
-results in the division of the nucleus and cell
-produces two genetically identical daughter cells
Interphase is broken down into 3 subphases
Gap 1 or G1 Phase
Synthesis or S Phase
Gap 2 or G2 Phase
G1 Phase:
Part of Interphase where the cell grows and performs normal cell functions
The first checkpoint
“point of no return”
G/S Checkpoint:
The barrier to the rest of the cell cycle after G1 Phase
Body makes sure the cell has enough space and is the right size.
S Phase:
Where DNA in the nucleus is replicated in preparation for Mitosis.
A copy is made so the daughter cells receives the same exact genetic information.
After the DNA has been replicated a cell must undergo cellular division.
G2 Phase:
Gap 2 is understood as the second part of Interphase where the cell continues to grow and perform cellular functions.
Replicates any organelles so each cell will have equal proportions of necessary structures after undergoing cellular division.
G2/M Checkpoint:
After the G2 Phase
Right before the cell commits to cell division in the process of Mitosis.
DNA is not easily…
visible inside the nucleus during Interphase. Instead of being condensed into easily identifiable chromosomes, the DNA is in a loosely bound state known as chromatin.
It is still wrapped around histones in the chromatin phase
Cell signaling plays a vital role in…
determining when a cell moves from one phase or cycle to another
Some cells exit the cell cycle in a period of rest following Mitosis known as…
G0 Phase
seen as an extended G1 Phase where the cell is neither dividing, nor preparing to divide.
Depending on the cell type the organism can be in G0 for a brief time or the rest of the organisms life
EX: Neurons, Multinucleated muscle cells
These cells are much less likely to re-enter the cell cycle to replace the damaged or destroyed cells.
Cellular reproduction occurs during a phase of the cell cycle known as…
Mitosis
Cellular division by a parent cell results…
in 2 genetically identical daughter cells
Involves only 1 division after replication occurs in the S phase.
daughter cells are identical to their parents cell and each other
After the division the daughter cells…
Maturation, (the cell re-enters Interphase and continues normal cell functions)
Reasons for Mitosis include:
Growth, Repair, Replacement and Maintenance
In order for one parent cell to divide into two identical daughter cells, the genetic material inside must be replicated.
In Interphase or S-Phase…
DNA replication takes place
Genome
This is the entire genetic material (DNA) for an organism or cell
DNA has two different…
appearances or forms within cell. It depends on what phase the cell is within the cell cycle as to how DNA looks.
Chromatin
Chromosmes
Chromatin:
The loose state of DNA. It is commonly described as resembling an untangled ball of yarn.
-Can be moved around to locate the gene segments of interest fir protein synthesis
Chromosome:
The tightly coiled state of DNA. It is commonly described as resembling corkscrew shaped pasta.
-DNA condenses into chromosomes for dividing equally and easily
Somatic cells
“soma” means body, (These are your normal body cells)
-These are the cells that make up the majority of an organism
-Their chromosomal content is 2n or diploid
(They get half from the mother (n) and half from the father (n))
-Also called autosomes.
Haploid
Opposite of Diploid
Haploid: 1 set of chromosomes
Sister Chromatids:
Tid means portion
This term refers to one-half of a replicated chromosome. The two halves are held together at the centromere (center unit), which is a group of proteins.
Mitosis means…
nucleus division
first divide the DNA, then secondly the cytoplasm
Prophase (first)
-Nuclear envelope is broken down and rearranged to make the spindle apparatus.
-The Chromatin condenses to form X shaped replicated chromosomes (two sister chromatids held together by one centromere. )
-Centrioles move towards the poles (present in animal cells only)…plants use cell wall.
-Centrioles anchor spindle fibers
Metaphase (middle)
-The replicated chromosomes lineup on the metaphase plate, which is found in the middle of the cell
-The spindle apparatus attaches to the Kinetochore (a part of the centromere and centrioles (the anchors)
-Third checkpoint occurs here. (Are all the chromosomes attached and lined up and ready to divide/separate, make sure spindles are attached to chromosomes.
Anaphase (seperate)
-Replicated chromosomes are pulled apart into sister chromatids and each chromatid moves towards opposite poles (ends) of the cell
-The spindle apparatus is being broken down as the two sister chromatids are “walked” towards the poles by the motor proteins using ATP.
Telophase (last) (opposite of prophase)
-The nuclear envelope is rebuilt by using broken down spindle apparatus pieces
-The chromatids begin to decondense back to their chromatin state
-A cleavage furrow (indent) begins to form using actin and myosin microfilaments.
Why is cytokinesis different from mitosis
They are different because mitosis is dividing the nucleus, and cytokinesis is dividing the membrane
plant cells do not have…
centrioles because they have cell walls to anchor to.
The new cell wall “Plate” develops, using small segments of cellulose, instead of a cleavage furrow
The main purpose of cellular division is for…
propagation of the lineage, or continuing to pass down a cell’s genetic traits from one generation to the next.
This occurs through the Cell Cycle, and all cells undergo some variation of this process.
Maturation:
occurs after division. (The cells grow, mature, and begin being able to perform their adult functions.)
Binary Fission:
-This is the process of genetic replication & cellular division in prokaryotes (bacteria).
-Much faster
-Prokaryotic DNA is shaped differently than eukaryotic DNA. It is arranged in a circular shaped chromosome.
-The two major steps in both are the same - DNA synthesis and division.
During the S phase of Interphase… (Binary fission)
DNA replication starts at the “origin” and works around the entire single circular chromosome. This results in two identical chromosomes in the nucleoid region.
Eukaryotic cells contain a nucleus. In order for these cells to divide, they undergo two unique division processes:
They must divide their nucleus (Mitosis) and the cell itself (cytokinesis).
During Mitosis, the cell’s DNA (which was replicated during the S phase of Interphase):
condensed into chromosomes and evenly separated through a series of specific steps. Those steps include: Prophase, Metaphase, Anaphase, and Telophase.
Following Mitosis, a second division process, known as…
cytokinesis, occurs. This process involves the division of the cellular organelles and cytosol.
During Cytokinesis…
microtubules & microfilaments that are part of the Spindle Apparatus push and pull against each other to form a cleavage furrow that continues to narrow until the one cell separates into two identical daughter cells.
Cell Cycle Checkpoints are designed to…
regulate or “control” the Cell Cycle.
-Regulation is crucial for normal growth and development.
-Regulation varies for each different cell type.
-The regulation is controlled by cell signaling molecules called Cyclins. (They control the Cell Cycle.)
The Cell Cycle is controlled by…
“STOP “& “GO” chemical signals at critical points. These signals indicate if key cellular processes have been completed correctly.
EX.. The replication of DNA and G2/M
Then 3 major checkpoints and what do they check for:
1. G1/S - can DNA synthesis begin?
2. G2/M - has DNA synthesis been completed correctly? This point is the regulation point where cells make a commitment to Mitosis.
3. Spindle Checkpoint (M-Checkpoint) - are all chromosomes attached to spindles? Can sister chromatids separate correctly?
Which checkpoint is the most critical and why:
G1/S is the most important because it begins the entire division process. It is the primary decision point or “restriction point.”
-If a cell receives the “GO” signal at the G1/S checkpoint, it divides.
-Several factors determine whether or not the “GO” signal is given. They include both internal signals: cell growth (size), cell nutrition and external signals: “growth factors”
-If a cell does not receive the “GO" signal, it exits the Cell Cycle & switches to G0 phase, or the non-dividing, working state.
How do cells know when to divide?
Cyclins, or cell communication signals specifically designed to tell the cell to divide are produced and released in the cytoplasm to give the cue to move from phase to phase
The cyclins usually…
activate or inhibit proteins (enzymes) called kinases within the cell.
When a cyclin binds with a kinase to activate or deactivate it, it is known as a…
Cyclin-dependent kinase, or CDK
The cell produces different types of cyclins at different points of the Cell Cycle that…
activate/deactivate distinctive kinases which perform the designated functions of the specific cellular division phase.
Ex. MPF, or Maturation Promoting Factor is a Cdk that is created at the G2/M checkpoint. It triggers the beginning of Mitosis.
The rise and fall of the amount of cyclin and CDK can be traced and correlates to…
specific phases of the cell cycle.
-CDKs & cyclin drive cell from one phase to next in cell cycle
Proper regulation of the cell cycle is so key to life that the genes for these regulatory proteins have been…
highly conserved through evolution. The genes are basically the same in yeast, insects, plants & animals (including humans).
A multicellular organism needs to coordinate cell division across…
different tissues & organs. This is critical for normal growth, development, & maintenance.
-Coordinate timing of cell division.
-Coordinate rates of cell division.
-Not all cells can have the same cell cycle
There are two irreversible points in the cell cycle:
The replication of genetic material and separation of sister chromatids.
-These points are known as checkpoints. This is where the process is assessed & possibly halted.
Cells use both internal signals (cyclins & Cdks) and external signals to…
determine whether or not to proceed through each checkpoint in the Cell Cycle.
-An example of an external signal that coordinates the timing of the Cell Cycle would be Growth Factors.
-Growth factors are protein signals released by somatic (body) cells that stimulate other cells to divide.
Growth factors, if improperly regulated, can…
create cancers.
Two categories of genes associated with growth factors have been identified as playing a major role in the formation of most types of cancers. Those two categories of genes include…
Proto-oncogenes and Tumor-suppressor genes.
Proto-oncogenes:
-normally activates cell division
-become oncogenes (cancer-causing) when mutated
-if switched “ON” can cause cancer
Tumor-suppressor genes:
-normally inhibits cell division
-if switched “OFF” can cause cancer
Cancer is essentially…
a failure of cell division control. It can be defined as unrestrained, uncontrolled cell growth
What control is lost when there is a failure of cell division control:
A cell loses control of multiple or all checkpoint stops during the Cell Cycle. The cell also loses control of Gene p53.
-Gene p53 plays a key role in G1/S restriction point. It codes for a protein that halts cell division if it detects damaged DNA.
-5. ALL cancers have to shut down p53 activity to grow and spread
If p53 proteins detect damaged DNA, they can cause several events to occur:
-They can stimulate repair enzymes to fix DNA.
-They can force the cell into the G0 resting stage.
-They can pause the Cell Cycle & keep the cell in G1 arrest
-They can cause apoptosis of the damaged cell.
Think of Gene p53 as a…
police officer or a security guard at a roadblock. Instead of checking for expired licenses, p53 checks the DNA at particular points in the Cell Cycle for damage.
Cancer develops only after…
a cell experiences anywhere from approximately 6 - 15 key mutations or changes in the DNA (It depends on the type of cell, location of the cell, and the type of mutation.)
Development of Cancer:
1.Cancer develops only after a cell experiences anywhere from approximately 6 - 15 key mutations or changes in the DNA
2.unlimited growth – turns on growth promoter
3.ignore checkpoints – turns off tumor suppressor genes (p53).
4.escape apoptosis - turns off apoptosis genes.
5.immortality = unlimited divisions - turns on chromosome maintenance genes.
6.promotes blood vessel growth - turns on blood vessel growth genes.
7.Overcome anchor & density dependence- turns off touch-sensor gene
8.Cancer cells ignore all checkpoints and begin to grow uncontrollably.
Mutations in the DNA are caused by:
Carcinogens.
Carcinogens or mutagens are defined as…
any substance that can cause or promote mutations in the DNA that ultimately lead to cancer.
Examples:
UV radiation, Chemical exposure, Radiation exposure and Heat
Tumors:
When cells ignore all checkpoints and begin to grow uncontrollably, it can lead to cancer. When cellular division occurs uncontrollably, a mass of abnormal cells will begin to grow. The mass is known as a Tumor.
There are two categories of tumors:
Benign & Malignant tumors.
Benign tumors:
-Are abnormal cells that remain at the original site as a lump or large mass of cells.
-Are not considered cancerous because they are not invasive - they do not spread to neighboring tissue.
-Most can be removed by surgery.
-Although not typically harmful by themselves, benign tumors have the potential to become very large and may pose possible health risks if they press against other organs and are not treated.
Malignant tumors:
-Cancer cells break away from the original tumor & leave their original site.
-Are carried by the cardiovascular & lymphatic systems to other tissues.
-Once they are in a new location, the cancer cells start more tumors. This is known as metastasis.
-Are considered invasive because they invade other cells’ living space, hijack their nutrients by stealing their blood supply, and grow until there is no room for the normal cells. The cancer cells eventually starve the healthy cells out and they begin to die.
-Cancer cells continue to grow, create tumors, and metastasize or move, until they have spread throughout multiple organ systems within the body.
-They impair functions of organs system-wide and ultimately cause a loss of homeostasis.
Gametes:
These are the sex cells.
n=haploid- half the number of chromosomes.
-egg- comes from female
-sperm- comes from male
Fertilization:
is the fusion of egg and sperm, must occur to be able to reproduce.
-This fusion between egg and sperm produces a single diploid cell called a zygote.
-The zygote goes on, through repeated mitosis, to produce the new organism
Homologous Chromosomes:
one from each parent that are similar in length, gene position, and centromere location. The position of the genes on each is the same, however the genes may contain different alleles. 44 = 22 pairs exist in all human cells.
The two types of reproduction that can occur by living organisms:
Asexual Reproduction and Sexual Reproduction
Asexual Reproduction:
1. This involves only one parent. The parent is producing genetic clones of itself. The parent and offspring are 100% identical in terms of DNA content and DNA nucleotide sequence.
Benefits – Reproduction can occur very quickly (Good for taking over a new area). It is a simple process. Only one parent is needed.
Risks – Every organism is the same. So if a disease affects one; it will affect all. (There is no variation!)
Sexual Reproduction:
1. This involves two parents to contribute DNA. This process “creates” variation, which is important in terms of survival in the environment.
Benefits – It produces variation. This is why some organisms have advantages over others within the same species in terms of survival and the ability to reproduce. Variety means there exists the possibility to evolve over time while living in an ever changing environment.
Risks – It takes two to be able to reproduce and they must be of the opposite sex for the Physical Reproduction to occur. This is not good for an endangered species. It also takes more time. It also involves a more complicated process to create the gametes that have half the DNA content.
Process of Gamete Formation:
-This process occurs in the sex organs of the organisms. These organs are called Gonads.
-Has 1 DNA replication followed by 2 cell divisions, therefore the result is 4 haploid cells.
a. Remember that the S phase doubles the amount of DNA. In humans, when all 46 chromosomes are replicated, therefore the parent cell has 2x (96 chromosomes) the DNA of a non-dividing cell.
Meiosis I:
(This division is the separation of chromosome pairs.) This takes the cell back to diploid (a full set of chromosomes (e.g. 46 in humans).
Meiosis II:
This division is the separation of sister chromatids.) In humans, 46 → 23 chromosomes.
In this process, Males produce 4 haploid sperm; each having 23 chromosomes.
In this process, Females produce 1 haploid egg with 23 chromosomes. The other three cells degrade.
Stages of Meiosis I: (Prophase)
-Pairing of homologous chromosomes occurs
-Each chromosome consists of two sister chromatids.
-As homologous chromosomes condense, they are bound together in a process called synapsis, which allows for crossing over.
i. Crossing over- chromosomal segments are exchanged between a pair of homologous chromosomes.
-Produces exchange of genetic information that creates new genetic combinations.
Stages of Meiosis I: (Post-Prophase)
2.Metaphase I
-Chromosome centromeres attach to spindle fibers.
-Homologous chromosomes line up as a pair at the equator.
3.Anaphase I
-Homologous chromosomes separate and move to opposite poles of the cell.
-The chromosome number is reduced to 2n to n when the homologous chromosomes separate.
4.Telophase I
-Chromosomes reach the cell’s opposite poles.
-Cytokinesis occurs
Stages of Meiosis II:
1.Prophase II
-A second set of phases begins as the spindle apparatus forms and the chromosomes condense.
2.Metaphase II
-Chromosomes are positioned at the equator.
3.Anaphase II
-Sister chromatids are pulled apart at the centromere by spindle fibers and move toward the opposite poles of the cell.
4.Telophase II
-The chromosomes reach the poles, and the nuclear membrane and nuclei reform.
5.Cytokinesis
-Results in four haploid cells, each with “n” number of chromosomes.
Nondisjunction:
The failure of homologous chromosomes or sister chromatids to separate properly during cell division.
Three Forms: (Nondisjunction)
a. Failure of a pair of homologous chromosomes to separate in meiosis I.
b. Failure of sister chromatids to separate during meiosis II.
c. Failure of sister chromatids to separate during mitosis.
-Results in daughter cells with abnormal chromosome numbers, a condition known as aneuploidy
When nondisjunction occurs during meiosis, it can result in…
gametes with the wrong number of chromosomes. If fertilized, the most common result is miscarriage. Most human embryos cannot survive with an abnormal chromosome number.
Karyotype:
-A simple picture of a person’s chromosomes that can diagnose many of genetic conditions caused by nondisjunction
-The chromosomes are isolated, stained, and examined under a microscope. A picture is then made of the chromosomes and they are cut up and arranged by size into homologous pairs. The chromosomes are lined up from largest to smallest.
-The autosomes are displayed as pairs 1-22 and the sex chromosomes are shown as pair 23. i. Female-XX ii. Male-XY
Major differences between Mitosis and Meiosis:
-The number of divisions (Mitosis has 1; Meiosis has 2).
-The final products of each process (Mitosis- “cloned” (identical) diploid daughter cells; Meiosis- variable haploid games).
-Crossover in Prophase 1, creates variation (no crossover in Mitosis)
-Chromosome pairs in mitosis vs. sister chromatids in meiosis separating in the second division to REDUCE to haploid.
Crossover “genetic swapping” between homologous chromosomes happens because…
This occurs to create variation from the parent’s genome. (They are then called Recombinant Chromosomes.)
-Occurs during Prophase I of meiosis.
-Increases genetic diversity among the resulting gametes.
Meiosis creates haploid gametes containing both…
paternal and maternal chromosomes.
Random fertilization due to sexual reproduction leads to…
the joining of gametes that contain different combinations of genes than the parents and therefore leads to genetic diversity.
Law of Segregation:
The alleles on homologous chromosomes move independently of one another increasing genetic diversity in gametes.
Law of Independent Assortment:
Occurs during Metaphase I of Meiosis. The maternal and paternal alleles from each pair assort independently from the alleles in the other pairs. All possible combinations of alleles occur in the gametes increasing genetic diversity.
Down Syndrome
Turner Syndrome
Klinefelter Syndrome
-abnormal cell division results in an extra full or partial copy of chromosome 21
-a completely or partially missing X chromosome
-occurs when boys are born with an extra X chromosome giving-XXY. - This is not an inherited condition and happens as a random error called chromosomal nondisjunction that occurs during cell division. This could also result in XXXY.
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