Cell Cycle Flashcards
What are the phases of the cell cycle?
- Interphase
- Mitosis
- Cytokinesis
How can interphase be divided?
- Gap 1 phase (G1)
- Synthesis Phase (S)
- Gap 2 phase (G2)
What happens in the gap 1 phase of interphase?
All organelles in the cell are doubled, including cytosol components and centrioles
- Cell growth
What happens in the synthesis stage of interphase?
The DNA replicates
- Genome replication
What happens in the gap 2 phase of interphase?
All the enzymes needed for cell division are produced
- Synthesise the microtubules/spindle fibers for mitosis
- Preparation for division
Which phase of the cell cycle do most eukaryotic cells spend their time?
Interphase, the cells still carries out the regular functions, it just also prepares for mitosis
What are the stages of mitosis?
- Prophase
- Metaphase
- Anaphase
- Telophase
What happens during prophase?
- Chromosomes condense and become visible
- The cytoskeleton disassembles as the spindle apparatus forms
- The centrioles move to the polar ends of the cell and the nuclear envelope breaks down,
- The centrioles extend their microtubules to contact with the sister chromatids kenetochore
Where are the two sister chromatids held together?
The centromere, a sort of junction holding the two sister chromatids together
How does the spindle fiber connect to a chromosome?
Proteins in the centromeres of the chromosomes called kinetochores attach to the microtubules
What happens during metaphase?
- The chromosomes are pulled by the microtubules into the middle of the cell, called the metaphase plate
What happens during anaphase?
- Chromosomes are split by the microtubules
- The sister chromatids are pulled to opposite poles of the cell
What happens during telophase?
- The cell is constricted in the middle and creates a pinch in the cell
- Chromosomes cluster at both poles
- Chromosomes decondense at both poles of the cell as the nuclear envelopes begin to reform
- The spindle apparatus disassembles as the microtubules are broken down into tubulin monomers
How does the cell pinch during telophase?
A ring of constricting actin and myosin filaments surrounding the inside of the circumference of the cell
What are the tubulin monomers reused for after the spindle apparatus is broken down?
To reconstruct the cytoskeleton of the new daughter cells
What is cytokinesis and what happens during this phase?
- Cell division
- The completion of cell division as the cleavage furrow is extended and separates the cells
- Cell membrane and wall are formed where necessary
What is the name of the pinch before the two daughter cells split?
Cleavage furrow
Why can’t plants use an actin cleavage furrow to separate the daughter cells?
The actin filaments are not strong enough to penetrate the cell wall
What do plants use instead of actin filaments during telophase?
Vesicles from the Golgi apparatus form an expanding membrane partician, kind of like a wall of vesicles between the two daughter cells
- Cytokinesis then splits the two cells at this wall of vesicles AKA the cell plate
- Inside the vesicles is the material to create the new cell walls
What is the G1 checkpoint in the cell cycle?
- A principal checkpoint
- Makes the decision whether the cell should divide, delay divisions or enter a resting stage
What is the G2 checkpoint in the cell cycle
- A principal checkpoint
- Assess the success of DNA replication and triggers the start of mitosis, As long as the preparation for mitosis is acceptable
What is the M checkpoint in the cell cycle?
- A principal checkpoint
- Assess the accuracy of mitosis,
- Checks to see if the chromosomes attached to the spindles properly
- Occurs during metaphase
- Triggers the exit of mitosis and cytokinesis and the beginning of G1
How many principal checkpoints are there in the cell cycle?
3
What allows the principal checkpoints to move onto the next step in the cell cycle?
- Acceptable levels of MPF
What is MPF?
- Maturation/Mitotic Promoting factor
- A protein complex made from CDK binding with cyclin
What happens to the cyclin after the threshold of MPF has been met and the cell cycle moves on?
The cyclin is broken down by a cyclin degrading protein
What is the Gap 0/ G0 phase?
A resting phase outside of the cell cycle where the cell just carries out its required duties
What determines if a cell passes the checkpoints?
- Amount of MPK
- Intracellular and extracellular chemical signals
- e.g. growth factor
What tells a cell to stop dividing and why?
Surface proteins
- when the environment is too crowded and cells are touching with too much pressure
What are kinases and their function in the cell cycle?
An enzyme that provides energy for the reactions needed for successful mitosis to occur
What are the two types of reproduction?
- Sexual
- Asexual
What is asexual reproduction?
Reproduction involving only one parent
- Produces an identical clone
- Genetic variation only from mutation
What types of cells use Asexual reproduction?
- Single-celled Eukaryotes
- Single-celled Prokaryotes
- Multicellular Eukaryotes (starfish)
- Some plants
What are the characteristics of DNA found in a prokaryote?
- Double-stranded
- Circular
What is Sexual reproduction?
Reproduction involving the fusion of gametes
- Two parents
- Results in genetic variation
What process produces gametes?
Meiosis
What are gametes and how do they differ from every other cell in your body?
- Sex cells with only 23 chromosomes
What is meiosis?
A process of cell division where the cell is divided and the daughter cells have only half the genetic information
- Involved in every sexual life cycle
What are somatic cells?
Cells not involved in reproduction
- Any cell but gametes
What is a homologous pair and how does it result in variation in offspring?
The pair of chromosomes that come from mother and father, found in somatic cells
- One homolog comes from the mother, the other from the father
- This creates genetic variation in offspring
What is a homologous chromosome?
A pair of the same chromosomes but of different genetic makeup (though the genes are in the same location)
- One from the father, one from the mother
How do gametes and somatic cells differ in chromosome structure?
- Somatic cells have homologous pairs
- Gametes do not have pairs of chromosomes, only one homolog from each pair
What does haploid mean?
Cells with a single set of chromosomes
What is formed during the fertilization of two haploid gametes?
A zygote
- The two gametes fuse and their chromosomes pair up, creating a somatic cell
- The zygote begins the diploid stage of the life cycle
What does diploid mean?
Cells with 2 complete sets of chromosomes
What are the two ways that gametes are formed?
- Immediately after meiosis
- Or the haploid cell produced by meiosis develops into a haploid organism and eventually produces gametes by mitosis
Can specialized cells divide?
No
Describe the structure of a chromosome.
- The DNA double helix wraps around a protein called histones
- This is called a nucleosome
- The nucleosomes then coil up into a 30nm fiber
- This 30nm fiber then folds into loops of 300nm
- These 300nm loops fold and compress into the typical X-shape chromosome
- Made of two sister chromatids, identical filaments of DNA connected at the centromere
- Kinetochores are protein plaques on either side of the centromere, holding the chromosome together
During what phase do chromosomes become visible?
Prophase, the DNA condenses and takes the typical X-Shape, and becomes visible under a light microscope
Where do microtubules attach to chromosomes?
Kinetochores attach to kinetochore spindles
What are the two mechanisms that move the chromosomes to opposite poles of the cell?
- The kinetochores have molecular motor proteins that allow the chromosome to move along the microtubule/spindle
- The microtubules also shorten at the centrioles and pull the chromosomes closer
What triggers the G1 checkpoint to stop the cell cycle?
DNA damage
What triggers the S checkpoint to stop the cell cycle?
incomplete replication or DNA damage
What triggers the G2 checkpoint to stop the cell cycle?
DNA damage
What triggers the M checkpoint to stop the cell cycle?
Chromosomes failing to attach to spindle fibers
How are CDK and cyclin used to regulate the cell cycle?
- Each CDK has its own cyclin to bind to, though the cyclin is not always available
- When the CDK does bind to the cyclin, producing MPF and signals for the cell cycle to continue if in abundance, then the cyclin is broken down by a protease
- The regulation of cyclin concentration is important b/c it controls the whole cell cycle
- Cyclins are synthesized in response to various signals, e.g. growth factor
What is the term for the genetically planned death of a cell?
Apoptosis
What are the reasons for apoptosis?
- The cell becomes useless (e.g. the connective tissue between the fingers of a fetus)
- Older cells are more prone to genetic damage, this can lead to cancer
Which cells are most prone to genetic damage and how does the body prevent it from happening?
Epithelial cells, these cells have a very short life span and experiences apoptosis in a few days or weeks
How is apoptosis triggered and how does a cell die?
- Internal signals that may be linked to cell age or damaged DNA
- Both internal and external signals trigger caspases which hydrolyze proteins in a cell
- Leads to cell death once proteins from the nuclear envelope, nucleosomes, and the cell membrane are targetted
How do tumors form?
When cells ignore restrictions of the cell cycle and continue to divide rapidly
What are the two types of tumors?
- Malignant (invasive)
- Benign (harmless)
What is a metastatic tumor?
Invasive malignant tumors that spread around the body via blood or lymph vessels
What is independent assortment?
The randomness anaphase 1 of meiosis involving the distribution of a single homologous pair (from mum or dad)
- The lineup of chromosomes in the middle of the cell is random
What is the relationship between chromosomes and genetic diversity?
The greater number of chromosomes, the more diverse a species can be
- Humans have 2^23 different combinations of chromosomes
What is nondisjunction?
When the chromosomes are not split properly in anaphase 1 or anaphase 2 of meiosis, leading to an increase in chromosomes/chromatids in the daughter cells
What is aneuploidy?
An abnormal amount of chromosomes
When a zygote is made up of a gamete with aneuploidy, what are the names for a zygote with an additional chromosome and a missing chromosome?
- Trisomy (additional/ 3 chromatids)
- Monosomy (missing/ 1 chromatid)
What is a karyotype?
Images of condensed chromosomes that are specific to an organism and are used to identify a specific chromosome by its size and centromere positions
How are karyotypes used?
Used to identify a trisomy or monosomy because they will not match the image of the chromosome as chromosomes are in homologous pairs.
What is the most common type of trisomy?
Trisomy 16
What is the trisomy for down syndrome?
Trisomy 21, is one of the few aneuploidies that allow a zygote to survive, most aneuploidies die as an embryo and is a large factor in causing miscarriages
What is a germ cell?
A reproductive cell
What is the law of independent assortment?
The inheritance of one allele will not influence the chance to inherit another allele
What is crossing over?
When two non-sister chromatids from a homologous pair trade genetic information
- Between the father chromosome and the mother chromosome
- The chromosomes now share DNA from both the mother and the father, this is done randomly and is the reason for genetic diversity between offspring
What is Meiosis I?
Reduction division
- The nucleus of the cell halves the chromosomal material between two haploid daughter cells
- Cell reduced from diploid to haploid, no longer in homologous pairs
What is Meiosis II?
Sister chromatids are split from the chromosomes, resulting in 4 haploid cells from the 2 previous cells
What is a tetrad?
Two duplicated homologous chromosomes (2 chromatids out of the 4) bound together by chiasmata during prophase.
- The outer chromatids are not connected at the chiasmata so they do not experience a cross over
- Google an image of chiasmata it helps lol
What are chiasmata?
- Points of connection between two homologous chromosomes, not at the centromere but along the X-Shape
- The site for cross overs
- Google an image of chiasmata it helps lol
What is nondisjunction?
An error in the separation of chromosomes.
When can nondisjunction take place?
- Meiosis I
- Meiosis II
- Mitosis
What happens if nondisjunction happens in meiosis I?
- The diploid cell splits its chromosomes unevenly between its two daughter cells
- Then meiosis II happens normally and you end with 4 haploids, all containing irregular numbers of chromosomes
- two gametes will have an extra chromosome alongside the complete set (2 trisomies), while the other two gametes will be missing a chromosome (2 monosomies)
What happens if nondisjunction happens in meiosis II?
- The diploid cells split their genetic information evenly between the two daughter cells in meiosis I,
- Then, in meiosis II, one of the cells split unevenly, this results in two gametes having irregular chromosomes, an additional one for one gamete and one missing for the other (1 trisomy and 1 monosomy)
Are there any cases where a human survives monosomy?
A woman can survive without the X chromosomes, but she will have Turner syndrome
What is a triploid?
When the subject has 3 sets of DNA from the fertilization of the egg using a germ cell with an irregular amount of chromosomes (one cell has double the chromosomes)
- This is lethal
What is a polyploid condition?
Cases where additional complete sets of chromosomes are present
- Different from aneuploid conditions because these conditions are caused by only a couple of chromosomes being added or taken away.
What is Kleinfelter syndrome?
When a man has an additional X chromosome
What is Cri du Chat?
When part of a chromosome is missing in the 5th pair
How are the karyotypes organized?
Size order, biggest to smallest
What are the stages of meiosis?
Meiosis I: - Prophase I - Metaphase I - Anaphase I - Telophase I Meiosis II: - Prophase II - Metaphase II - Anaphase II - Telophase II
Describe the process of meiosis I.
- Interphase, doubles genetic material
- Prophase I, DNA condenses into chromosomes and from homologous pairs, they also perform crossing over, Spindle forms and moves to opposite poles of the cell and being to line up the chromosomes
- Metaphase I, the homologous pairs line up down the metaphase plate
- Anaphse I, each pair is taken by spindle fibers shortening to opposite poles of the cell
- Telophase I, a nuclear envelope encapsulates the separated chromosomes and pinches at the cleavage furrow
- Cytokinesis, the cells split into two daughter cells with 46 chromosomes each
Describe the process of meiosis II.
- Prophase II, the two daughter cells lose nuclear envelope and the spindles form, moving the chromosomes towards the middle of the cell
- Metaphase II, Chromosomes line up single file down then metaphase plate, connected to spindles by a centromere
- Anaphase II, the chromatids are pulled from their kinetochores to opposite ends of the cell by the spindle fiber shortening
- Telophase II, a nuclear envelope envelopes the separated chromatids, and the cell pinches at the cleavage furrow.
- Cytokinesis splits the two daughter cells into 4 gametes with 23 chromosomes, all genetically unique
How are chromosomes counted?
By the centromere
Describe the process of mitosis.
- Interphase, G1 where the cell grows and doubles its contents, S where the DNA is replicated, G2 where the enzymes for mitosis and made and spindle fibers are synthesized
- Prophase, DNA condenses into chromosomes and nuclear envelope disappears, spindle fibers move with centrioles to opposite poles of the cell
- Metaphase, spindle fibers line up the chromosomes in a single file line at the equator (metaphase plate), spindle fibers attach to the centromere
- Anaphase, Chromatids are pulled apart from the centromere and to opposite ends of the cell, the spindle fibers shorten and the chromosomes move down the fibers as well using a motor action
- Telophase, a nuclear envelope forms around each of the chromosomes sets, and a pinch in the cell begins to form called the cleavage furrow
- Cytokinesis, the cell splits into two identical daughter cells
- Checkpoints at G1, G2, and M, these all have to be met for mitosis to be carried out properly, they are regulated by CDK and Cyclin creating MPF which levels control if the checkpoints have been met.
