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Applied Science Unit 11 - Learning Aim A
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Cell Division. Unit 11 Learning Aim B Whole Assignment. DISTINCTION graded. References included.
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Cell division (in eukaryotic cells)Cell theory states that all cells are created by already existing cells. For a cell to produce another cell, it
must undergo the cell cycle, during which a cell replicates itself. Body cells undergo the cell cycle occurs
when an organism receives growth factors or old/damaged cells need to be replaced to repair tissue.
Sex cells undergo the cell cycle when two gametes fuse via fertilization and produce more gametes for
sexual reproduction. The series of steps within the cell cycle are as follows:
https://www.inspiritvr.com/general-bio/cell-biology/the-cell-cycle-study-guide
The two main stages of cell replication are DNA replication and cell division. To understand these stages,
you must know how DNA is stored within the cell when the cell is not undergoing the cell cycle:
A genome is the complete set of genetic information of an organism. Most of this information is stored
within the nucleoplasm of the nucleus as structures called chromatin; chromatin consists of DNA
molecules wrapped around histone proteins. The purpose of the chromatin structure is to compress the
DNA, allowing the DNA to fit within the nucleus. It is a long strand which forms a shapeless network.
,https://www.alamy.com/nucleus-membrane-bound-organelle-that-contains-the-cells-chromosomes-
nucleoplasm-nucleolus-nuclear-membrane-chromatin-nucleus-and-nucleolus-image438682898.html
The structure and location of the DNA changes as a cell undergoes the cell cycle. There are checkpoints
(also known as restriction points) which are control mechanisms that monitor the cell cycle's progress.
The two main checkpoints occur after G1 and G2, but there are others such as those during early G1
(G0) and halfway through mitosis. They prevent uncontrolled cell division which forms tumors and
detect and repair damage to DNA. Certain regulatory chemicals trigger the checkpoints: p35 gene,
otherwise known as the tumor suppressor gene, and proteins which are cyclins and cyclin-dependent
kinases (CDKs). These have no effect without the other; cyclins are the regulatory sub-units, and the
kinases are the catalytic sub-units. When they are bound, they phosphorylate (add phosphate groups to)
target proteins which activate or deactivate them to trigger the next phase of the cell cycle. Cyclins are
synthesized in response to growth factors or various signaling molecules that attach to receptors in the
cell’s plasma membrane. Molecular events such as the checkpoints that occur in a specific sequence to
ensure the cycle cannot be reversed and the genome is only duplicated once per cycle.
G0 is considered a resting phase which occurs at early G1. Some cells such as neurones remain
within this phase most of the time or indefinitely while other cells such as epithelial cells lining
the gut do not go through this phase. Cells in this phase can undergo apoptosis (programmed
cell death), senescence (ageing) or differentiation.
During G1, the cell experiences cell growth and prepares for DNA replication. Genes are
transcribed to make RNA, biosynthesis such as protein synthesis occurs (and the synthesis of
enzymes required for DNA synthesis during DNA replication). Also, organelles such as
ribosomes, mitochondria and chloroplasts duplicate (mitochondria and chloroplasts are derived
from bacteria and contain DNA meaning they divide by binary fission and their DNA duplicates).
A checkpoint of Cyclin-CDK ensures the cell is prepared for the next phase by promoting the
expression of transcription factors which trigger the expression of S-cyclins and enzymes needed
for DNA replication.
During the S (synthesis) phase, DNA replication occurs. Before the cell can divide, each DNA
molecule in the parent cell must replicate. To do so, the chromatin divides and condenses into
linear structures called chromosomes. Each chromosome consists of one DNA molecule which
contains specific lengths of DNA, each of which codes for a regulatory length of RNA or a
, protein; these are called genes. A chromatid is an unduplicated chromosome, so DNA replication
is the process of a single chromatid chromosome becoming a sister chromatid chromosome. The
sister chromatids are joined together at the centromere. (Chromatids also have protective
endings at both ends called telomeres)
https://rsscience.com/what-is-mitosis-food-model-of-mitosis/
https://www.thecosmictreehouse.com/post/unknotting-the-double-helix-where-knot-theory-meets-
biology
If the cell is an animal cell, it is likely to have an organelle called the centrosome which is located within
the cytoplasm near the nucleus. Its structure is a pair of centrioles positioned 90 ° to each other within a
dense material called centrosome. Centrioles consist of 9 sets of microtubule triplets arranged in a
cylinder. So, if this organelle is present, it is also replicated during the S phase.
During G2, the cell grows again and prepares to divide. A checkpoint of Cyclin-CDK complexes
ensures the cell is prepared to divide by stimulating proteins that are involved in condensing
chromosomes and making the spindle (the cytoskeletal structure made of microtubules and
motor proteins which separates sister chromatids).
During the M phase (mitosis/meiosis), cell division occurs. The cell division that occurs depends
on the type of cell: Mitosis is the division of body cells and Meiosis is the division of sex cells.
During Mitosis, the cell divides once resulting in two daughter cells while during Meiosis the cell
divides twice resulting in 4 daughter cells. So, the purpose of mitosis is to replace old/damaged
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