Notesale: Turn your study into money

Document Preview

Extracts from the notes are below, to see the PDF you'll receive please use the links above

---

Cell Cycle


The cell cycle is the series of phases and steps that a cell goes through to replicate itself



It is important for replicating cells and controlling cell growth



It consists of interphase and mitosis



Interphase is divided into three phases: G1, S, and G2



Mitosis is divided into four phases: prophase, metaphase, anaphase, and telophase

Cell


A cell is the basic unit of all living things



A cell has three main components: cell membrane, nucleus, and cytoplasm



The cell membrane is a phospholipid bilayer that surrounds the cell




The nucleus contains genetic material in the form of chromatin, which is DNA wrapped around
histone proteins
The cytoplasm is the fluid inside the cell

Interphase


Interphase is the first part of the cell cycle



It is divided into three phases: G1, S, and G2



During G1 phase, the cell prepares to replicate by getting ready to duplicate its DNA



During S phase, the DNA is duplicated to form a new double-stranded DNA



During G2 phase, the cell prepares for mitosis by checking for any errors in the DNA and
making sure everything is ready for cell division

Mitosis


Mitosis is the second part of the cell cycle



It is divided into four phases: prophase, metaphase, anaphase, and telophase



During prophase, the chromatin condenses to form chromosomes and the nuclear membrane
begins to break down



During metaphase, the chromosomes align at the center of the cell



During anaphase, the sister chromatids separate and move to opposite ends of the cell



During telophase, the nuclear membrane reforms around the separated chromosomes and the
chromosomes begin to decondense

Replication of DNA


During the cell cycle, the DNA is duplicated to form a new double-stranded DNA



This process is called replication



It occurs during the S phase of interphase

Cell duplication and mitosis



In order to pass chromosomes down, cells must duplicate and have a total of 46 chromosomes
(diploid)




This duplication is called mitosis, and it starts with interphase
During interphase, the cell increases the number of organelles and prepares for DNA
replication

Preparing for DNA replication



Cell synthesizes proteins and enzymes necessary for DNA replication
Most cells exist in the G1 phase of the cell cycle, but certain types of cells, such as labile or
proliferative cells, constantly replicate

Types of cells



Labile or proliferative cells constantly replicate, such as skin cells and blood cells
Skin cells are constantly shed and replaced, and blood cells are constantly produced by
hematopoietic stem cells

Hematopoietic Stem Cells


Located in the red bone marrow



Two basic types: labile proliferative cells and stable cells



Labile proliferative cells constantly go through the cell cycle



Stable cells replicate only when there is a strong enough stimulus
Examples of Stable Cells



Hepatocytes in the liver: can regrow themselves when a portion of the liver is removed



Epithelial cells in the kidney tubules: can replicate when stimulated



Alveolar cells in the lungs: do not replicate again once they have gone through the cell cycle



Neurons, skeletal muscle cells, and cardiac muscle cells: do not undergo mitosis
G1 Phase of the Cell Cycle



Cells make more organelles and synthesize proteins and enzymes



Cells also prevent or repair damage, such as thymidine dimers, before replicating DNA
Cell Cycle and DNA Replication



Cell cycle consists of four phases: Gap 1 (G1), Synthesis (S), Gap 2 (G2), and Mitosis/Cytokinesis



In G1 phase, cells prepare for DNA replication by making more organelles, proteins, and
enzymes



The purpose of making more organelles is to have enough for two cells after DNA replication



S phase stands for synthesis, the phase where DNA replication occurs




During S phase, the DNA is opened up to form a replication bubble and a new strand is
synthesized
DNA replication is maintained by specific types of enzymes called DNA polymerases



Errors in DNA replication are rare, but tumor suppressor genes and DNA repair genes help
ensure accurate replication




S phase takes approximately six hours in most cells
Before entering S phase, there is a G1 to S phase checkpoint to ensure that the DNA is in good
condition
Replication Process



During DNA replication, the DNA is unzipped and two new strands are synthesized



The new strands are complementary to the original strands, following the base pairing rules (AT, C-G)



The process of DNA replication is semi-conservative, meaning that each new DNA molecule
contains one original strand and one newly synthesized strand



The enzyme DNA polymerase is responsible for adding new nucleotides to the growing strands



DNA polymerase can only add nucleotides in one direction, 5' to 3'



To replicate the leading strand (the strand that runs in the same direction as the replication
fork), DNA polymerase simply adds nucleotides in a continuous manner



To replicate the lagging strand (the strand that runs in the opposite direction of the replication
fork), DNA polymerase has to work in short, discontinuous stretches called Okazaki fragments



RNA primers are used to start the synthesis of both leading and lagging strands



After DNA replication is complete, the RNA primers are removed and replaced with DNA
nucleotides by another enzyme called DNA polymerase I



The remaining gaps between the Okazaki fragments are filled in and the DNA is sealed by an
enzyme called DNA ligase
Regulation of DNA Replication



The regulation of DNA replication is crucial to ensure that cells divide only when necessary



The G1 to S phase checkpoint is a regulatory point in the cell cycle where the cell checks for
any damage or problems with the DNA before entering the S phase



If there are any problems, the cell will not enter the S phase and will either attempt to repair
the damage or undergo programmed cell death (apoptosis)



The regulation of DNA replication is tightly controlled by various proteins and signaling
pathways to ensure that cells divide only when necessary and in a controlled manner

Cell Cycle and Mitosis


Cell cycle consists of G1, S, and G2 phases, also known as Interphase



G1 phase: checks DNA for integrity, ensures enough proteins and organelles for replication



S phase: DNA replication occurs



G2 phase: cell grows in size to prepare for mitosis



G1/S checkpoint: a regulatory point before moving from G1 to S phase



Mitosis, or M phase, consists of Prophase, Prometaphase, Anaphase, and Telophase



Prophase: chromatin condenses into chromosomes for proper separation



Prometaphase: nuclear membrane breaks down, allowing chromosomes to move



Anaphase: sister chromatids separate and move to opposite ends of the cell



Telophase: nuclear membrane reforms around separated chromosomes



Cytokinesis: cytoplasm divides equally to form two separate cells

Regulation of Cell Cycle


Cell cycle is regulated by checkpoints, including G1/S checkpoint



Tumor suppressor genes and proto-oncogenes play a role in regulation



Regulation ensures proper DNA replication and cell division

Importance of Cell Cycle and Mitosis


Cell cycle and mitosis are essential for growth, development, and tissue repair



Proper regulation of cell cycle and mitosis prevents uncontrolled cell growth, such as cancer



Phases of Cell Division


In the S phase, DNA is replicated



The nuclear envelope will dissolve to allow for separation of chromosomes



The microtubule organization center (MTOC) will form and connect to the chromosomes to
help separate them

Phases of Mitosis


Prophase: chromatin is condensed, the nuclear envelope begins to dissolve, and the MTOC
forms



Metaphase: the nuclear envelope is dissolved, and the MTOC takes up residence in the
opposite ends of the cell

Components of the MTOC


The MTOC is formed by centrioles or centrosomes



From the MTOC, polar and astral microtubules form and connect to the chromosomes

Process of Dissolving the Nuclear Envelope



Special types of cyclin dependent kinases and enzymes phosphorylate different proteins of the
nuclear envelope
Phosphorylation activates proteases to break down the proteins of the nuclear envelope
Functions of Chromatin



Chromatin is the structure in which DNA is organized and stored in the nucleus



Chromatin condenses and becomes chromosomes during cell division to ensure proper
separation of genetic material

Significance of Chromatin



Proper condensation and separation of chromatin is crucial for the accurate transmission of
genetic information during cell division



Understanding chromatin structure and function is essential for understanding the processes of
gene expression and inheritance
Microtubules Organization Centers (MTOCs)



Cell has two poles, each with an MTOC



Microtubules are connected to chromosomes at the centromere




Polar microtubules connect to the kinetochore, a protein structure on the outside of the
centromere
Polar microtubules hook and separate sister chromatids
Chromosomes



Made up of chromatin, DNA, and histone proteins



Short arm and long arm, with the centromere in the middle



Ends of the chromosome are called telomeres



Two identical copies of a chromosome are called sister chromatids
Metaphase



Polar microtubules connect to the kinetochores of the chromosomes



Polar microtubules hook and separate sister chromatids

The Kinetochores and Chromosome Separation


Kinetochores are connected to the chromosomes and polar microtubules



During metaphase, the chromosomes align perfectly on the metaphase plate



Anaphase is the stage where the chromosomes separate and move away from one another



Cohesin, a protein connecting the sister chromatids, is split during anaphase



The chromatids are pulled towards opposite poles by motor proteins
Motor Proteins



Motor proteins, such as dynein and kinesin, help move the chromatids towards the opposite
ends of the poles



They can walk along the microtubules carrying structures with them towards a specific direction



Dynein and kinesin are two motor proteins that play a role in this process
Significance of the Process



The separation of chromosomes is important for the cell to divide and create new cells



The process is regulated by specific proteins and mechanisms
Implications of Understanding the Process



Understanding the process can lead to a better understanding of cell division and its regulation



It can also lead to a better understanding of diseases related to cell division, such as cancer

Cell Division: Cytokinesis


During cytokinesis, the cell divides to form two separate cells



A constriction ring made of actin and myosin proteins forms and contracts, creating a cleavage
furrow



The cytoplasm is equally distributed between the two cells through the process of cytokinesis

Nuclear Envelope and Chromosomes


As cytokinesis occurs, the nuclear envelope begins to reform



Chromosomes, which are present in the form of 46 chromatids (2n) before division, are equally
distributed between the two cells



After division, the chromatin (the loose form of chromosomes) becomes visible in the
telophase stage

Other Cell Components


An equal amount of ribosomes and mitochondria are present in both cells after division



The cytoplasm, which is the fluid inside the cell, is also equally distributed between the two
cells
Cell Membrane



The cell membrane remains intact throughout the process of cell division, enclosing the
cytoplasm and organelles in both cells
End Result



At the end of cytokinesis, two separate cells are formed, each with an equal amount of
organelles and cytoplasm
Number of chromosomes in a human cell




A human cell has 46 chromosomes, which means it is diploid (2n)
Even though the cells are not perfectly identical in size, they should have the exact same
amount of cytoplasm and organelles



---