Unit 1 notes for IB Biology HL students. I took my exams in July 2022 and got a 7 in Biology HL. The notes are a combination of information from different sources with a main focus on the IB Biology Textbook.
1.6 Cell Division
Objectives:
❏ Understanding:
❏ Mitosis is the division of the nucleus into two genetically identical daughter nuclei
❏ Chromosomes condense by supercoiling during mitosis
❏ Cytokinesis occurs after mitosis and is different in plant and animal cells
❏ Interphase is a very active phase of the cell cycle with many processes occurring in the nucleus
and cytoplasm
❏ Cyclins are involved in the control of the cell cycle
❏ Mutagens, oncogenes and metastasis are involved in the development of primary and secondary
tumours
❏ Applications:
❏ The correlation between smoking and incidence of cancers
❏ Skills:
❏ Identification of phases of mitosis in cells viewed with a microscope
❏ Determination of a mitotic index from a micrograph
❏ Nature of Science
❏ Serendipity and scientific discoveries: the discovery of cyclins was accidental
The Cell Cycle:
- Cell cycle = an ordered set of events which culminates in the division of a cell into two daughter cells
- Cell cycle is roughly divided into 3 main phases:
1. Interphase = the stage in the development of a cell between two successive divisions
a. Longest phase of the cell cycle
b. This phase of the cell cycle is a continuum of 3 distinct stages:
c. G1 - First intermediate gap stage in which the cell grows and prepares for DNA
replication
i. Some cells do not progress beyond G1 because they are never going to divide
ii. These enter a phase called G0 which may be temporary or permanent
iii. E.g. nerve cells, mature cardiac muscle
d. S - Synthesis stage in which DNA is replicated
e. G2 - Second intermediate gap stage in which the cell finishes growing and prepares for
cell division
Phase Location Activities
G1 Cytoplasm - The cell grows and functions normally undergoing everyday
processes
- Rapid protein synthesis takes place allowing the cell to grow
in size
- Proteins required for DNA synthesis (the next phase) are
made
- Mitochondria and chloroplasts (in case of plant cells) are
replicated. This also continues in S phase
S Nucleus - The amount of DNA doubles as DNA replication takes place
- The genetic material is duplicated but no chromosomes are
, formed yet
G2 Cytoplasm - Protein synthesis occurs to produce the proteins needed for
cell division (e.g. microtubule proteins that will make up a
mitotic spindle)
- The cell is actively preparing for cell division
2. Mitosis
a. Nuclear division whereby DNA (as condensed chromosomes) is separated into two
genetically identical daughter nuclei
b. Involves separation of sister chromatids into individual chromosomes which are then
distributed among the daughter nuclei
3. Cytokinesis
a. Once mitosis has been completed, the formation of two nuclei with identical sets of
chromosomes, the cell enters cytokinesis
b. The cytoplasm of a parental cell is divided between the two daughter cells
c. Cellular contents are segregated and the cell splits into two
The role of mitosis:
- Mitosis = the division of the nucleus into two genetically identical daughter nuclei
- Interphase
- Before mitosis can occur, all of the DNA in the nucleus must be replicated
- Each chromosome is converted from a single DNA molecule into two identical DNA molecules
(chromatids)
- During mitosis, one of the chromatids passes to each daughter nucleus
- Mitosis is involved whenever cells with genetically identical nuclei are required in eukaryotes (e.g. during
embryonic development, growth, tissue repair, asexual reproduction)
- Majority of cells spend most of their time in interphase (state where they are not ready for cell division)
- Mitosis is a continuous process but it is divided into four phases:
1. Prophase
, 2. Metaphase
3. Anaphase
4. Telophase
Interphase:
- Very active phase in the life of a cell when many metabolic reactions occur
- Some of these (e.g. reactions of cell respiration) also occur during cell division but DNA replication in the
nucleus and protein synthesis in the cytoplasm only happen during interphase
- During interphase, the number of mitochondria in the cytoplasm increases (due to growth and division of
mitochondria)
- In plant cells and algae, the number of chloroplasts increase in the same way
- Plants also synthesise cellulose and use vesicles to add it to their cell walls
- Key processes (DOCTOR)
1. DNA replication - DNA is copied during the S phase of interphase
2. Organelle duplication - Organelles must be duplicated for twin daughter cells
3. Cell growth - Cytoplasmic volume must increase prior to division
4. Transcription - Key proteins and enzymes must be synthesised
5. Obtain nutrients - Vital cellular material must be present before division
6. Respiration (cellular) - ATP production is needed to drive the division process
Supercoiling of chromosomes:
- Chromosomes condense by supercoiling during mitosis
- Supercoiling = Repeatedly coiling DNA molecule to make chromosome shorter and wider
- Condensation:
- Occurs during first stage of mitosis by a complex process of coiling
- Chromosomes become shorter and fatter
- DNA molecules in chromosomes are immensely long (more than 50,000 µm long, human nuclei
only 5 µm)
- It fits easily inside because it is so narrow (width of 0.002 µm)
- Essential to package chromosomes into much shorter structures
- Proteins called histones help with supercoiling, enzymes are also involved
- Histones = A simple protein bound to DNA, involved in the coiling of chromosomes
- Chromatin vs Chromosome
- Chromatin:
- DNA is usually loosely packed within the nucleus as unravelled chromatin
- In this unravelled form, the DNA is accessible to transcriptional machinery and so
genetic information can be translated
- DNA is organised as chromatin in all non-dividing cells and throughout the process of
interphase
- Chromosome:
- DNA is temporarily packaged into a tightly wound and condensed chromosome prior to
division (via supercoiling)
- In this condensed form, the DNA is able to be easily segregated however is inaccessible
to transcriptional machinery
- DNA is organised as chromosomes during the process of mitosis (condense in prophase,
decondense in telophase)
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