Quiz 1 - Mitosis & Meiosis Flashcards
Basic steps in cell division?
1) copying of DNA
2) separating DNA copies
3) dividing cytoplasm to create two complete cells
What is mitosis and binary fission?
both types are asexual reproduction which is cell division that results in genetically identical daughter cells
mitosis: asexual reproduction in eukaryotes
binary fission: asexual reproduction in prokaryotes
What is sexual reproduction called in eukaryotes?
meiosis (involves gametes - sperm/egg cell division)
What are the types of sexual reproduction in prokaryotes?
Transformation = prokaryote takes up DNA found within the environment that has originated from other prokaryotes
Transduction = prokaryote is infected by a virus which injects short pieces of chromosomal DNA from one bacterium to another
Conjugation = DNA is transferred between prokaryotes by means of a sex pilus
What is chromosomes and chromatin fiber?
- A chromosome is a single long double helix of DNA wrapped around proteins called histones
- When cell is not dividing, chromosome is in form of long, thin chromatin fiber
What helps form the chromosome structure?
Chromosomes form almost normally without histones, so it is actually condensin proteins that promote DNA interactions which shape chromosomes, while histones just add compaction and protection
What does the cell cycle consist of?
1) Mitotic (M) phase (mitosis and cytokinesis)
2) Interphase (cell growth and copying of chromosomes in preparation for cell division)
What is the interphase cycle?
Interphase (about 90% of the cell cycle) can be divided into sub-phases:
– G1 phase (“first gap”): organelles duplicate
– S phase (“synthesis”)
– G2 phase (“second gap”):
** The cell grows during all three phases, but chromosomes are duplicated only during the S phase
What are the phases of mitosis?
prophase, prometaphase, metaphase, anaphase, telophase
What is the mitotic spindle?
apparatus of microtubules controlling chromosome movement during mitosis (includes the centrosomes, spindle microtubules, and asters)
What are centrosomes?
Organelle that serves as a microtubule organizing center (MTOC) (spindle microtubules grow out from them) replication leads to two centrosomes that migrate to opposite ends of cell
What are the types of spindle microtubules?
Kinetochore microtubules = capture sister chromatids by binding to kinetochore proteins
Non-kinetochore microtubules (also known as polar microtubules) = do not capture sister chromatids
What are asters (astral microtubules)?
A radial array of short microtubules that extends from each centrosome, which connect to proteins on inner surface of cell membrane to anchor the centrosomes
What are somatic cells and gametic cells?
Somatic cells (non-reproductive cells) have two sets of chromosomes – mitosis leads to the generation of somatic cells!
Gametic cells (reproductive cells: sperm and eggs) have half as many chromosomes as somatic cells – meiosis leads to the generation of gametes!
What happens during g2 interphase?
nuclear envelope intact and nucleoli present; centrosome is duplicated; chromatin duplicated during S-phase
What happens during early prophase?
nucleolus disappears; chromatin fibers become tightly coiled and condense to form chromosomes (which first become visible using light microscope); centrosomes move away from each other as microtubules lengthen
What happens during late prophase (prometaphase)?
nuclear envelope breaks down; chromosomes become completely condensed; microtubules (kinetochore and non-kinetochore) invade nuclear space [kinetochore microtubules attach to chromosomes]
What happens during metaphase?
centrosomes at opposite ends of cell (held in place now by astral microtubules); formation of spindle apparatus is now complete; chromosomes settle at metaphase (equatorial) plate
What happens during anaphase
sister chromatids move to opposite poles of cell as kinetochore microtubules shorten; non-kinetochore microtubules grow longer which helps elongate cell
What happens during telophase?
chromosomes decondense into chromatin; nucleolus and nuclear envelope reappears; spindle microtubules disappear
What is cytokinesis and how does it occur in animal vs plant cells?
- Separation of cytoplasm into two daughter cells
- animal: occurs by a process known as cleavage, forming a cleavage furrow on cell surface
- plant: a cell plate forms during cytokinesis (vesicles -containing cell wall materials- from Golgi move along microtubules to middle of cell)
How is the cell-cycle controlled?
- Sequential events of the cell cycle are directed by a distinct cell cycle control system driven by specific chemical signals present in the cytoplasm
What is the G1 checkpoint?
- For many cells, the G1 checkpoint seems to be the most important one
- pass checkpoint if: cell size adequate, nutrients are sufficient, social signals are presents, DNA is undamaged
- If a cell receives a go-ahead signal at the G1 checkpoint, it will usually complete the S, G2, and M phases and divide
- If the cell does not receive the go-ahead signal, it will exit the cycle, switching into a non-dividing state called the G0 phase (permanent or temporary)
What is the g2 checkpoint?
Second checkpoint between G2 and
M phases
– Chromosomes must be replicated successfully and DNA is not damaged to pass checkpoint
– Activated MPF is present (M phase-promoting factor)
What is MPF, Cyclin/Cdk?
Two types of regulatory proteins are involved in cell cycle control: cyclins and cyclin-dependent kinases (Cdks)
* Level of cyclin fluctuates during the cell cycle while level of Cdk remains fairly constant
* MPF (M phase-promoting factor) is a cyclin-Cdk complex that triggers a cell’s passage past G2 checkpoint into M phase
What are the two M checkpoints?
- Between metaphase and anaphase:
> Delay occurs as all kinetochores have not yet attached to spindle microtubules
> This keeps the anaphase-promoting complex (APC) in an inactive state
> When all kinetochores are attached,
the APC activates, triggering breakdown of securin and activation of separase to degrade cohesin proteins (hold sister chromatids together) so they can separate during anaphase
– Between anaphase and telophase (ensures that chromosomes have fully separated)
What are examples of internal cell-cycle control signals?
- production of high levels of MPF: important for passing G2 checkpoint
- activation of anaphase promoting complex: important for passing the first M checkpoint (between metaphase and anaphase)
What are examples of external cell-cycle control signals?
essential nutrients, growth factors, density-dependent inhibition, anchorage dependence
What are growth factors?
proteins released by certain cells that stimulate other cells to divide
* For example, platelet-derived growth factor (PDGF) stimulates division of human fibroblast cells in culture
– Fibroblast = cell responsible for making extracellular matrix and collagen
What is density-dependent inhibition?
crowded cells stop dividing
What is anchorage dependence?
in which they must be attached to a substratum for division
What is the difference between benign/malignant tumours?
If abnormal cells remain at the original site, the lump is called a benign tumor
* Malignant tumors invade surrounding tissues and can metastasize, exporting cancer cells to other parts of the body, where they may form secondary tumors
How do cancer cells/tumours form?
Cancer cells do not respond normally to the body’s control
mechanisms; exhibit NEITHER density-dependent inhibition nor
anchorage dependence
* Cancer cells may not need growth factors to grow and divide
* Cell transforms into cancer cell – cells divide rapidly (proliferate) in uncontrolled manner to form tumor
List the differences between cancer cells and normal cells?
- Normal cells exhibit density-dependent inhibition and anchorage dependence but cancer cells do NOT
- Normal cells have controlled rate of cell division (NO loss of cell cycle function, so
they divide until there are enough cells present) but cancer cells don’t have controlled rate of cell division (loss of cell cycle function which causes cells to divided rapidly before having had a chance to mature) - Either repaired or die (undergo apoptosis) when they are damaged or get old (For example, there is a p53 protein that has the job of checking to see if a cell is too damaged to repair and, if so, advise cell to kill itself), but cancer cells either NOT repaired or DO NOT undergo apoptosis when they are damaged or get old (inactive/mutated p53 protein)
- Normal cells have surface adhesion molecules that keep them bonded to neighboring cells but cancer cells do not
- Normal cells DO NOT have the ability to metastasize (spread) – they stay in the area of the body where they
belong but cancer cells can metastasize to other regions of the body by travelling through blood stream or lymphatic system - Develop into specialized cells (it means they can easily develop into brain cells, heart cells, lung cells or other specific types of cells) but cancer cells DON’T develop into specialized cells
- Normal cells undergo angiogenesis only as part of normal growth and development and when new tissue is needed to repair damaged tissue but cancer cells undergo angiogenesis even when growth is not necessary!
What is a life cycle?
the generation-to-generation sequence of stages in
the reproductive history of an organism
• Fertilization and meiosis alternate in sexual life cycles
What are homologous chromosomes?
two chromosomes in
each pair are called
homologous chromosomes,
or homologs…one
chromosome is from the
mother and other
chromosome is from the
father…homologous
chromosomes can exist as
unreplicated or replicated
Homologous chromosomes
are same size (i.e. length),
and they have genes which
are arranged in same orde
How is a karyotype developed?
Karyotypes can be
performed using
amniotic fluid,
blood, bone marrow,
placenta
• After growing cells, a
drug called
colchicine is added
before karyotype
processing
– Colchicine arrest
the cell at
metaphase during
cell division
– At metaphase, the
chromosomes are
most condensed
What is Down syndrome?
Down syndrome due to three copies of
chromosome 21
– Most common chromosome abnormality in
humans
– Typically associated with a delay in cognitive
ability and physical growth
What is Klinefelter syndrome
Klinefelter syndrome, an XXY male
– Individual has male sex organs, but are sterile
– May display feminine characteristics
– Intelligence is normal
What is turner syndrome?
Turner syndrome, an XO female
– Do not mature sexually during puberty
(sterile)
– Short stature and normal intelligence
What is the difference between meiosis I and meiosis II (# of daughter cells, type of division, etc)?
Meiosis I starts with diploid parent cell containing a homologous pair of replicated chromosomes: homologous chromosomes separate into two haploid daughter cells with replicated chromosomes; it is called reductional division
Meiosis II starts with haploid cells with just one homologous chromosomes: sister chromatids of each
replicated chromosome separate. Results in four haploid daughter cells with unreplicated chromosomes; it is called equational division?
What happens during prophase I
Prophase I typically occupies more than 90% of time required for
meiosis
• Replicated chromosomes condense and spindle microtubules begin
to form from centrosomes
In synapsis, homologous replicated chromosomes loosely pair up,
aligned gene by gene
• Each pair of homologs forms a bivalent (each bivalent usually has
one or more chiasmata, regions where crossing over occurred)
• In crossing over, non-sister chromatids exchange DNA segments to
produce chromosomes with
a combination of maternal
and paternal alleles
• Cells in prophase I are diploid
What happens during metaphase I
Paired homologs line up at metaphase plate at random (cells in
metaphase I are diploid)
• Microtubules from one pole are attached to the kinetochore of one
replicated chromosome of each bivalent
• Microtubules from the other pole are attached to the kinetochore
of the other replicated chromosome of each bivalent
What happens during anaphase I
paired homologs separate (cells in anaphase I are
diploid)
• One chromosome moves toward each pole, guided by the spindle
microtubules (kinetochore microtubules shorten while non-
kinetochore microtubules lengthen)
• Sister chromatids remain attached at the centromere
What happens during telophase I and cytokinesis
In the beginning of telophase I, the
homologs (as replicated chromosomes)
finish migrating to the poles of the cell
(cells in telophase I are diploid)
• Afterwards, cytokinesis occurs,
ultimately forming two haploid daughter
cells
• In animal cells, a cleavage furrow forms;
in plant cells, a cell plate forms
What happens during prophase II
In prophase II, a spindle microtubules form
• In late prophase II, replicated chromosomes (each still composed of
two sister chromatids) move toward the metaphase plate
What happens during metaphase II?
• Replicated chromosomes are arranged at metaphase plate
• Due to crossing over in meiosis I, the two sister chromatids of each
replicated chromosome are no longer genetically identical
• Kinetochore microtubules attach to sister chromatids
What happens during anaphase II?
• In anaphase II, the sister chromatids separate
• The sister chromatids of each replicated chromosome now move as
two newly individual chromosomes toward opposite poles of cell
What happens during telophase II and cytokinesis?
In telophase II, the chromosomes
(which appear unreplicated) arrive at
opposite poles
• Nuclei form, and the chromosomes
begin decondensing
• Each cell then undergoes cytokinesis
• At the end of meiosis, there are four
daughter cells, each with a haploid set
of unreplicated chromosomes
• Each daughter cell is genetically distinct
from the others and from the parent cell
What three mechanisms contribute to genetic variation?
Independent assortment of chromosomes
– Crossing over
– Random fertilization
What is independent assortment of chromosomes and what are the number of combinations possible?
Homologous pairs of replicated chromosomes (as bivalents) orient
randomly at metaphase I of meiosis
• Each bivalent sorts maternal and paternal homologs (as replicated
chromosomes) into daughter cells independently of each other
The number of combinations possible when replicated
chromosomes assort independently into gametes is 2n, where n is
the haploid number
• For humans (n = 23), there are ~ 8.4 million (223) possible haploid
combinations of unreplicated chromosomes that gametes can
receive
How does crossing over contribute to genetic variation?
Crossing over produces recombinant chromosomes, which
combine genes inherited from each parent
• Crossing over begins very
early in prophase I, as
homologous replicated
chromosomes pair up
• In crossing over, homologous
portions of two non-sister
chromatids trade places
• Crossing over contributes to
genetic variation by
combining DNA from two
parents into a single chromosome
How does random fertilization contribute to genetic variation and what are the number of possible combinations?
Random fertilization adds to genetic
variation because any sperm can fuse
with any ovum (unfertilized egg)
• The fusion of two gametes (each
sperm or egg cell has one of ~ 8.4
million possible haploid chromosome
combinations from independent
assortment)
– 223 = ~ 8.4 million
• Zygote can have one of ~ 70 trillion
possible diploid combinations
– 8.4 million x 8.4 million - ~70 trillion
