Meiosis

Meiosis Overview

• Homologous chromosomes
  • Greek word derivation:
    • Homologos: agreeing
    • Chroma: color
    • Soma: body
    • “Colored bodies that are the same”
  • Almost all eukaryotic organisms have this arrangement—chromosomes have a “buddy,” i.e. they occur in homologous pairs.
  • These are chromosomes which are similar but not identical.
    • One chromosome came from the mother, and the other in the homologous pair came from the father.
    • They code for the same genes (i.e. the same gene loci occur), but may represent different alleles.
• Sister chromatids
  • Identical copies of a chromosome.
  • Initially, they are joined at the centromere.
• The goal of meiosis: reduce the chromosome number by half to create gametes.

Figure 13.5, page 231, Campbell's Biology, 5th Edition

Two Types of Reproduction

• Asexual Reproduction (via mitosis)
  • One cell produces two identical cells.
  • Simply duplicate the chromosomes and divide.
• Sexual Reproduction (via meiosis)
  • Two organisms combine their genetic material to produce a new cell.
  • More complex:
    • Chromosome number has to be reduced by half.
    • Find a partner and combine genetic material to make offspring.
    • You can't do it yourself!

Two Types of Cells

• Somatic cells
  • From Greek: soma means body.
  • 99.9 % of all cells in an organism
  • They make up the body of the organism.
  • Reproduce by mitosis.
• Reproductive cells (gametes)
  • From Greek: gamos means marriage.
  • There are two types of gametes:
    • Eggs
    • Sperm
  • Only found in special organs that produce them. In humans, these are:
    • Ovaries (eggs)
    • Testes (sperm)
  • Reproduce by meiosis.

Chromosomes Come in Pairs

• In most (but not all) organisms, chromosomes come in pairs.
• Thus, an organism will always have an even number of chromosomes.
• These pairs are known as homologous chromosomes.
  • See above for more details.
  • Every chromosomes has a buddy that is similar but not identical.
  • We can think of it as redundancy in the system—two copies of everything just in case it is needed.
• Example:
  • Lets say we have 4 chromosomes.
  • We immediately think “2 homologous pairs.”
  • In this case each chromosome in the pair arises from duplication, so duplicated chromosomes pair up into tetrads.
    • There are two tetrads (or homologous pairs) in the picture below.
    • The formation of tetrads occurs in prophase I of meiosis, shown here.

Unknown source

Diploid and Haploid

• Diploid Cell (2n)
  • A cell containing two sets of chromosomes (2n), one set from each parent
  • Each chromosome is present with its homologue (buddy).
  • The human diploid number of chromosomes is 2n = 46.
    • Note that it is an even number.
    • It has to be, because it is 2n and n is a whole number.
  • All somatic cells (body cells) have a diploid number of chromosomes.
• Haploid Cell (n)
  • A cell containing only one set of chromosomes (n)
  • Each chromosome's homologue is not present.
  • The human haploid number of chromosomes is n = 23.
  • All reproductive cells (gametes) have a haploid number of chromosomes.

The Human Life Cycle

• Meiosis
  • Occurs in the testes and ovaries.
  • Reduces chromosome number by half.
    • Diploid to haploid
    • 46 (2n) to 23 (n)
  • The resulting sperm or eggs have a haploid number of chromosomes (n = 23).
• Sex: fertilization brings the chromosome number back to the normal diploid number (2n = 46).
• Mitosis
  • Once the egg is fertilized, the first cell is formed—called a zygote.
  • This zygote now begins a process of mitotic cell division.
  • This form of cell division happens trillions of times during your life.

Figure 13.3, page 230, Campbell's Biology, 5th Edition

Meiosis

The exciting and unique events of meiosis that require explanation occur in prophase I.

• Chromosomes appear, and one can see that they have duplicated because each chromosome is a pair of sister chromatids.
• Homologous chromosomes are paired. This is called synapsis.
• The pair of homologous chromosomes in their duplicated state (as sister chromatids) is called a tetrad since there are four chromatids.
• Chromatids within the tetrads cross-over between the two chromosomes.
  • They form chiasmata (Greek for “cross”).
  • This allows them to exchange DNA with each other.
  • It also helps hold the tetrads together.

Part of figure 13.7, page 234, Campbell's Biology, 5th Edition

Meiosis vs. Mitosis Comparison

This web page contains an animation comparing the two processes.

Figure 13.7, page 234, Campbell's Biology, 5th Edition

Clarifying Remarks

• Take n to be the number of homologous chromosomes pairs in a normal body cell (e.g. in humans, n = 23).
• What can be confusing about meiosis is that at different times, the number of chromatids per a chromosome changes.
• Diploid and haploid are terms that apply only to the cell—chromosomes are not diploid or haploid!
  • One way to get them straight is to think in terms of the number of distinct “things” present:
    • Remember that sister chromatids are identical copies: they do not affect the number of distinct “things.”
    • Remember that homologous chromosomes are similar but not identical: they do affect the number of distinct “things.”
  • Haploid means there are n distinct “things” present: n nonhomologous chromosomes, or n nonhomologous chromosomes with sister chromatids.
  • Diploid means there are 2n distinct “things” present: n homologous chromosome pairs (thus 2n total), or n homologous chromosome pairs with sister chromatids.