Cell Division

The purposes of cell division

1. growth and development

2. tissue regeneration/repair

3. reproduction

asexual reproduction

one organism makes 2 (or more) copies of itself

accomplished through → **mitosis** and **binary fission**

sexual reproduction

two different individuals contribute genetic info to make new genetically unique organism(s)

accomplished through → **meiosis**

Why do cells need to divide eventually? Explain surface-to-volume ratio & genome-to-volume ratio explanation (also diffusion problems as it grows)

**surface to volume ratio:** as a cell grows in size its contents (volume) can become too much to hold inside it

SA = s^2\*6

vol = s^3

**genome to volume ratio:** as a cell grows and its cytoplasm and organelles increase, it becomes harder and harder for the nuclear instructions to control it

**diffusion rates** are affected by cells that keep growing in size

It is ideal for the time **diffusion** takes to be **as little as possible** so it doesn’t take too long for particles to **make their way through the cell**

origin of the name chromosomes

chromo_somes\~colored body

some guy stained some cells and the dye stuck to the DNA → colored bodies

Chromosome

packages of DNA that gets condensed when cell division is about to start

during **interphase** it is more loosely arranged in the nucleus

during **mitosis** chromosomes exist as **condensed chromatin**

chromatid

A ***chromatid*** (Greek khrōmat- 'color' + -id) is one half of a duplicated chromosome. Before replication, one chromosome is composed of one DNA molecule

sister chromatids

refers to the identical copies formed by the DNA replication of a chromosome, with both copies joined together by a common **centromere**. In other words, a sister chromatid may also be said to be **'one-half'** of the **duplicated chromosome**. A pair of sister chromatids is called a **dyad**.

chromatin

**Chromatin** is a complex of nucleic acids (e.g. DNA or RNA) and proteins (e.g. histones).

condensed chromatin

DNA+protein and looks like the “X“ shape which is technically a **duplicated condensed chromosome.**

centromere

it links the sister chromatids together

the region of a chromosome to which the microtubules of the spindle attach, via the **kinetochore**, during cell division

telomere

molecular protective caps of repetitive DNA at the ends of chromosomes

kinetochore

the intersection of microtubules (from the spindle fiber) and centromere

non-kinetochore

the part of the microtubule that does not intersect with the centromere

What is the Cell Cycle?

a means of regulating often how quickly a cell divides

has multiple phases that can most generally be divided into interphase and M-phase

why are checkpoints important in the cell cycle

checkpoints have to occur before one phase leads to the next

G1 Checkpoint

Checks for:

cell size

nutrients

growth factor

DNA damage

G2 Checkpoint

Checks for:

cell size

DNA replication

Spindle Assembly Checkpoint

Checks for:

chromosome attachment to spindle

Interphase

the __in-between__ divisions phase → a cell does its regular cellular activities and __prepares__ for division in the future

Tends to be the majority of the cell cycle

has 3 subphases:

G1, S, G2

G1 (growth phase 1)

increases in size + G1 Checkpoint

S Phase

DNA is synthesised

G2 (Growth Phase 2)

cells grows some more G2 checkpoint

G0 (resting state)

the cell is “resting“ and not participating in the cell cycle

it is a cellular state that is not in the cell cycle

Mitosis (M Phase)

cell division in which 1 parent cell divides once to make 2 genetically identical daughter cells

There are 6 phases of mitosis:

• prophase

• prometaphase

• metaphase

• anaphase

• telophase

• cytokinesis

What are somatic cells?

body cells

Prophase

the chromosomes condense

nuclear envelope breaks down

nucleolus disappears

spindle apparatus begins to form between the centriole pairs

Prometaphase

spindle fibers (microtubules) attach to the centromeres of the chromosomes to begin to move them where they need to be in the next phase of mitosis

Metaphase

chromosomes are moved to the equator (metaphase plate) by the spindle

Anaphase

__shortening__ of the microtubules pulls apart the **sister chromatids**__,__ moving them towards the poles

**non-kinetochore** microtubules __elongate__ relative to the shortening of the kinetochore ones

this __elongates__ the cell as well

Telophase

__opposite__ of prophase

chromosomes begin to unravel/decondense

nuclear envelopes reform

spindle apparatus breaks down

nucleoli reform

Cytokinesis

separation of __cytoplasms__ to complete the mitosis process

-Cytokinesis → how does it differ in animal vs. plant cells?

**animal cells: (cell membrane)**

a cleavage furrow exists at the pinching in point. Microfilaments pull in on the plasma membrane to accomplish this.

**plant cells: (cell wall)**

vesicles filled with cellulose align along the equator and fuse together to make a new cell wall

-Binary fission → which cells do this? Why don’t we use the phases (Prophase, Metaphase, etc.) for this?

how bacteria divides

no phases of mitosis because bacteria doesn’t have multiple chromosomes and no need to segregate the chromatids etc.

DNA replication happens like in eukaryotes but the splitting of the parent cell happens pretty quickly

tumor suppressor genes

suppress the likelihood of a tumor developing

oncogenes

correlated with the incidence of cancer

some tumor suppressor genes can mutate into an oncogenes

Meiosis

cell division that involves a diploid cell dividing 2 times to produce 4 genetically unique daughter cells that have half the number of chromosomes (haploid)

What are gametes?

female egg cell and male sperm cell

Why would this be referred to as “reduction division”?

because it results in the cells have half the number of chromosomes as the parent cell

Diploid

2 sets of chromosomes

**somatic** cells are diploid cells

in humans the 2n = 46

Haploid

1 set of chromosomes

**gametes** = sperm, egg, pollen, etc.

in humans the n = 23

tetrads

Two pairs of sister chromatids (a dyad pair) aligned in a certain way and often on the equatorial plane during the meiosis process.

Homologous Chromosomes

the pairs of chromosome you have (one from each bio parent)

different pairs are different sizes (pair 1 is the largest… pair 22 is very small)

What is genetic recombination (aka “crossing over”) and when does it happen?

genetic recombination/crossing over is when chromatids from the homologous chromosomes overlap randomly during **Prophase 1** and exchange parts - this create much more genetic variation in the resulting gamates

What is independent assortment?

randomly lining up and segregation of chromosomes from each parent during the formation of sex cells

How do they contribute to genetic variation in the resulting cells?

**Meiosis I** involves crossing over and independent assortment. Crossing over occurs in prophase I and this results in the exchange of DNA between homologous chromosomes. This creates new combinations of alleles. Independent assortment describes the different ways in which chromosomes can assemble along the metaphase plate. This occurs in metaphase I. **Meiosis II** involves independent assortment but not crossing over.

What happens in Meiosis I?

Prophase 1 → tetrads and crossing over

Metaphase 1 → they line up homologous chromosomes pairs etc. ie. diploid 6, haploid 3 3 lined up next to 3

Anaphase 1 → homologous chromosomes split up

Telophase 1 they split with 3 haploids in each

Meiosis II

Prophase II → the exact same thing

Metaphase II → they line up 3 in a line along the equator

Anaphase II → they split like they usually do

Telophase II they split into the like chromatid things

chiasmata

chiasm → singular

the crossover points between homologous chromosomes

exact number that will be made is **not yet predicatable**

gonads

sexual organs

-What are similarities and differences between mitosis and meiosis?

**differences:**

^^__mitosis__:^^

involves 1 cell division to make 2 cells and meiosis involves 2 cell division to make 4 cells

clones of parent cells

takes place everywhere where cells are dividing in an organism (somatic cells with diploid # of chromosomes

^^__meiosis__:^^

genetic recombination written tetrads of homologous chromosomes to make genetically unique cells

takes place in gonads to produce gametes (haploid #)

**similarities:**

cell division

cell cycle basic phases (prophase, metaphase, anaphase, telophase)

synthesis of DNA

Gametogenesis: Oogenesis and Spermatogenesis

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Oogenesis and Spermatogenesis: Similarities and Differences

**Similarities:**

both start with diploid cells and make genetically unique haploid cells through meiosis

both require fertilization to occur to create a new individual (haploid gamete + haploid gamete = diploid cell, the 1st cell of new life)

**Differences:**

oogenesis begins much earlier (**oocytes** are paused meiosis I and the “play“ button isn’t pressed in a chemical sense until ovulation/fertilization by sperm)

oogenesis produces only viable **ovum** per meiosis event, spermatogenesis produces 4 viable sperm each time. **Polar bodies** are not viable **ova**

**ova** are MUCH larger and have all the important organelles of the cytoplasm - sperm are very small. Besides the haploid nucleus mitochondria in the mid-piece, and the flagellum they have hardly any other major organelles