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Meiosis and Mitosis

Fall Biology

Table of contents
  1. Introduction
  2. 13.1: How Does Meiosis Occur?
    1. Chromosomes Come in Distinct Sizes and Shapes
    2. The Concept of a Ploidy
    3. An Overview of Meiosis
      1. Meiosis Consists of Two Cell Divisions
      2. Meiosis I is a Reduction Division
    4. Phases of Meiosis I
      1. Early Prophase I
      2. Later Prophase I
      3. Metaphase I
      4. Anaphase I and Telophase I
      5. Meiosis I: A Recap
    5. The Phases of Meiosis II
      1. Prophase II
      2. Metaphase II
      3. Anaphase II and Telophase II
    6. Mitosis versus Meiosis
  3. 13.2: Meiosis Promotes Genetic Variation
    1. Independent Assortment
    2. Crossing Over
    3. How Does Fertilization Affect Genetic Variation?
  4. Meiosis: Where the Sex Starts, Crash Course Biology
  5. Khan Academy Fertilization Terms
  6. Chromosomal Crossover in Meiosis I
  7. Genetic Variation
  8. Allele Shuffling
  9. What is inheritance?


  • 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.