Meiosis: Cell Division and Genetic Variation

• Meiosis is a cell division process that produces gametes or sex cells.
• Gametes in organisms are sperm and egg cells.
• Meiosis results in daughter cells which are genetically distinct from parent cells and contain half the number of chromosomes.
• Somatic cells (body cells) are produced through mitosis, maintaining an identical chromosome number between parent and daughter cells.
• Somatic cells are diploid, while gametes are haploid.

Meiosis 1

• Each cell contains two copies of each chromosome (one from each parent).
• The nuclear membrane dissolves, and centrosomes split, with centrioles moving to opposite cell sides.
• Homologous chromosomes pair up to form a bivalent.
• A chiasma forms where homologous chromosomes overlap, leading to crossing over.
• Genetic material is exchanged during crossing over, forming recombinant chromosomes.

Metaphase 1

• Chromosomes align at the cell's equator.
• Each chromosome in a homologous pair attaches to spindle fibres via the centromere.
• Random arrangement of homologous chromosomes along the equator creates genetic variation.

Anaphase 1

• Spindle fibres contract, pulling one entire chromosome from each pair to opposite poles.

Telophase 1

• A nuclear membrane develops around the chromosomes at each pole.

Cytokinesis

• The cell divides into two haploid daughter cells containing unique chromosomes.

Meiosis 2

• Begins with two unique, haploid daughter cells from meiosis 1.
• The nuclear membranes of the daughter cells dissolve, and centrioles move to opposite ends.
• No crossing over occurs because cells are haploid, lacking homologous chromosome pairs.

Metaphase 2

• Chromosomes in both daughter cells align at the cell's equator.
• Spindle fibres attach to the centromeres.

Anaphase 2

• Spindle fibres contract, separating chromatids and pulling them to opposite poles.

Telophase 2

• Nuclear membranes form around the daughter cells.

Cytokinesis

• The cytoplasm divides, forming four unique, haploid daughter cells, which develop into gametes.

Importance of Reduction Division

• Reduction division ensures consistent chromosome numbers across species members.

Gametogenesis

• The process of forming gametes
• Two types include spermatogenesis and oogenesis

Spermatogenesis

• Sperm cells form through spermatogenesis.
• It starts with a diploid primary spermatocyte, which divides during meiosis 1 into two haploid secondary spermatocytes.
• Secondary spermatocytes divide again in meiosis 2, forming four haploid spermatids.
• Spermatids mature into genetically unique sperm cells.

Oogenesis

• Egg cells form through oogenesis.
• It starts with a single diploid primary oocyte, which divides into a secondary oocyte and a polar body during meiosis 1.
• The secondary oocyte divides during meiosis 2, forming an ovum and another polar body, while the first polar body divides into two more polar bodies.
• The final result is one haploid ovum and three polar bodies.

Random Arrangement

• During Metaphase 1 and 2, the chromosome alignment at the cell equator impacts the genetic information in each gamete.
• Random arrangement results in four genetically unique gametes.

Crossing Over

• Homologous chromosomes pair up to form a bivalent during Prophase 1.
• Genetic information is exchanged at the chiasma, creating recombinant chromosomes.

Non-disjunction

• Occurs when errors happen during meiosis
• Errors during meiosis lead to Non-disjunction

Meiosis 1

• Homologous chromosomes fail to separate during Anaphase 1.
• One daughter cell ends up with both chromosomes, and the other gets none.
• The resulting gametes are not haploid, leading to monosomic or trisomic zygotes.

Meiosis 2

• Chromatids fail to separate during Anaphase 2.
• One daughter cell produces abnormal gametes.
• One gamete contains one chromatid, and the other contains two chromatids.
• Gametes with an incorrect chromosome number are aneuploid.
• These gametes produce monosomic and trisomic zygotes.
• Non-disjunction leads to zygotes with extra or missing chromosomes.
• Monosomic zygotes have only one copy of a chromosome.
• Trisomic zygotes contain three copies of a chromosome.

Chromosome Abnormalities

• Monosomy results in a missing chromosome.
• Most monosomic zygotes spontaneously abort due to developmental issues.
• Turner Syndrome is the exception, occurring in females with a missing X chromosome, causing a webbed neck.
• Trisomy occurs when an individual has three sets of chromosomes instead of the usual two.
• Pregnancies with extra chromosomes 13, 18, or 21 may not spontaneously abort.

Trisomy 13

• Results in Patau Syndrome

Trisomy 18

• Causes Edwards Syndrome

Trisomy 21

• Results in Down Syndrome
• Mostly occurs during oogenesis
• Karyotypes are used to examine the number and structure of chromosomes in cells.

Conditions of Down Syndrome

• Include weaker immune systems and a greater risk of heart defects

Characteristics of Down Syndrome

• Include slanted eyes, protruding tongue, short stature, flat faces, learning difficulties, and delayed cognitive and emotional development.