LIFESCI 7A Lecture Notes - Lecture 9: G2 Phase, G1 Phase, Nuclear Membrane
WEEK 9
11.1: Cell Division
● Prokaryotic cells reproduce by binary fission
○ cell replicates its DNA, increases in size, and
divides into two daughter cells.
○ Process similar in bacteria, archaeons, and
mitochondria/chloroplasts
○ FtsZ gene encodes proteins that assemble and
form ring at the constriction site where the new cell
wall forms; fundamental role in p-cell division
■ Circular genome attached by proteins to cell
membrane
■ DNA replication initiated at origin of
replication
■ Cell elongates, DNA copies separate
■ Constriction forms in the middle of
cell, new membrane and cell wall
form, dividing cell into two daughters
w/ identical genetic info
● Eukaryotic cells reproduce by mitotic cell division
○ mitosis (divide nucleus) and cytokinesis
(divide cytoplasm)
○ Nuclear envelope breaks down, then other
mechanisms allow DNA to duplicate and cell
to split
○ Some unicellular eukaryotes replicate
similarly to prokaryotic cell division
■ Suggests eukaryotic mitosis evolved
from prokaryotic binary fission
● The cell cycle describes the life cycle of a eukaryotic
cell.
○ M phase The stage of the cell cycle
consisting of mitosis and cytokinesis, in
which the parent cell divides into two daughter cells
■ Lasts about an hour in mammalian cells
○ interphase The time between two successive M phases.
■ replication of the DNA in the nucleus so that each daughter cell receives a copy of the
genome
■ increase in cell size so that each daughter cell receives sufficient amounts of
cytoplasmic and membrane components to allow it to survive on its own
■ G1 phase The gap phase in which the size and protein content of the cell increase and
specific regulatory proteins are made and activated in preparation for S-phase DNA
synthesis.
● Many of the proteins (kinases) created promote DNA synthesis
■ S phase The phase of interphase in which the entire DNA content of the nucleus is
replicated.
■ G2 phase The gap phase in which the size and protein content of the cell increase in
preparation for M-phase mitosis and cytokinesis.
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○ Not all the cells in your body are actively dividing since not all tissues require the rapid
replenishing of cells
■ G0 phase The gap phase in which cells pause in the cell cycle between M phase and S
phase; may last for periods ranging from days to more than a year; have exited the cell
cycle
9.1 Principles of Cell Communication
● Cells communicate using chemical signals that bind to specific receptors.
○ signaling cell The source of the signaling molecule.
○ signaling molecule The carrier of information transmitted when the signaling molecule binds to
a receptor; also referred to as a ligand.
■ Vary immensely: peptides, lipids, gases
■ Carries info from one cell to another
○ receptor protein The molecule on the responding cell that binds to the signaling molecule.
■ If a specific hormone is released, only cells with that hormone’s specific receptor will
receive the signal
○ responding cell The cell that receives information from the signaling molecule.
○ Cell signalling with these four elements is universal among both prokaryotes and eukaryotes,
unicellular and multicellular
○ quorum sensing bacteria are able to determine whether they are at low or high population
density and then turn on specific genes across the entire community.
● Signaling involves receptor activation, signal transduction, response, and termination.
○ Signalling molecule binds to receptor of responding cell…..
■ receptor activation The “turning on” of a receptor, which often occurs when a signaling
molecule binds to a receptor on a responding cell.
● Receptors can bind to/activate proteins in responding cell
● Or are enzymes that change shape/activity when signal is received
● Or are channels that open/close in response
■ signal transduction receptor transmits information through the cytoplasm.
● one molecule activates the next molecule, which activates the next, and so on
● signal is often amplified at each step in the pathway; even a low signal
concentration can have big effect on cell
■ response A change in cellular behavior, such as activation of enzymes or genes,
following a signal.
■ termination the stopping of a signal.
● Response can be terminated at any point along pathway
● protects the cell from overreacting to existing signals and therefore helps the cell
to have an appropriate level of response
● Allows cell to receive new signals
12.1: DNA Replication
● During DNA replication, the parental strands separate and new partners are made.
○ Parent DNA serve as template strands for the synthesis of daughter strands
■ Daughter strands are complementary and antiparallel to parents
■ Parent DNA splits and forms replication fork
○ DNA replicates semi-conservatively, in which each strand of a parental DNA duplex serves as
a template for the synthesis of a new daughter strand.
■ So each new DNA strand has one strand from parent and one completely new strand
○ This method of DNA replication is UNIVERSAL!
● New DNA strands grow by the addition of nucleotides to the 3′ end.
○ DNA is not synthesized in the direction of the replication fork; it’s impossible!
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Document Summary
Cell replicates its dna, increases in size, and divides into two daughter cells. Process similar in bacteria, archaeons, and mitochondria/chloroplasts. Ftsz gene encodes proteins that assemble and form ring at the constriction site where the new cell wall forms; fundamental role in p-cell division. Circular genome attached by proteins to cell membrane. Dna replication initiated at origin of replication. Constriction forms in the middle of cell, new membrane and cell wall form, dividing cell into two daughters w/ identical genetic info. Eukaryotic cells reproduce by mitotic cell division. Mitosis (divide nucleus) and cytokinesis (divide cytoplasm) Nuclear envelope breaks down, then other mechanisms allow dna to duplicate and cell to split. Some unicellular eukaryotes replicate similarly to prokaryotic cell division. The cell cycle describes the life cycle of a eukaryotic from prokaryotic binary fission cell. M phase the stage of the cell cycle consisting of mitosis and cytokinesis, in which the parent cell divides into two daughter cells.
