Meiosis and Mitosis

Fall Biology

Table of contents

Introduction
13.1: How Does Meiosis Occur?
13.2: Meiosis Promotes Genetic Variation
Meiosis: Where the Sex Starts, Crash Course Biology
Khan Academy Fertilization Terms
Chromosomal Crossover in Meiosis I
Genetic Variation
Allele Shuffling
What is inheritance?

Introduction

People have known that during sexual reproduction, a male reproductive cell (sperm) and a female reproductive cell (egg) unite in fertilization.
The formation of gametesexplain why each gamete has half the usual number of chromosomes. ** Meiosis** is a nuclear division that leads to a halving of the chromosome number.
- Ultimately leads to the production of a sperm and egg.

13.1: How Does Meiosis Occur?

To understand meiosis, we must first understand ideas about chromosomes.
Consider this observation:
- Each organism has a characteristic number of chromosomes.
- e.g. your cells have 46 chromoomes, dogs have 78, some ferns have 1000.

Chromosomes Come in Distinct Sizes and Shapes

American cell biologist Nettie Maria Steven, 1906.
Found that although a specific fly species had 8 chromosomes, there were five distinct types.
- In males, two chromosomes (known now a X and Y) were unpaired.
- In females, there was a pair of X chromosomes (and no Y chromosome).
X and Y chromosomes are sex chromosomes; any other chromosomes are autosomes.
Chromosomes that are the same size and shape are homologous chromosomes (also referred to as homologs).
- A pair is a homologous pair.
- Contain the same gene in the same positions along the genes.
- However, are not identical.
A gene is a segment of DNA found at a place in the chromosome that influences a trait.
- e.g. chromosome 2 in Drosophila contains a gene that influences eye color.
- An allele is a version of a gene.

The Concept of a Ploidy

Stevens identified the fruit fly karyotype by identifying the number and types of chromosomes present.
Many organisms (including humans) have two version (homologs) of each chromosome; these are diploid.
- One allele is carried on each of the homologs.
Bacteria, archaea, etc. have cells that contain only one type of chromosome, called haploid (‘single-form’).
- Have only one copy of each chromosome; an individual has only one allele of each gene.
Notation used by biologists:
- n represents number of distinct type of chromosomes in a cell, called the haploid number.
  - In humans, n=23.
- For sets of chromosomes, use 2n, 3n, etc.
The number of chromosome ets is the cell’s ploidy.
- Diploid cells are 2n because there are two chromosomes.
  - Maternal chromosome from the mother and paternal chromosome from the father.
  - Note that in human, 2n = 46.
Many species have >2 types of a chromosome.
- These are polyploid (‘many form’).
  - Polyploid species can be triploid (3n), tetraploid (4n), hexaploid (6n), octoploid (8n), etc.

An Overview of Meiosis

Cells replicate each of their chromosomes before starting meiosis.
An unreplicated eukaryotic chromoome begins as a single, long DNA double helix wrapped around histones (proteins).
- DNA is replicated during the S phase of the cell cycle.
- Replicated chromosome consists of two sister chromatids.
- Each sister chromatid contains an identical copy of the DNA double helix.
An unreplicated chromosome is never called a chromatid; it is only used to describe an individual chromosome after replication.

Meiosis Consists of Two Cell Divisions

Back-to-back cell divisions occur in meiosis, meiosis I and meiosis II.
Homologs of each chromosome pair are separated in meiosis 1.
- Homologs go to respective daughter cells.
At the end of meiosis I, each daughter cell has one of each chromosome instead of two.
- (half as many chromosomes are the parent cell).
- Diploid parent cell produces two haplooid daughter cells.
- Note: each chromosome still has two siter chromatids.
During meiosis II, sister chromatids of each chromosome separate.
- Sister chromatid becomes an individual chromosome (called daughter chromosome).
- Each daughter cells has one copy of the daughter chromosome.
- Sister chromatids separate into daughter chromosomes during meiosis II.
  - Note: this is what happens during mitosis.
Chromosome movement during meiosis I and II:
- …depends on microtubules of the spindle apparatus.
- these attach to the kinetochores.
- are located at the centromere of each chromosome.

Meiosis I is a Reduction Division

Reduction of chromosome number in meiosis I makes the division different from meiosis II or mitosis.
Original cell entering meiosis is diploid; four daughter cells are haploid.
Some or all haploid daughters go on to become egg cells or sperm cells.
- Process of gametogenisis (“gemete-origin”)
When two haploid gametes fuse during fertilization, the chromosomes form as diploid cells again.
- Resulting diploid cell from fertilization is a zygote.

Phases of Meiosis I

Meiosis is a set of continuous events.

Early Prophase I

Nuclear envelop breaks down, spindle apparatus forms.
Siter chromaetids are held by proteins called cohesins.
Homologous chromosome pairs come together.
- Result is the synapsis.
- Pairing of homologous chromosomes along their regions.
THe homologs are held together by the synaptonemal complex, a net of proteins.
Structure that results from synapsis is bivalent; consits of paired homologous replicated chromosomes.
Chromatids from different homologs are referred to as non-sister chromatids.

Later Prophase I

The nuclear envelope is complete broken down.
Each homolog in the bivalent come to be attached to microtubule fibers that come from a spindle poole.
- One homolog is attached to each.
- This attachment is essential for separating homologous pairs.
The synaptonemal complex disassembles in late prophase I, and the homologs begin to separate at many points along their length.
- Are joined by the X-shaped structure (chiasmata, singular: chiasma)
One chiama forms in every pair of homologous chromosomes.
- Chiasmata mark sites of DNA breakage and rejoining.
Chromatids that form a chiasma are non-sister chromatids.
- At each chiasma, there is an exchange of parts of chromosomes; known as crossing over.

Metaphase I

Kinetochore microtubules move pairs of homologous chromosomes (the bivalents) to the metaphae plate.
- The place in between the poles of the spindle apparatus.
Important points about chromosome movement:
- In metaphase I, each bivalent is ‘on’ the metaphase plate (one homolog on one side and the other homolog on the other).
- The alignment of each bivalent is independent of other bivalents.
  - Rephrased, alignment of bivalent is important.

Anaphase I and Telophase I

Anaphase I begins as homologs move to different poles of the spindle apparatus.
- Kinetochores of each homolog attach to spindle fibers; each homolog is attached to a different spindle pole.
- Two homologous chromosomes in the bivalent separate from each other.
Chiasmata are broken during anaphae I. (cohesin proteins are removed).
Separating homologs are a combination of maternal and paternal segments.
During telophase I, homologs finish moving to opposite sides of the spindle.
Cytokenisis (division of the cytoplasm) occurs and two haploid daughter cells form.

Meiosis I: A Recap

Chromosome movement happens through assembly and dissassembly of microtubules attached to the kinetochore.
During meiosis I, a diploid parent cell produces two haploid daughter cells.
- Sister chromatids remain attached to each chromosome.
- Chromosomes in each daughter cell are a random assortment of maternal and paternal chromosomes because of crossing over and independent alignment.

The Phases of Meiosis II

Chromosome replication occurred before meiosis I.
- There is no DNA replication before meiosis II. (critical difference)
Meiosis II separates the sister chromatids into individual cells.
- Each cell contains unreplicated daughter chromosomes
After meiosis II, there are four haploid cells.
- Meiosis II occurs in both daughter cells produced by meiosi.
The overall process of Meiosis produces four daughter cells from a parent cell.

Prophase II

A spindle apparatus forms in both daughter cells.
Microtubules that polymerize from the two spindle pokes attach to the kinetochores on opposite sides of every chromosome.
- These begin to move the chromosomes towards the middle of the cell.

Metaphase II

The chromosomes are lined up at the metaphase plate.
- Each chromosome is attached by spindle fibers to both of the poles.

Anaphase II and Telophase II

The sister chromatids are separated in anaphase II.
These move to different daughter cells in telophase II.
Each chromatid is considered to be an independent daughter chromosome.

Mitosis versus Meiosis

13.2: Meiosis Promotes Genetic Variation

Because of the crossing over and random shuffling components of meiosis I, chromosomes in one gamete are different from chromosomes in every other gamete.
Fertilization creates genetically varied diploid offspring.
- Does this by bringing together two haploid sets of parental chromosomes.
Chromosome sets and varied combinations occur only in sexual reproduction.
- Asexual reproduction produces offspring without the fusion of gametes, and is based of mitosis. Aexually produced offspring are clones (exact genetic copies).
- Sexual reproduction is the production of offspring through the generation and fusion of gametes. Sexual reproduction results in offspring that have chromosome complements.

Independent Assortment

Each somatic cell in your body has 23 homologous pairs of chromosomes (46 in total).
- Half the chromosomes come from your mother, half come from your father.
When pairs of homologous chromosomes line up during meiosis I, different combinations of maternal and paternal chromosomes result.
- This phenomenon is known as independent assortment.
The creation of new combinations of alleles is known as genetic recombination.
A diploid organism can produce 2^n combinations (n is the haploid chromosome number).`
- 8.4 million combinations for humans.

Crossing Over

Segments of paternal and maternal chromatids exchange when crossing over.
Crossing over produces new combinations of alleles within a chromosome.
Genetic recombination is important because it creates genetically diverse gametes.
- Independent asortment generates varied combinations.
- Crossing over produces new combinations of alleles in each chromosome.

How Does Fertilization Affect Genetic Variation?

Crossing over and the independent assortment of maternal and paternal chromosomes enure every gamete is unique.
Random fertilization means that the sperm and the egg can come together, regardless of which alleles they carry.
A human can produce ~8.4 million gamete by independent asortment.
- Two parents can produce 8.4m * 8.4m = 70.6 * 10^12 genetically distinct offspring.

Meiosis: Where the Sex Starts, Crash Course Biology

Available here

Sexual reproduction; cells split repeatedly to form a human.
Most of your somatic (body) cells replicate through cell identification.
Mitosis replicates a cell with 46 chromosomes into two identical cells.
- However, you cannot clone yourself.
- Mitosis is not the only cell division method we have.
All of your body cells have the same DNA; 46 chromosomes in 23 pairs.
- Contain versions of the same alleles; one in each pair from your mom and one from your dad.
- Similar; pairs are ‘homologous chromosome pairs’.
Special cells that have only 1/2 of the amount: sperm and egg cells.
- Haploid cell (half the full set of chromosomes).
- Need each other to combine to complete the full 46 chromosomes.
Mitosis needs meiosis.
- Diploid cell splits in half twice, producing four cells.
Raw material for meiosis: in ovaries or the testies.
- Primary Oocytes or Primary Spermatocytes.
Interphase: long strings of DNA begin to replicate.
- Centrosomes: being to replicate as well, are a set of cylinder-like proteins that regulate how materials are moved around along ropey proteins called microtubeles.
Prophase 1: centrosomes begin heading to their corners of the cell, unspooling the microtubeles.
- Each DNA clumps with proteins into chromosomes; each chromosome is linked with its duplicate copy to form X-shaped double chromosome.
- Each of the chromosomes when attached are chromatids.
  - Each double chromosome has two chromatids.
- Crossover: Each double chromosome lines up with its homolog.
  - Double chromosomes have 4 chromatids between them.
  - One chromatid in each X gets tangled up with the other; forming crossover.
- Homologous Recombination
  - During crossover, exchange DNA (recombination).
If we simply cloned to ourselves, we would not have variation for natural election.
- All 4 chromatids are now different; each chromatid will end up in a separate sex cells.
One pair of chromoomes does not always go through crossover or recombination: 23rd chromosome, sex chromosomes.
- Female: two complete cells.
- Male: only one X chromosome, and the other doesn’t do with the Y (not homologous)
- Half of resulting sperm are X and half will be Y.
Metaphase 1: each chromosome lines to its homologous pair partner.
Anaphase 1: These get pulled apart to separate ends of the cell.
Telophase 1: A nuclear membrane forms around each; cleavage forms between the cells and the two cells form. Cytokenisis.
- Two haploid cells have been formed.
- Centromeres till look like X.
- Aim is to end up with four cells.
Aim: not to duplicate double chromosomes, but to pull them apart into separate single-strand chromosomes.
Prophase II: just bunch into separate ends.
Metaphase II: chromosomes are moved into alignment into the middle of the cell.
Anaphase II: the two trands are pulled apart, the crease forms, and four total cells are produced.
Result: four cell with 23 chromosomes each.
- Half will be male, half will be girls.
During telophase I, more of the cytoplasm, organelles, etc. head into one of the cells; same with telophase II.
- Forms 1 egg (with more nutrients and organelles to make the new embryo) and 3 polar bodies (esentially useless).

Khan Academy Fertilization Terms

Access here

When the sperm and the egg cell fuse, this is fertilization.
- Produces a cell that differentiates into all the cells of our body.
Each of the sex cells (sperm cell and the ovum) are called gametes.
- Each gamete has half the number of chromosomes as the somatic cells of your body.
Fertilization setup.
- 23 chromosomes from your father.
  - 23rd chromosome is X or Y; if it is X, it is female; if it is Y, it i male.
- Fuses with the ovum (egg that the mother is contributing), which has 23 additional chromosomes.
  - 23rd chromosome is X.
When the two gametes are fused:
- The ‘fertilized egg’ iss the zygote.
- 23 chromosomes from the father and 23 chromosomes from the mother form 23 chromosome pairs.
  - Pairs are homologous chromosomes.
  - Means that these two chromosomes code for the same proteins, but there are variants for how they code for those variants.
Haploid and diploid number/cells.
- Human haploid number is 23 (‘hapl’ - single), number of chromosomes in each of the gametes.
  - Referred to as n.
- Human diploid number is 46 (‘di’ - two), number of cells in zygote.
  - Referred to as 2n.

Chromosomal Crossover in Meiosis I

Access here.

Germ cell; a cell that can go through mitosis to produce germ cells or undergo meiosis to produce other germ cells, or to undergo meiosis to produce gametes.
Hypothetical: Diploid Number is 4.
Interphase: grows and replicates DNA; after replicating, it is still one chromosome.
- Made up of two sister chromatids.
At this point, either mitosis or meiosis can happen.
- This will focus on meiosis.
Prophase I
- Nuclear membrane begins to dissolve.
- DNA begin to bunch up into ‘condensed form’ into ‘X’s.
- Centromere is at the middle of the Xs.
- DNA has been replicated; in each of the chromosomes exist two siter chromatids.
- In a homologous pair, there are four chromatids. Often called a tetrad.
- Genetic/homologous recombination; may contain different DNA but code for the same genes.
Recombination; DNA is swapped at certain sections.
- Happens fairly often, and is a way to get variation.
- Exchange of information between these chromosomes.
- Generally happens at fairly ‘clean’ points, called the chiasma.
  - plural: chiasmata.

Genetic Variation

By Utah Learn Genetics, access here

Genetic variation happens through mutation and recombination
A gene variation that makes it unable to see could be harmful, for example.
A gene variation that makes it more attractive to pollinators allows its genes to be pased down.
Most gene variation are not good or bad (neutral).
- Don’t effect survival, so usually stay in a population.
Harmful variations get weeded out.
Variation is necessary to life.

Allele Shuffling

By Utah Learn Genetics, accesshere

Alleles have variations in their DNA sequences.
- May change amino acid, or when, where, and how much protein is made.
A population can have many alleles for each gene.
The traits are influenced by combinations of alleles.
Pairs of chromosomes have pairs of genes arranged in the same order.
Chromosomes are copied, recombined, shuffled, and split into four haploid cells (sperm/eggs).

What is inheritance?

By Utah Learn Genetics, accesshere

When things reproduce, they pass DNA to their offspring.
Some living things like bacteria can reproduce without a partner.
People reproduce with a partner (sexual reproduction)
- We all have two copies of each gene.
- This influences our inherited traits.
Children resemble their genes, but unique gene combinations give children a unique set of characteristic.