HER - Reproduction and Meiosis [LESSON]

Reproduction and Meiosis

Types of Reproduction

There are two major forms of reproduction: asexual and sexual.

Asexual reproduction requires only one parent.

There are many types of asexual reproduction. The major advantage of this type of reproduction is that it is much faster than sexual reproduction and does not require finding a mate. Four major types are:

1. 1. Binary fission: Single parent cell doubles its DNA, then divides into two cells. Usually occurs in bacteria.
   2. Budding: Small growth on surface of parent breaks off, resulting in the formation of two individuals. Occurs in yeast and some animals.
   3. Fragmentation: Organisms break into two or more fragments that develop into a new individual. Occurs in many plants, as well as some animals (like coral, sponges, and starfish).
   4. Parthenogenesis: An embryo develops from an unfertilized cell. Occurs in invertebrates, as well as in some fish, amphibians, and reptiles.

Sexual reproduction requires two parents. Each parent contributes a gamete - a sex cell that has half of the normal DNA of a regular body cell. In males, the gametes are sperm and in females, the gametes are eggs. When these two gametes combine during fertilization, the result is a zygote, which then continues to develop into an embryo.  The major advantage of this type of reproduction is increased genetic variety which allows survival of the species against a threat.

Every species has a specific number of chromosomes in its cells. Human cells contain 46 chromosomes, paired up as two sets of 23. The number 46 is a human's diploid number. Their cells contain two homologous sets of chromosomes, 23 chromosomes from the male parent that correspond to 23 chromosomes from the female parent.

The cells involved in sexual reproduction, called gametes, contain half the number of chromosomes as diploid cells. These haploid cells contain a single set of chromosomes to pass on to their offspring. The process for producing gametes, meiosis, reduces the diploid number of chromosomes by half to produce haploid sperm and egg cells. The haploid number (abbreviated n) for a human is 23. This means 23 chromosomes are in the male sperm and 23 chromosomes are in the female ova.

Diploid and haploid numbers are represented by the algebraic symbols 2N and N respectively. The diploid cell of a given organism always contains twice the number of chromosomes as the haploid cell, so if we know one number, we can determine the other. For example, if an organism's haploid number (N) is 4, the diploid number (2N) is 2 × 4, or 8. If the diploid number is 20, the haploid number is 20/2, or 10.

Through sexual reproduction, haploid cells from each parent fuse together in a process called fertilization. The union of these gametes forms a diploid zygote. It is important that the resulting gametes only have one set of chromosomes to prevent the doubling of chromosomes when two gametes come together during fertilization. If the chromosomes were doubled, the type of species created would be different from the parent species.

Try the learning activity below by classifying the description box into Sexual or Asexual in the sorting activity.

Meiosis

The purpose of meiosis is to produce gametes or sex cells.  Meiosis is the first part of sexual reproduction and the second is fertilization which is the joining of the gametes. During meiosis, four daughter cells are produced, each of which is haploid (containing half as many chromosomes as the parent cell). 

Watch the Meiosis Part 1 video below to learn about the basics of meiosis.

Meiosis contains two separate cell divisions, meaning that one parent cell can produce four gametes (eggs in females, sperm in males). In each round of division, cells go through four stages: prophase, metaphase, anaphase, and telophase.

Before entering meiosis I, a cell must first go through interphase. This is the same interphase that occurs before mitosis. The cell grows, copies its chromosomes, and prepares for division during the G1, S, and G2 phases of interphase.

Meiosis I

Meiosis I is the first round of cell division, in which the goal is to separate homologous pairs.

During prophase I, differences from mitosis begin to appear. As in mitosis, the chromosomes begin to condense, but in meiosis I, they also pair up. Each chromosome carefully aligns with its homologue partner so that the two match up at corresponding positions along their full length. During metaphase I, homologue pairs—not individual chromosomes—line up at the metaphase plate for separation. In anaphase I, the homologues are pulled apart and move apart to opposite ends of the cell. The sister chromatids of each chromosome, however, remain attached to one another and don't come apart. Finally, in telophase I, the chromosomes arrive at opposite poles of the cell. In most organisms, the nuclear membrane re-forms and the chromosomes decondense as a cytokinesis event typically occurs between meiosis I and meiosis II.

Meiosis II

Cells move from meiosis I to meiosis II without copying their DNA. Meiosis II is a shorter and simpler process than meiosis I, and you may find it helpful to think of meiosis II as “mitosis for haploid cells." The cells that enter meiosis II are the ones made in meiosis I. These cells are haploid—have just one chromosome from each homologue pair—but their chromosomes still consist of two sister chromatids.

During prophase II, chromosomes condense and the nuclear envelope breaks down if needed. The centrosomes move apart, the spindle forms between them, and the spindle microtubules begin to capture chromosomes. The two sister chromatids of each chromosome are captured by microtubules from opposite spindle poles. In metaphase II, the chromosomes line up individually along the metaphase plate. In anaphase II, the sister chromatids separate and are pulled toward opposite poles of the cell. In telophase II, nuclear membranes form around each set of chromosomes, and the chromosomes decondense.

Cytokinesis splits the chromosome sets into new cells, forming the final products of meiosis: four haploid cells in which each chromosome has just one chromatid. In humans, the products of meiosis are sperm or egg cells. There are four sperm produced in humans but just one egg (ovum) per round of meiosis.

Try the Meiosis Matching. First, match the phases to the correct event, then click the arrow at the bottom of the activity.

Meiotic Processes

There are two processes that occur during meiosis that ensure that the genetic material is mixed up as well as possible. It ensures that even from the same organism, no two gametes are the same.

Are all of the gametes in the same parent genetically the same?

NO – meiosis ensures that every gamete is unique.  Think about the fact that siblings are genetically unique from each other even if they share the same parents (identical twins would be an exception to this).

Watch the Meiosis Part 2 video below to learn more about these very important processes.

 

Crossover

Crossover (or crossing over) occurs when the two homologous pairs of chromosomes align on top of each other to exchange DNA at different points of attachment, each called a chiasma.  There are multiple chiasmata on each homologous pair (as much as 25 points per chromosome). Remember, crossover exchanges alleles but within the same set of genes.

For example, in the image, the letters A, B, and C represent genes found at particular spots on the chromosome, with capital and lowercase letters for different forms, or alleles, of each gene. The DNA is broken at the same spot on each homologue—here, between genes B and C—and reconnected in a crisscross pattern so that the homologues exchange part of their DNA. 

Independent Assortment

When the homologous pairs line up at the metaphase plate, the orientation of each pair is random. For instance, in the meiosis I image above, the pink version of the big chromosome and the purple version of the little chromosome happen to be positioned towards the same pole and go into the same cell. But the orientation could have equally well been flipped, so that both purple chromosomes went into the cell together. This allows for the formation of gametes with different sets of homologues.

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