Cell & Nuclear Division (DP IB Biology)

Equal cytokinesis is when the cytoplasm divides evenly between two daughter cells, resulting in cells of similar size.

Mitochondria are needed for cellular respiration, and chloroplasts are needed for photosynthesis in plant cells. A cell (or whole organism) could not function without these two processes.

Unequal cytokinesis occurs when the cytoplasm divides unevenly, resulting in daughter cells of different sizes.

Oogenesis in humans and budding in yeast are examples of unequal cytokinesis.

During oogenesis, a primary oocyte divides to form a secondary oocyte and a polar body, followed by another division to form an ovum and a second polar body.

Polar bodies degenerate and become part of the final ovum during oogenesis.

Two processes by which the nucleus of a eukaryotic cell can divide are mitosis and meiosis.

The nucleus must divide before cell division to avoid the production of anucleate cells, which are cells without a nucleus.

Mitosis gives rise to genetically identical cells and is used for growth, repair of damaged tissues, replacement of cells, and asexual reproduction.

Mitosis maintains the chromosome number by producing daughter cells that are diploid (2n) and genetically identical to the parent cell.

Meiosis gives rise to genetically different cells and is used to produce gametes (sex cells).

Meiosis produces haploid (n) daughter cells with half the chromosome number of the parent cell, leading to genetic variation.

DNA replication, during interphase, must occur to create two identical strands of DNA called chromatids. This must happen before mitosis and meiosis.

During anaphase one chromatid from each chromosome is pulled to opposite poles of the cell, eventually becoming individual chromosomes.

DNA condensation is necessary to fit the long DNA molecules into the small nuclei of cells.

Supercoiling, aided by histone proteins and enzymes, repeatedly coils the DNA molecule to condense it into chromosomes.

Microtubules are responsible for the movement of chromosomes by lengthening and shortening to facilitate their movement.

Microtubule motors are motor proteins that carry chromosomes along the microtubules to the equator of the cell in preparation for nuclear division.

Mitosis is the process of nuclear division by which two genetically identical daughter nuclei are produced, each having the same number of chromosomes as the parent cell.

There are four main stages of mitosis: prophase, metaphase, anaphase, and telophase.

During prophase chromosomes condense and become visible. Each chromosome consists of two identical sister chromatids joined at the centromere.

During prophase the two centrosomes move toward opposite poles of the nucleus, and spindle fibers begin to emerge from them.

During prophase, the nuclear envelope breaks down into small vesicles, and the nucleolus disappears.

During metaphase chromosomes line up at the equator of the spindle, also known as the metaphase plate.

During metaphase, spindle fibres attach to the centromeres of the chromosomes via kinetochores. Each sister chromatid is connected to a spindle fibre from opposite poles.

The key event of anaphase is when the sister chromatids separate at the centromere and are pulled to opposite poles by the shortening spindle fibres.

During telophase, chromosomes arrive at opposite poles and begin to de-condense.

Nuclear envelopes reform around each set of chromosomes, spindle fibres break down, and new nucleoli form within each nucleus.

The stages of the cells X, Y and Z are:

• X = anaphase

• Y = metaphase

• Z = anaphase

Meiosis is a type of cell division that produces genetically different cells and is used to produce gametes (sex cells).

Meiosis consists of two rounds of division: Meiosis I and Meiosis II.

Meiosis produces haploid gametes, reducing the chromosome number by half, which is essential for maintaining the species' chromosome number across generations during sexual reproduction.

During Meiosis I, homologous chromosomes are separated, resulting in two haploid nuclei, each still containing chromosomes with two chromatids.

A bivalent is a pair of homologous chromosomes that come together during prophase I of meiosis.

Meiosis I is called reduction division because it reduces the chromosome number by half, from diploid (2n) to haploid (n).

During Meiosis II, the chromatids of each chromosome are separated, resulting in four haploid nuclei, each containing chromosomes with a single chromatid.

False.

There is no chromosome replication between Meiosis I and Meiosis II.

Crossing over results in the exchange of genetic material between homologous chromosomes, creating recombinant chromosomes and increasing genetic variation.

The random orientation of bivalents during metaphase I leads to the independent assortment of chromosomes, which generates a variety of possible genetic combinations in gametes.

Genetic variation is important for natural selection and increases a species' ability to adapt and survive in changing environments.

Non-disjunction occurs when chromosomes fail to separate properly during meiosis, leading to gametes with an abnormal number of chromosomes.

Down Syndrome, also known as Trisomy 21, occurs when non-disjunction results in an extra copy of chromosome 21, leading to a total of 47 chromosomes in the affected individual, instead of 46.

The risk of non-disjunction increases with the age of the mother, older ova are more likely to undergo incorrect chromosome separation.