Meiosis Flashcards

Summary of Presentation

• Structure of Chromosomes

• Chromosomes in Somatic and Sex Cells

• Meiosis

• Differences between Meiosis I and II

• Reasons for Variation

• Importance of Meiosis

• Differences between Mitosis and Meiosis

• Similarities between Meiosis and Mitosis

• Consequences of Abnormal Meiosis

Understanding Chromosomes & Cell Types: The Basis for Meiosis

Chromosome Structure

• Chromosomes before replication:- Each chromosome (before DNA replication for meiosis) consists of one DNA molecule which is tightly coiled around proteins; the arms are connected by a centromere.

• Chromosomes after replication:- After replication (e.g., in Prophase I of meiosis), each chromosome consists of two DNA molecules.

• The two halves of a replicated chromosome are called identical sister chromatids.

• The identical sister chromatids are connected, in between the arms, by one centromere.

• Chromosomes carry genes, which are small portions of DNA that code for specific traits.

Diagram of a Replicated and Condensed Metaphase Eukaryotic Chromosome:

1. Chromatid

2. Centromere

3. Short arm

4. Long arm

• One un-replicated chromosome, with one DNA molecule: one chromatid

• One replicated chromosome, with two DNA molecules called sister chromatids.

• Figure 15: Before DNA replication, each chromosome comprises one DNA molecule. After replication, each chromosome comprises two DNA sister chromatids.

• Figure 20: Relationships between molecules. The two halves of a replicated chromosome are called chromatids.

• Cell --> Nucleus --> DNA, genes, & chromosomes

• DNA

  • Nitrogenous bases

  • Adenine

  • Guanine

  • Thymine

  • Cytosine

  • Sugar-phosphate backbone

• Chromosome

• Gene (segment of DNA)

Understanding Chromosomes & Cell Types: The Basis for Meiosis

Types of Cells in the Human Body

• Somatic Cells (Body Cells)- These are all cells in the body except sex cells (e.g., skin, lung, liver cells).

• Somatic cells are diploid (2n), meaning they contain two complete sets of chromosomes.

• In humans, somatic cells have 46 chromosomes, arranged as 23 pairs.

• Sex Cells (Gametes)- These are reproductive cells: sperm cells (male) and egg cells (female).

• Gametes are haploid (n), meaning they contain one complete set of chromosomes.

• In humans, gametes have 23 chromosomes.

Types of Cells in the Human Body

• Somatic Cell

  • Diploid

  • 46 chromosomes (2n) in human

  • Skeletal and muscle cells

  • Blood cells

  • Stem cells

  • All other cells

  • Organ and tissue cells

  • Fat cells

  • Neuron cells

  • SOMATIC CELLS- Can be found in several parts of the body excluding the reproductive parts.

  • Somatic cells are diploid, meaning they have two sets of chromosomes.

  • Somatic cells contribute to the overall genetic makeup of the individual.

  • Formed through the process of mitosis.

  • Somatic cells and gametes share common cellular structures, such as the nucleus, cytoplasm, and cell membrane.

  • Both somatic cells and gametes ultimately originate from germ cells.

• Gamete Cell

  • Germ line (germ cells)

  • Haploid

  • 23 chromosomes (n) in human

  • Sperm

  • Ovum (egg)

  • Fertilized egg

  • GAMETES- These are reproductive cells, specifically the egg cells (ova) in females and sperm cells in males.

  • Gametes are haploid, meaning they have only one set of chromosomes.

  • Gametes are involved in sexual reproduction and contribute half of the genetic material to the offspring.

  • Formed through the process of meiosis.

Understanding Chromosomes & Cell Types: The Basis for Meiosis

Human Chromosome Organization

• Homologous Chromosomes:-

• In diploid cells, chromosomes exist in pairs.

• One chromosome of each pair comes from the mother, and one from the father.

• These pairs are called homologous chromosomes.

• Homologous chromosomes have the same shape, size, and carry genes for the same characteristics at the same positions (loci).

Human Chromosome Organization

• Homologous Chromosomes

  • Homologous chromosomes

  • Haploid (N)

  • Diploid (2N)

  • Triploid (3N)

  • Tetraploid (4N)

  • same length

  • Replication

  • Centromere

  • same gene location

  • sister chromatids

Understanding Chromosomes & Cell Types: The Basis for Meiosis

Human Chromosome Organization (continued)

• Karyotype:-

• This is the number, appearance, and arrangement of a full set of chromosomes in the nucleus of a somatic cell.

• Human karyotypes show 23 pairs of chromosomes.

• Autosomes:-

• There are 22 pairs of autosomes in humans.

• These control general body characteristics, structure, and functioning.

• They are numbered 1 to 22.

• Gonosomes (Sex Chromosomes):-

• The 23rd pair determines sex.

• Females have two X chromosomes (XX) – an identical pair.

• Males have one X chromosome and one Y chromosome (XY) – a non-identical pair.

Human Chromosome Organization (continued)

• Karyotype

  • Normal male

  • Klinefelter syndrome; (XXY) or XYY syndrome

  • Y chromosome rearrangements ; X del(Yq)

  • Normal female

  • Turner syndrome; X0 or XX-X or Trisomy X syndrome; XXX

Table of Common and Uncommon Chromosomal Abnormalities

Terminology:

• TERM: Gametes

• DEFINITION: Is another word for sex cells.

• USE IN SENTENCE: The gamete found in the human female is called the egg or ovum.

• TERM: Somatic cells

• DEFINITION: These are normal body cells.

• USE IN SENTENCE: All the other cells in the body are called somatic cells.

• TERM: Karyotype

• DEFINITION: Is the number and appearance of chromosomes in the nucleus

• USE IN SENTENCE: A karyotype can be used to detect any genetic diseases.

• TERM: Haploid

• DEFINITION: Refers to having just one set of chromosomes.

• USE IN SENTENCE: Gametes have a haploid number of chromosomes.

• TERM: Diploid

• DEFINITION: Having a double set of chromosomes

• USE IN SENTENCE: Somatic cells have a diploid number of chromosomes.

• TERM: Autosomes

• DEFINITION: All the chromosomes except the sex chromosomes.

• USE IN SENTENCE: In a karyotype there are 22 pairs of autosomes.

• TERM: Gonosome

• DEFINITION:
  These are the sex chromosome

• USE IN SENTENCE:
  In a female the gonosome is XX.

Overview of Meiosis

Definition:

• Meiosis is a type of cell division where one parent cell divides to form four daughter cells.

• These four daughter cells are genetically different from each other and from the original parent cell.

• Crucially, these four daughter cells possess half the chromosome number (meaning they contain one complete set of chromosomes) of the original parent cell (they are haploid).

• Thus:

  • Meiosis reduces the chromosome number by half.

  • Meiosis produces four genetically distinct haploid cells (gametes or spores).

  • Meiosis is essential for sexual reproduction to maintain a constant ploidy number across generations.

Primary Outcome of Meiosis:

• Meiosis results in the formation of gametes (sex cells) in animals and humans, or spores in some other organisms like plants.

Occurrence of Meiosis

• Meiosis occurs in both plant and animal cells.

• In plants:

  • It occurs in the anther to produce pollen grains (which contain male gametes).

  • It also occurs in the ovary of plants to produce egg cells (female gametes).

• In humans:

  • In females, meiosis occurs in the ovary during the process of oogenesis to produce an egg cell (ovum).

  • In males, meiosis occurs in the testes during the process of spermatogenesis to form spermatozoa (sperm cells).

Significance of Meiosis in Sexual Reproduction:

• Produces Haploid Gametes:- The gametes (sperm and egg) produced by meiosis have half the number of chromosomes as the original body (somatic) cell meaning they contain one complete set of chromosomes. This means they are haploid (n).

• Restoration of Diploid Number at Fertilisation:- This halving of chromosomes is essential so that when fertilisation occurs (the fusion of a haploid sperm with a haploid egg), the resulting zygote is diploid (2n).

• The diploid zygote contains the correct, full number of chromosomes (meaning they contain two complete sets of chromosomes) characteristic of the species.

• Development of Multicellular Adult:- The diploid zygote then divides by mitosis (a different type of cell division that maintains chromosome number) to develop into a multicellular adult organism.

Meiosis

• Human Life Cycle

  • 46 chromosomes

  • Male individual

  • Female individual

  • MEIOSIS

    • 23 chromosomes

    • Sperm

    • 23 chromosomes

    • egg

    • FERTILISATION

    • 46 chromosomes

      • Zygote

      • Mitosis (growth)

      • 46 chromosomes in each somatic cell in the new individual

• Human life cycle

  • Haploid gametes (n = 23)

  • Ovum (n)

  • Sperm cell (n)

  • Ovary; Testis

  • Diploid (2n)

    • Multicellular diploid adults (2n = 46)

    • Diploid zygote (2n = 46)

Meiosis I (Reduction Division)

Interphase

• Nuclear envelope

• Centrosomes (with centriole pairs)

• Chromatin

• Chromosomes duplicate

• Interphase is the preparatory phase that occurs before Meiosis I begins.

• Key event: DNA Replication-

  • The DNA within the cell replicates (makes identical copies of itself).

  • As a result, the genetic material in the chromatin network is doubled.

  • This means each chromosome becomes double-stranded, consisting of two identical sister chromatids joined by a centromere.

• Meiosis I is known as Reductional Division primarily because the number of chromosomes (ploidy level) in the cells is reduced by half from the beginning to the end of this stage.

Prophase I

• Chiasmata

• Spindle

• Sister chromatids

• Tetrad

• Homologous chromosomes

• Bivalents form; arrange in pairs.

• centriole

• spindle fibres

• Homologous chromosomes aligned

• chromosome

• Chromosome condensation

  • The chromatin network (diffuse DNA) condenses and coils tightly.

  • As a result, chromosomes become shorter, thicker, and individually visible under a microscope.

• Pairing of homologous chromosomes (synapsis)

  • Homologous chromosomes (one maternal, one paternal chromosome of each type) find each other and arrange themselves in pairs.

  • A bivalent is formed when a pair of homologous chromosomes lie closely together, side-by-side.

Crossing Over

• Homologous chromosomes aligned

• Chromosome crossover

• Representation of crossing over

• Homologous chromosomes arrange themselves in pairs

• Bivalent forms

• Sister chromatids are visible

• Crossing over and exchange of chromatid segments occur

• Homologous chromosome pair after crossing over

• This is a critical event for genetic variation.

• It takes place between non-sister chromatids of homologous chromosomes within a bivalent.

• One chromatid from the maternal chromosome overlaps with a chromatid from the paternal chromosome.

• The point(s) where this overlapping and exchange occurs is called a chiasma (plural: chiasmata).

• At the chiasma, segments of the chromatids break off and are exchanged, attaching to the corresponding position on the other chromatid.

• After crossing over, when the homologous chromosomes eventually separate, each chromosome (specifically, its chromatids that participated in the exchange) will now possess a new combination of genetic material, containing segments from both the maternal and paternal chromosome.

• This results in the reshuffling of genetic material between homologous chromosomes.

• Other cellular changes in Prophase I

  • In animal cells, centrioles move to opposite poles of the cell.

  • Spindle fibres (microtubules) begin to form from these poles.

  • The nuclear membrane (nuclear envelope) starts to break down and disappear.

  • The nucleolus also begins to disappear.

Importance of Crossing Over

• Exchange of genetic material- Crossing over results in a physical exchange of segments of DNA between homologous chromosomes.

• Therefore, after crossing over, individual chromatids (and thus the chromosomes they are part of) can have a combination of genes from both the maternal and paternal parent.

• Creation of genetic variation- This exchange means that the gametes eventually produced will be genetically different from each other and from the parent cell.

• Crossing over is a primary mechanism responsible for generating genetic variation among offspring in sexually reproducing organisms.

Metaphase I

• Microtubule attached to kinetochore

• Metaphase plate

• Bivalents on equator

• Chromosome in double row

• Centromere (with kinetochore)

• Spindle fibres are fully formed and attach to the centromeres of each chromosome.

• Homologous chromosome pairs (bivalents) move to the equator (middle plane) of the cell.

• The two homologous chromosomes of each pair lie parallel to each other, on opposite sides of the equator.

• Random arrangement (or independent assortment) of these homologous pairs occurs:

  • Which chromosome of a pair lies on which side of the equator is completely by chance.

  • This random arrangement is a key source of further genetic variation in the gametes.

Anaphase I

• Sister chromatids remain attached

• One chromosome of the bivalent moves to one pole, and its partner moves to the other pole.

• Chromosomes group at the poles.

• Homologous chromosomes separate

• Pairs of homologous chromosomes split up

• The spindle fibres contract and shorten.

• This pulls one whole chromosome from each homologous pair to opposite poles of the cell.

• This process separates the homologous chromosomes, with one chromosome from each pair moving to each pole.

• Important: Sister chromatids do not separate at this stage; entire replicated chromosomes move.

Telophase I

• Cleavage furrow forms

• Two daughter cells with half the chromosome number (n) are formed.

• Groups of replicated chromosomes (each consisting of two sister chromatids) arrive at each pole.

• A new nuclear membrane forms around the group of chromosomes at each pole.

• The nucleolus may reappear within each new nucleus.

• Spindle fibres disappear.

• Cytokinesis (division of the cytoplasm) occurs, splitting the original cell into two daughter cells.

• Final outcome of Meiosis I

  • This results in two daughter cells.

  • Crucially, each new daughter cell now has half the number of chromosomes (is haploid, n) compared to the original diploid mother cell.

  • The cells are genetically different from each other and from the original parent cell. This is due to:

  • Crossing over (which occurred in Prophase I, reshuffling alleles between homologous chromosomes).

  • Random arrangement of homologous chromosomes at the equator during Metaphase I.

Meiosis II Equational Division

• Meiosis II begins with the two haploid (n) daughter cells produced during Meiosis I.

• Unlike Meiosis I, there is no Interphase before Meiosis II, meaning DNA does not replicate again.

• The process of Meiosis II is very similar to mitosis. Each of the two haploid cells from Meiosis I will divide.

• Meiosis II is known as Equational Division primarily because the number of chromosomes (ploidy level) in the cells remains the same (equal) from the beginning to the end of this stage.

Prophase II

• The chromosomes (each still composed of two chromatids joined by a centromere) become visible.

• Important: Chromosomes are NOT in homologous pairs at this stage.

• In animal cells, the centrioles (from the division of the centrosome) move to opposite poles.

• Spindle fibres begin to form from the poles.

• The nuclear membrane and nucleolus start to disappear (if they reformed in Telophase I).

Metaphase II

• Single chromosomes (each consisting of two chromatids) align themselves randomly along the equator (middle plane) of each cell in a single row.

• The centromere of each chromosome is in line with the equatorial plane.

• Spindle fibres from opposite poles attach to the centromeres of each chromosome (specifically, to the kinetochores).

• The orientation of which chromatid faces which pole is random, contributing further to genetic variation.

Anaphase II

• The centromeres of each chromosome split in half.

• The sister chromatids are pulled apart by the contracting spindle fibres.

• These separated chromatids (now considered individual, single-stranded chromosomes or unreplicated chromosomes) move to opposite poles of the cell.

Telophase II

• A complete set of unreplicated chromosomes (formerly chromatids) is now present at each pole.

• Spindle fibres disappear.

• A new nuclear membrane forms around each group of chromosomes.

• A nucleolus reforms in each new nucleus.

• Cytokinesis then occurs, dividing the cytoplasm of each cell.

• Outcome of Meiosis II

  • Since Meiosis II occurred in the TWO cells produced by Meiosis I, the final result is FOUR daughter cells.

  • Each of these four daughter cells is haploid (n), meaning it has half the number of chromosomes as the original parent cell that entered Meiosis I.

  • All four daughter cells are genetically different from each other and from the original parent cell (due to crossing over in Prophase I and random assortment in Metaphase I & II).

Cytokinesis: Division of the Cytoplasm

• Cytokinesis is the physical process of cell division, which divides the cytoplasm of a parental cell into daughter cells. It typically occurs after the nucleus has divided (karyokinesis).

• In animal cells:

  • Once the two new nuclei are formed, the cell membrane constricts or cleaves (pinches inwards) around the middle of the cell.

  • This constriction, often called a cleavage furrow, deepens until it meets in the middle, eventually forming two separate daughter cells.

• In plant cells:

  • A cell plate (a new cell wall precursor) forms between the two newly formed nuclei, along the equator of the cell.

  • This cell plate grows outwards, eventually dividing the original cell into two new daughter cells, each with its own cell wall.

Mnemonic for Meiosis Stages (PMAT)

• Prophase – Chromosomes PAIR up (in Meiosis I, crossing over occurs).

• Metaphase – Chromosomes move to the MIDDLE (equator).

• Anaphase – Chromosomes move APART to the poles.

• Telophase – TERMINAL phase where daughter cells are formed.

• (Note: This mnemonic applies to both Meiosis I and Meiosis II, but the specifics of what pairs up or moves apart differ between the two stages).

Summary: Meiosis I Reductional Division

• Interphase (Before Meiosis I):

  1. DNA replicates (chromosomes become two sister chromatids).

• Prophase I:

  1. Chromosomes condense.

  2. Homologous chromosomes pair up (synapsis) forming bivalents.

  3. Crossing over occurs at chiasmata between non-sister chromatids, leading to genetic recombination.

  4. Nuclear envelope breaks down; spindle fibres form.

• Metaphase I:

  1. Bivalents (homologous pairs) align randomly at the cell's equator.

  2. Independent assortment of homologous pairs occurs (key for variation).

  3. Spindle fibres attach to each chromosome of a homologous pair.

• Anaphase I:

  1. Homologous chromosomes separate and move to opposite poles.

  2. Sister chromatids remain attached at their centromeres.

• Telophase I & Cytokinesis:

  1. Chromosomes (each still composed of two sister chromatids) gather at poles.

  2. Nuclear envelopes may reform.

  3. Cytoplasm divides.

• Result:
  Two genetically different haploid (n) daughter cells. (Goal: Separate homologous chromosomes; diploid (2n) → two genetically different haploid (n) cells; chromosomes remain replicated).

Summary: Meiosis II Equational Division

• Prophase II:

  1. Chromosomes re-condense (if they decondensed).

  2. Nuclear envelope breaks down (if reformed).

  3. New spindle fibres form in each cell.

• Metaphase II:

  1. Individual replicated chromosomes (each with two sister chromatids) align randomly at the equator of each cell.

  2. Spindle fibres attach to the centromeres (kinetochores of sister chromatids).

• Anaphase II:

  1. Centromeres divide.

  2. Sister chromatids separate and are pulled to opposite poles.

  3. Separated chromatids are now considered individual unreplicated chromosomes.

• Telophase II & Cytokinesis:

  1. Unreplicated chromosomes gather at poles.

  2. Nuclear envelopes reform.

  3. Chromosomes decondense.

  4. Cytoplasm divides for each cell.

• Result:
  Four genetically different haploid (n) daughter cells. (Goal: Separate sister chromatids; results in four genetically different haploid (n) cells; chromosomes become unreplicated. Similar to mitosis but starts with haploid cells.)

Note: Interkinesis may occur between Meiosis I & II – NO DNA replication.

Importance of Meiosis

1. Production of gametes

2. Halving of the chromosome number (the halving effect)

3. Introduction and promotion of genetic variation

• Production of gametes

  • Meiosis is essential for producing gametes (sex cells, e.g., sperm and egg cells).

  • Typically, four daughter cells (gametes) are formed from one parent cell undergoing meiosis.

• Halving of the chromosome number (the halving effect)

  • Meiosis reduces the chromosome number by half, transforming diploid cells (2n) in the sex organs into haploid sex cells (n).

  • This halving effect is crucial because it ensures that when haploid gametes fuse during fertilisation, the resulting zygote is diploid (2n).

  • Consequently, the species-specific chromosome number remains constant from generation to generation and is not doubled with each fertilisation.

• Introduction and promotion of genetic variation

  • Meiosis is a key mechanism for introducing genetic variation within a population.

  • This genetic variation is vital as it can lead to increased survival rates if environmental conditions change, as some individuals may have advantageous traits.

  • Individuals produced are genetically different from their parents and siblings.

Mechanisms Causing Genetic Variation During Meiosis & Sexual Reproduction

1. Crossing Over (during Prophase I)

   • Crossing over is the exchange of segments of genetic material between non-sister chromatids of homologous chromosomes when they are paired up as bivalents.

   • Importance:

     • It results in the formation of new genetic combinations on the chromosomes.

     • Chromatids (and therefore the resulting chromosomes in gametes) will contain a mixture of maternal and paternal genetic information.

     • This means the gametes formed will be genetically different from each other and from the parent cell's original chromosomes.

2. Random Arrangement of Chromosomes (during Metaphase I and Metaphase II)

   • Metaphase I:-

     • Homologous chromosome pairs (bivalents) align randomly at the equator of the cell. The way one pair lines up does not affect how other pairs line up (independent assortment). This means paternal and maternal chromosomes are mixed up in terms of which pole they will eventually move to.

   • Metaphase II:-

     • Individual chromosomes (each made of two chromatids) also align randomly at the equator before chromatids separate. The orientation of which chromatid faces which pole is by chance.

   • Both random arrangements contribute to different combinations of chromosomes (and alleles) in the resulting gametes.

3. Random Fertilisation

   • During sexual reproduction, any sperm cell can fuse with any egg cell.

   • This chance fusion of genetically different gametes results in unique genetic combinations in each zygote, further increasing genetic variation among offspring.

Differences Between Meiosis I and II

Terminology

• TERM: Crossing over

• DEFINITION: Process involving the exchange of genetic material between members of the homologous pair.

• USE IN SENTENCES: Crossing over brings about variation.

• TERM: Homologous pair

• DEFINITION: Refers to a pair of identical chromosomes, one of paternal origin and one of maternal origin.

• USE IN SENTENCE: During prophase I of meiosis, the chromosomes arrange themselves in homologous pairs.

• TERM: Variation

• DEFINITION: Refers to the variety of appearance shown by organisms of the same species.

• USE IN SENTENCE: Variation would ensure the survival of the species.

Abnormal Meiosis: Errors in Chromosome Separation

Nondisjunction

• Is the primary cause of abnormal meiosis

• Nondisjunction is the failure of chromosomes