Week10a Flashcards
Cell division
-reproduction of cells
• The continuity of life is based upon cell division
Unicellular organisms:
Multicellular organisms
➢ Unicellular organisms: Reproduction by cell division (e.g.
binary fission)
➢ Multicellular organisms depend on cell division for:
– Growth
– Development from a fertilized cell
– Repair of damaged tissues
Mitosis:
Meiosis:
– Mitosis: production of somatic cells (diploid cells)
– Meiosis: production of gametes (haploid cells)
Difference between
Mitosis:
and
Meiosis:
• Mitosis: conserves the chromosome number of the cells
=> production of 2 genetically identical cells that are also
genetically identical to the parental cell
• Meiosis: reduces the chromosome number in half
=> production of gametes in the gonads
=> Fertilization: A male and female gamete (haploid cells) fuse
producing a zygote with a complete set of chromosomes (diploid)
Cell cycle
define?
Stages?
• The functional process that a cell goes through until it is
divided in 2 identical daughter cells
• Phases (stages) of the cell cycle:
- G1 (gap 1): preparation of the cell for DNA replication
- S phase (synthesis): DNA replication
- G2 (gap 2): preparation for cell division
- M phase (mitotic phase): cell division (mitosis)
Two phases of the cell cycle
– Interphase: G1, S and G2 phases
– Mitotic phase: Mitosis and Cytokinesis
Mitosis includes what?
Prophase, Prometaphase, Metaphase, Anaphase, Telophase
G0 phase:
found on top of G1 in pic btw
- Resting phase: non-dividing cells are resting at this
phase
• Differentiated cells enter from G0 to G1 after the action of growth factors
• Cells exit G1 and enter G0
(G1 → G0) in order to
differentiate
Cell cycle control:
signal types?
Εxtracellular signals (e.g. presence of growth factors)
- Intracellular signals (e.g. cell size)
Cell types according to their cell division potential
- Post-mitotic cells:
- Cells that divide upon appropriate stimulation (signal):
- Cells with high mitotic activity:
Post-mitotic cells:
what is it?
example?
: terminally differentiated cells which have lost their ability to replicate => permanently arrested at G0 phase
- Example: neural cells, cardiac muscle cells, red blood cells
Cells that divide upon appropriate stimulation (signal):
examples?
- Example: most of the cells in our body only divide upon
stimulation by growth factors or other signal - e.g. lymphocytes upon antigenic presentation
Cells with high mitotic activity:
e.g. germ cells, stem
cells, epithelial cells
What is Interphase
3 facts?
Stages?
- The period between cell divisions
- The larger phase of the cell cycle
- The cell prepares for cell division
• The cell decides whether or not it will proceed with cell
division
• Includes the 3 first phases of the cell cycle:
- G1, S, G2 phases
What do cells have to do Before cell division (during interphase):
- Cell components have to replicate (organelles,
membranes, proteins) - Chromosomes need to replicate
➢ Genetic material needs to replicate in order for daughter
cells to have the same genome as the parental cell
➢ This ensures their survival
the 3 sub- phases of INTERPHASE:
• G1 phase: preparation for DNA replication
- Protein synthesis, organelle production
• S phase: DNA synthesis (replication)
• G2 phase: preparation for cell division (mitosis)
- Protein synthesis, organelle production
Eukaryotic chromosomes:
– Consist of chromatin, a complex of DNA and proteins
(histones)
– Each chromosome carries a few hundred to a few
thousand genes
Somatic cells:
Gametes (reproductive cells):
Somatic cells: diploid cells (2n, n =23 chromosomes)
- have 2 sets of 23 chromosomes = 46 total => 23 chromosome pairs
- Each homologous chromosome pair has 1 paternal
and 1 maternal chromosome
Gametes (reproductive cells): haploid cells (n, n =23
chromosomes)
– have one set of chromosomes = 23 total
– Have only 1 chromosome (either paternal or maternal)
from each homologous chromosome pair
Somatic cells:
Somatic cells: diploid cells (2n, n =23 chromosomes)
- have 2 sets of 23 chromosomes = 46 total => 23 chromosome pairs
- Each homologous chromosome pair has 1 paternal
and 1 maternal chromosome
Gametes (reproductive cells):
Gametes (reproductive cells): haploid cells (n, n =23
chromosomes)
– have one set of chromosomes = 23 total
– Have only 1 chromosome (either paternal or maternal)
from each homologous chromosome pair
Differnces in
-Interphase
and
-Mitosis
• Interphase: chromosomes are not condensed
– G1 phase: each chromosome consists of one
chromatid (not replicated yet)
– S phase: DNA is replicated
– G2 phase: Each duplicated chromosome has 2 sister
chromatids
• Mitosis (cell division):
- the chromosomes condense => can be seen with a light microscope
- sister chromatids separate => each future daughter
cell receives one chromatid
(Mitosis)
1.Prophase
- The chromatin fibers condense into distinct chromosomes
- The nucleoli disappear, nuclear membrane degradation begins
• The mitotic spindle (composed of centrosomes and
microtubules) begins to form
(Mitosis)
2.Prometaphase
• The nuclear envelope fragments
• The microtubules extending from each centrosome can
invade the nuclear area and bind to the chromosomes
• The chromosomes become more condensed
(Mitosis)
- Metaphase
• The centrosomes are now at opposite poles
• The chromosomes are aligned on the metaphase plate
imaginary plane equidistant between the spindle’s two poles
(Mitosis)
- Anaphase
- The sister chromatids of each chromosome move towards opposite poles
- By the end of anaphase the two poles of the cell have equivalent collections of chromosomes (chromatids)
(Mitosis)
- Telophase
- Nuclear envelops reform
- Nucleoli reappear
- Two daughter nuclei form in the cell
- Chromosomes decondense
• Mitosis (the division of one nucleus into two genetically identical
nuclei) is now complete
- Cytokinesis
after mitosis
• Cytokinesis: the cells are completely separated.
• The division of the cytoplasm usually begins by late telophase so the
two daughter cells appear shortly after the end of mitosis.
• In animal cells, cytokinesis occurs by a process known as cleavage,
forming a cleavage furrow.
(Prophase)
- Chromosomal condensation:
- Chromosomal condensation:
- Interphase: the chromosomes are decondensed (loose form
of chromatin)
=> to help replication and transcription - Μitosis: chromosomal condensation => begins at
prophase - Prophase: each chromosome consists of 2 sister
chromatids (it is duplicated)
- Sister chromatids = DNA copies (replicated during S
phase)
- sister chromatids are joined by centromeres
- Centromeres: consist of repetitive DNA sequences
(Prophase)
- Centrosomes move towards the opposite poles of the cell
- Centrosomes move towards the opposite poles of the cell
- Usually found next to the nucleus (cell center)
- Centrosome replication: during S phase
2 new centrosomes during mitosis
After mitosis: 1 centrosome per cell
(Prophase)
- Mitotic spindle formation:
- Mitotic spindle formation:
- Mitotic spindle begins to form by the polymerisation of
microtubules - Microtubule polymerisation starts from the centrosome
- Κinetochores: protein structures found at the centromere
of each chromosome - 1 kinetochore/chromatid
- mitotic spindle microtubules attach to the kinetochores
of the chromatids
=> move chromosomes towards the metaphase plate - Attachment point for motor proteins
(Prophase)
3 types of mitotic spindle microtubules :
(a) Αstral microtubules:
- radial (star-like) structure around the centrosome
- Function: positioning of the spindle in the cell
(b) Kinetochore (chromosomal) microtubules:
- join the centrosome with the kinetochores (centromeres) of the
chromosomes - Function: chromosomal movement
(c) Polar microtubules:
- start from the centrosome but do not attach to the chromosomes
- interact with other polar microtubules projecting from the other pole
- Function: maintain the integrity of the spindle
The mitotic spindle:
- an apparatus of microtubules that controls chromosome
movement during mitosis - arises from the centrosomes or other MTOC
- includes spindle microtubules (kinetochore and polar)
and astral microtubules
• Animal cells: MTOC= centrosome made by two
centrioles
• Plant cells: have different type of MTOC (lack centrioles)
(Prophase)
- Nuclear envelope and organelle degradation:
- Nuclear envelope and organelle degradation:
- Nuclear lamin phosphorylation (nuclear envelope
components) - Lamins: intermediate filaments present in the nuclear
lamina
Nuclear envelope degradation and packaging into vesicles
- Endoplasmic reticulum and Golgi apparatus are also
degraded and packaged into vesicles
=> they will later be separated into the daughter cells
How do prokaryotes reproduce?
• Prokaryotes (bacteria) do not have an organized
reproductive cell cycle => do not reproduce by mitosis
• Prokaryotes reproduce by another type of cell division
called binary fission
• Binary fission is a very simple cell division process
=> much faster than eukaryotic cell cycle
– bacterial replication time is 1-3 h
In binary fission:
- The bacterial chromosome replicates
– The two daughter chromosomes actively move apart
• Cell division of unicellular eukaryotes:
- Certain protists
- Certain protists exhibit types of cell division that seem intermediate between binary fission and mitosis carried out by most eukaryotic cells (e.g. multiple fission)
• Cell cycle stages:
➢ G1 phase: preparation for DNA replication
➢ S phase: DNA replication
➢ G2 phase: preparation for cell division (mitosis)
➢ M phase: cell division (mitosis)
• G0: Non-dividing cells
Mitosis:
: division of nucleus (Prophase, prometaphase,
metaphase, anaphase, telophase)
Cytokinesis:
division of cytoplasm
