D2.1 Cell and Nuclear Divsion

New cells are produced by cell division, ensuring continuity of genetic information and enabling growth, repair and reproduction.

Cytokinesis is the physical division of a parent cell's cytoplasm and plasma membrane into two separate daughter cells, occurring after nuclear division (mitosis). While mitosis separates sister chromatids into two nuclei, cytokinesis splits the rest of the cell, using a cleavage furrow in animals or a cell plate in plants.

Animal cells divide by formation of a cleavage furrow caused by contraction of a ring of microfilaments.
Plant cells form a cell plate from vesicles which develops into a new cell wall between daughter cells.

In animal cells, actin microfilaments contract to pull the membrane inward forming a cleavage furrow.
In plant cells, vesicles fuse at the cell centre forming a cell plate which grows outward and forms a new wall.

Mitochondria contain enzymes for aerobic respiration and cannot be formed de novo, so each daughter cell must inherit at least one.

In yeast budding, a small daughter cell forms receiving less cytoplasm.
In human oogenesis, one large ovum and small polar bodies form due to unequal cytoplasmic division.

Mitosis produces genetically identical diploid cells maintaining chromosome number.
Meiosis produces genetically different haploid cells reducing chromosome number

Anucleate cells form if cytokinesis occurs without nuclear division.
Such cells cannot transcribe DNA or synthesise proteins and therefore cannot survive long.

DNA replication occurs during the S phase of interphase before mitosis and meiosis.

DNA replication produces identical sister chromatids joined at the centromere.
Cohesin proteins hold the chromatids together until separation during anaphase.
Condensation of chromosomes allows efficient movement during cell division.

the failure of homologous chromosomes or sister chromatids to separate properly during cell division (meiosis or mitosis). This error results in daughter cells with an abnormal number of chromosomes—either too many or too few—

Spindle microtubules attach to kinetochores at the centromere and pull sister chromatids or homologous chromosomes toward opposite poles during division.

Prophase:
Metaphase:
Anaphase:
Telophase

Prophase: chromosomes condense, nuclear envelope breaks down, spindle forms.
Metaphase: chromosomes align at the equator attached to spindle microtubules.
Anaphase: sister chromatids separate and move to opposite poles.
Telophase: nuclear envelopes reform and chromosomes decondense.

Slide 24

SEE SLIDE 24

A pair of chromosomes with the same genes at the same loci, one inherited from each parent.

Diploid cells contain two sets of chromosomes (2n = 46 in humans).
Haploid cells contain one set of chromosomes (n = 23 in humans).

Meiosis I separates homologous chromosomes, reducing the chromosome number from diploid to haploid.

Mitosis involves one division producing two genetically identical diploid cells.
Meiosis involves two divisions producing four genetically different haploid cells.
Homologous chromosomes pair and crossing over occurs only in meiosis.

Meiosis I:
– homologous chromosomes pair and crossing over occurs
– bivalents align at the equator
– homologous chromosomes separate
– two haploid cells form

Meiosis II:
– chromosomes align at the equator
– sister chromatids separate
– four haploid genetically different cells form

Homologous chromosomes pair → crossing over occurs → bivalents align → homologous chromosomes separate → sister chromatids separate → four haploid cells form. SEE SLIDE 25

Nondisjunction is failure of homologous chromosomes or sister chromatids to separate during cell division, producing cells with abnormal chromosome numbers.

In meiosis I, failure of homologous chromosomes to separate produces four abnormal gametes (two n+1 and two n−1).
In meiosis II, failure of sister chromatids to separate produces two normal gametes and two abnormal gametes (one n+1 and one n−1).

Down syndrome is caused by nondisjunction resulting in trisomy of chromosome 21.
Symptoms include characteristic facial features, hearing loss, heart + vision disorders

Genetic variation is produced by crossing over between non-sister chromatids in prophase I and by random orientation of homologous chromosome pairs at metaphase I.

A bivalent is a pair of homologous chromosomes that have synapsed in prophase I.
Crossing over is exchange of DNA between non-sister chromatids producing new allele combinations.

Homologous chromosome pairs align randomly at the equator during metaphase I.
This random orientation results in different combinations of maternal and paternal chromosomes in gametes after separation.
The number of possible chromosome combinations due to independent assortment is 2ⁿ (where n is the haploid number).

The number of possible chromosome combinations in gametes due to independent assortment is 2ⁿ, where n is the haploid number.

In humans → 2²³ possible combinations.