CHAPTER 14: Meiosis: Getting Ready for Baby summary


  • For organisms to reproduce sexually, each parent donates a cell to produce a new organism.
  • A sperm cell joins an egg cell to combine genetic information.
  • Meiosis is a reproduction cycle that involves a combination of an egg and a sperm cell.
  • Meiosis occurs in gonads of organisms that reproduce sexually.

Life cycle

  • Somatic cells have twice the genetic information of cells participation in meiosis.
  • Fertilization is the process by which gametes combine to form somatic cells.
  • This creates a life cycle.
The human life cycle steps
  • Somatic cells produce gametes through the process of meiosis.
  • Eggs and sperm cells combine in the process of fertilization to produce the first called a zygote.
  • The zygote divides to make a multicellular organism by mitosis.

Homologous chromosomes

  • Homologous chromosomes are matching chromosome pairs.
  • Homologous chromosomes contain the same gene types as other cells.
  • Diploid organisms acquire each pair of homologous chromosomes from each parent.
  • Many physical differences help in identification of types of chromosome:
    • Chromosome length: chromosomes can vary in length from very small to fairly long.
    • Centromere position: chromosomes can be seen when cells are almost dividing.
    • Staining pattern: some stans can attach differently to chromosomes, creating dark and light patterns.


  • Meiosis is a cell division process that separates homologous chromosomes to allow gametes to receive each type of the chromosomes.
  • Each of the twenty-three chromosomes has different information.

Similarities between meiosis and mitosis

  • Cells that proceed to meiosis receive a signal that provokes them to exit the G1 interphase to enter the S phase.
  • Cells copy chromosomes during the S phase chromosomes with similar sister chromatids joined to the centromere.
  • The cells proceed to meiosis through the G2 phase.
  • Chromosomes move around the spindle during meiosis.

Meiosis overview

  • The meiosis part that generates the first division is known as meiosis I.
  • Meiosis I helps in separating homologous chromosome pairs.
  • The part of meiosis that generates the second division is referred to as meiosis II.
  • Meiosis II helps in separating sister chromatids.

Meiosis I

  • The phases of meiosis include the following: prophase I, metaphase I, anaphase I and telophase I.
  • Prophase I: There is a breakdown of the nuclear membrane.
    • There is the disappearance of the nucleoli.
    • The chromosomes undergo condensation and become more visible.
  • Metaphase I: during this phase, there is lining up of chromosome pairs in the middle.
  • Anaphase I: during this phase, homologous chromosomes are separated from each other and each pair goes to opposite regions of the cell.
  • Telophase I: during this phase, nuclear membranes form and create haploid two nuclei.
  • Cytokinesis: it results in the formation of new cells.

Meiosis II

  • It includes the following events:
  • Prophase II: spindles form on each cell and joins to chromosomes.
  • Metaphase II: during this phase, the chromosomes are arranged in the middle of the cell.
  • Anaphase II: in this phase, sister chromatids separate and migrate to the opposite side of the cell.
  • Telophase II: there are several events that occur in this phase.
    • The chromosomes uncoil.
    • Breakdown of spindle fibers.
    • The nuclear membrane is reformed.
    • There is the reappearance of the nucleoli.
  • Cytokinesis: This process results in 4 cells from the original one cell. The 4 cells finally develop into gametes.

Crossing over

  • Crossing over involves the following steps:
    • Homologous chromosomes are joined along their length. Synaptonemal complex proteins stick the chromosomes together.
    • Small cuts are induced in the DNA backbone of the homologous chromosomes.
    • Proteins seal the DNA backbone breaks by attaching homologous chromosome pieces to each other.


  • The nondisjunction process leads to the formation of gametes with the wrong chromosome number.
  • It occurs in chromosomes that fail to separate correctly.
  • Nondisjunction can occur in meiosis I.


Prophase I
Each chromosome comes near its replicated chromosome pair. Nucleus dissolves at this stage. Crossing Over (exchange of genetic material) occurs at this phase.

Metaphase I
The pairs of homologous chromosomes line up in the center of the cell. The centromere of each chromatid pair attaches to one spindle fibre.

Anaphase I
Homologous chromosomes are pulled to opposite ends of the cell. Note that chromatids do not separate – each duplicated chromosome still has two chromatids.

Telophase I
The cytoplasm divides and two new cells form. Each new cell has one duplicated chromosome from each similar pair.

Prophase II
The duplicated chromosomes and spindle fibers reappear in each new cell.

Metaphase II
The duplicated chromosomes move to the centre of the cell. Each centromere attaches to two spindle fibres instead of one.

Anaphase II
The centromere divides. The chromatids seperate and move to opposite ends of the cell. Each chromatid is now an individual chromosome.

Telophase II
The spindle fibres disappear, and a nuclear membrane forms around each set of chromosomes.

Homologous chromosomes
Chromosomes that have the same sequence of genes, that have the same structure, and that pair during meiosis.

Crossing Over
Process in which homologous chromosomes exchange portions of their chromatids during meiosis. This increases genetic variation.

Chromosomes (uncondensed in this phase) replicate in preparation for meiosis. At this point they are long and thing and called “chromatin”.

Chromosomes duplicate before dividing in meiosis.

Error in meiosis in which homologous chromosomes fail to separate, resulting in gametes with too many or too few chromosomes

A cell that contains only one set of chromosomes instead of the normal pair. Gametes, which are sex cells like sperm and eggs, are haploid cells.

A cell that contains two sets of chromosomes; one inherited from the mother and one inherited from the father. Most body cells (nerve, brain, muscle, skin, etc.) are considered diploid cell.

Male gamete (sex cell)

Female gamete (sex cell)

The result of meiosis is 4 gametes, or sex cells, that each contain half of the genetic information in the parent organism.

A process in cell division during which the number of chromosomes decreases to half the original number. It occurs by two divisions of the nucleus and results in the production of 4 sex cells (gametes).

what is the difference between Meiosis I and II?
homologous pairs (I) vs. sister chromatids (II)

What is the ploidy of the cells at the end of Meiosis II?
haploid (23 chromosomes)

  • Egg or Sperm are haploid, then when sperm and egg combine they become diploid, which is 46 chromosomes that make up another human*

During Mitosis, what structures separate?
sister chromatids (single chromosome X)

During Meiosis I and II, what structures separate?
homologous pairs
(two chromosomes hanging out together XX & XX)

How is “crossing over” beneficial to an organism?
-natural selection/evolution

what are some disadvantages that crossing over might impart to an organism?
-genetic mutation
-“bad” genes

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