D2.1.1—How are new cells generated in living organisms?

In all living organisms, a parent cell often referred to as a mother cell divides to produce two daughter cells via cell division through mitosis and cytokinesis;

D2.1.2—How do the new cells split the cytoplasm between them?

In animal cells a ring of contractile actin and myosin proteins pinches a cell membrane together to split the cytoplasm;

in plant cells, vesicles assemble sections of membrane and cell wall to achieve splitting;

D2.1.3—Equal and Unequal Cytokinesis

The division of cytoplasm is usually even. both daughter cells must receive at least one mitochondrion and any other organelle that can only be made by dividing a pre-existing structure;

• in egg formation in humans (and other animals);called oogenesis it is uneven, with one cell getting almost all the cytoplasm;

  • oogenesis has two divisions (egg cells are produced through meiosis)

  • first division produces one large cell with nearly all the cytoplasm, and one small polar body

  • the large cell further divides—produces one large cell and one polar body

  • the large cell is the only one that develops into a mature oocyte

• when yeast buds (produces asexually) by mitosis, a small outgrowth of the mother cell is formed. Yeast does not double in size before dividing

  • outgrowth receives one nucleus, and a small share of cytoplasm

  • a dividing wall is constructed between the two cells

  • the small cell splits away

D2.1.4—Roles of mitosis and meiosis in eukaryotes

Nuclear division is needed before cell division to avoid production of anucleate cells (cells without a nucleus).

• Mitosis ,

  • maintains the chromosome number and genome of cells

  • occurs in somatic cells

  • daughter cells have the same number of chromosomes (diploid) as the parent cell

  • allows successful genome to be inherited without changes by the offspring (in asexual reproduction)

• Meiosis,

  • halves the chromosome number and generates genetic diversity

  • occurs in gametes

  • daughter cells are haploid, essential for fertilisation

  • allows evolution by natural selection as it generates variation

D2.1.5—DNA replication as a prerequisite for mitosis and meiosis

• a cell preparing for nuclear division replicates all its DNA

  • to ensure all daughters receive a full complement of genes

  • to be able to perform any function required

• before replication DNA is in the form of a long, single molecule called a chromosome

• after replication DNA is a pair of identical molecules, held together by loops of cohesin, also called chromosomes

• sister chromatids are held together by cohesin loops

D2.1.6— Condensation and movement of chromosomes as shared features of mitosis and meiosis

• to separate and move DNA molecules without knots/tangles/breaks they must be packaged into shorter structures

• chromosomes are condensed by being shortened

  • initial shortening: double helix wrapped around histone proteins to form nucleosomes, linking nucleosomes together

  • additional shortenings occur (currently very little research exists about them)

• chromosomes are moved using microtubules (hollow cylinder of tubulin proteins)

  • which act as a cytoskeleton during interphase

  • microtubules are assembled, reach equator of the cell, and form a spindle-shaped array

  • kinetochores are assembled on the centromere of each chromatid,

    • act as a motor for microtubules by removing tubulin subunits from ends of microtubules, putting them under pressure

  • microtubules move by getting shortened or lengthened

  • microtubules link up with kinetochores or other microtubules

D2.1.7—Phases of mitosis

• interphase occurs before mitosis, prepares the cell to undergo nuclear division

  • chromosomes dispersed through nucleus, not individually discernible

  • DNA replicated, each chromosome is to elongated chromatids with identical DNA

• Prophase - Phase 1

  • Supercoiling (condensation) of chromosomes,

  • breakup of nuclear membrane

  • growth of microtubules at the poles from MTOCs to form a spindle-shaped array

• Metaphase - Phase 2

  • attachment of microtubules to chromosomes, put under tension to test the attachment

  • Chromosomes line up at the equator

• Anaphase - Phase 3

  • cohesin loops cut, sister chromatids become separate chromosomes

  • centromeres divide

  • kinetochores shorten microtubules, pulled to opposite poles

• Telophase - Phase 4

  • reformation of nuclear membranes around chromosomes at each pole

  • chromosomes decondense

  • two daughter cells produced through cytokinesis

D2.1.8—Identification of mitotic phases

PROPHASE: Supercoiling (condensation) of chromosomes; breakup of nuclear membrane; growth of microtubules; attachment of microtubules to chromosomes;

METAPHASE: Chromosomes line up at the equator

ANAPHASE: division of centromeres; sister chromatids move to opposite poles;

TELOPHASE: reformation of nuclear membranes; around chromosomes at each pole;

D2.1.9—Meiosis as a reduction division

Diploid means two sets of each chromosome; 2n; they are in pairs; haploid means one set of chromosomes; meiosis involves two divisions of a diploid cell; to produce four haploid nuclei; in meiosis chromosome pairs (a pair of the same chromosomes) separate in the first division and sister chromatids are pulled apart in the second;

STAGES OF MEIOSIS

prophase I; chromosomes condense, (homologous) chromosomes pair up, crossing over happens as chiasmata form

metaphase I; movement of pairs of chromosomes known as bivalents to the equator in random assortment of chromosomes along equator;

anaphase I; movement of half of the chromosomes to each pole

telophase I, chromosomes uncoil; reduction of chromosome number to haploid;

metaphase II; (double-stranded) chromosomes line up on the equator and are attached to both poles at the centromere;

anaphase II; sister chromatids move to opposite poles in

telophase II; decondensation and uncoiling in reformation of nuclear membranes; cytokinesis then occurs; tetrad of haploid daughter cells formed.

D2.1.10—Down syndrome and non-disjunction

• non-disjunction is when chromosomes and or chromatids do not separate correctly; and therefore go to the same pole

  • non-separation of (homologous) double-stranded chromosomes during anaphase I; also known as bivalents (due to incorrect spindle attachment) ALSO due to non-separation of chromatids during anaphase II; due to centromeres not dividing; occurs during gamete formation; less common in sperm than egg formation;

• Down syndrome due to extra chromosome 21; called trisomy 21;

  • sperm or egg receives two chromosomes of same type;

  • offspring has three chromosomes of same type;

D2.1.11—Why is meiosis a source of genetic diversity? When do the events responsible happen?

• due to crossing over; during prophase I; leading to new combinations of alleles/genes on chromosomes;

• random orientation of chromosomes during metaphase I; leading to different chromosomes (maternal or paternal) being selected for each gamete; almost infinite variety created;

HL ONLY - D2.1.12—What is cell proliferation? How is it used for growth, cell replacement and tissue repair?

proliferation (making more cells) is used for growth within plant meristems; the regions of growth at the end of shoot tips and root tips; and early-stage animal embryos; in skin, cell proliferation is used during routine cell replacement; and during wound healing;

HL ONLY - D2.1.13—What is the cell cycle? What are the phases?

Cell cycle is the period between one cell division and the next; Interphase is what happens between mitosis; Composed of G1, G2 and S; G1: the cell grows; duplicates organelles e.g. mitochondria and extra cytoplasm including enzymes; synthesises proteins; S: The stage during which DNA is replicated; G2: more growth

HL ONLY - D2.1.14— How does the cell grow during interphase?

interphase is a metabolically active period; there is biosynthesis of cell components including proteins and DNA; Numbers of mitochondria and chloroplasts increase; through growth and division of these organelles;

HL ONLY - D2.1.15—How is the cell cycle controlled?

Cyclins are proteins that control the movement through different phases of the cell cycle; e.g. from G1 to S; the concentration of different cyclins increases and decreases during the cell cycle; and a threshold level of a specific cyclin is required to pass each checkpoint in the cycle.; if this doesn't happen, the division will stop;

HL ONLY - D2.1.16—How can mutations in genes that control the cell cycle caused cancer?

mutations in proto-oncogenes; convert them to oncogenes; which lead to tumour formation; and mutations in tumour suppressor genes; which normally stop tumour formation; result in uncontrolled cell division; known as cancer;

HL ONLY - D2.1.17 — How are tumour cells different than normal ones? How do they spread? Which tumours are not usually harmful? Which are?

• benign tumours are not normally harmful as they do not spread, invading other tissues;

• malignant tumours however can spread and invade other tissues;

• a primary tumour is the site of the first cancer;

• a secondary tumour is one that has moved to a different site;

HL ONLY - D2.1.17 — What is a mitotic index?

the mitotic index is the number of cells in mitosis (prophase, metaphase, anaphase, telophase and cytokinesis); divided by the total number of cells; gives an indication of how many cells out of the total are in a state of cell division; high mitotic index in root tips; and other regions of cell division; or tumours;