Biology: Cellular Reproduction Cheat Sheet

• Humans have 46 chromo­somes that are in 23 pairs in a cells nucleus
• Autosomes are the 22 pairs of chromo­somes that control traits that do not relate to gender
• Sex chromo­somes are the 1 pair that contain the genes that do control gender
• Body cells (somatic cells) have 46 (2n) chromo­somes in pairs called diploid
• Sex cells (gametes) have 23 (n) chromo­somes not in pairs called haploid

• External signals (hormones) can stimulate a cell to divide
• Genes stimulate the cell cycle:
• Proto-­onc­ogenes stimulate cell cycle
• Tumour­-su­ppr­essor genes inhibit the cell cycle

There are many variations of each gene, called alleles

Gene combin­ations we inherit from our parents depend on how their genes were sorted during meiosis process

Each sperm or egg that are made will always be different

Crossing over happens between two non-sister chromatids of homologous chromo­somes. They break at the same point, exchanging sections of chromo­somes, resulting in hybrid chromo­somes.

In metaphase I, how homologous chromo­somes pair up is random. Different arrang­ements of which homologous chromosome goes to which pole lead to gametes with various combin­ations of parental chromo­somes. This randomness is called indepe­ndent assort­ment.

Sexual reprod­uction signif­icantly influences species evolution by quickly creating new genetic combin­ations, leading to genetic diversity. Three methods contribute to this diversity:

1. Indepe­ndent assortment
2. Crossing over
3. Random fertil­iza­tion, where any of 8 million sperm can fertilize the egg, further enhances genetic variation.

Prophase
• Chromosome are visible and condensed.
• The nuclear membrane and nucleolus breaks down
• The centrioles migrate to opposite poles of the cell and spindle fibres (micro­tub­ules) form

Metaphase (Middle)
• Chromo­somes line up in the middle of the cell attached to the microt­ubules
• The centro­meres are aligned along the equatorial plane (equator) that divides the cell in half

Anaphase (away)
• Sister chromatids separate to opposite poles
• The spindle fibres start to disint­egrate near the centriole, leading the shortening spindles to tug the chromo­somes toward the cell’s poles.

Telophase (“two” – two cells)
• A new nuclear membrane surrounds the set of chromo­somes at each pole
• Chromo­somes begin to uncond­ensed
• Nucleolus reforms
• There are now two daughter nuclei - not daughter cells

Cytoki­nesis
• The cytoplasm divides equally
• There are now two daughter cells
• In animal cells, actin filaments contract and pinch the cell in two – this forms a cleavage furrow
• ATP is required for the contra­ction – two new identical, somatic, diploid daughter cells are produced

Nondis­jun­ction is the failure of chromo­somes to separate correctly during either meiosis I or meiosis II

This leads to an abnormal chromosome number

When a gamete has an abnormal number of chromo­somes, it's called aneupl­oidy, which means there's an abnormal chromosome count.
If fertil­ization involves gametes with abnormal chromosome numbers, normal develo­pment typically isn't possible, leading to early death.
However, if there's an extra chromosome 21, or less commonly, an extra chromosome 22, develo­pment can proceed.
A child with an extra chromosome 21, known as trisomy 21 or Down syndrome, experi­ences delayed develo­pment and mental impair­ment.

• Contri­butes to genetic variety
• Makes only germ cells – gametes
• Is a reduction division 46 -> 23 chromo­somes

• Meiosis involves 2 divisions
• Meiosis I – separates the homologous pairs
• Meiosis II – separates the sister chromatids

Prophase I
• Same as mitosis
• The pairin­g/i­nte­rtw­ining of homologous chromo­somes is Synapsis

Metaphase I (middle)
• The homologous pairs with their attached sister chromatids that line up.

Anaphase I (Away)
• The spindle fibers start to break down near the centri­oles, causing the shrinking spindles to pull the homologous chromo­somes towards the cell's poles, while the sister chromatids remain connected.

Telophase I
• Homologous chromo­somes are found at both ends of the cell, and the spindle fibers break apart. New nuclear membranes form around each set of chromo­somes at both ends of the cell, the chromo­somes start to relax, and the nucleolus reappears.

Cytoki­nesis
• The cytoplasm is evenly split, ensuring each new cell gets its share of organelles and fluid. Two haploid cells are made, but since each homologous chromosome stays connected to its sister chromatid, each cell has 46 chromo­somes. So, another cell division happens to give each gamete just 23 chromo­somes.

Prophase II
Before the next prophase, DNA doesn't replicate. Cells from meiosis I proceed directly to meiosis II, skipping a second S phase. DNA condenses, the nuclear membrane and nucleolus break down, centrioles move, and spindle fibers form.

Metaphase II (middle)
Spindle fibres connect to the kineto­chore, and the chromo­somes line up at the equatorial plane, but now in meiosis II, the chromo­somes with their sister chromatid all line up.

Anaphase II (Away)
Spindle fibres begin to break down at the centriole end, which causes the sister chromatids to separate toward each end of the cell.

Telophase II
At each end of the cell, sister chromatids are present, and the spindle fibers break apart. A fresh nuclear membrane forms around each set of 23 chromo­somes on both ends of the cell, the chromo­somes start to relax, and the nucleolus reappears.

Cytoki­nesis:
In males, meiosis II results in four sperm cells, each containing 23 chromo­somes. In females, during each cell division, one cell receives more cytoplasm, becoming the egg cell, while the other becomes a polar body that dissolves. However, females don't finish the final meiotic division unless fertil­ization happens. Eggs remain paused in metaphase II until ovulation and won't complete the last division if not fertil­ized.