The Biology of Reproduction

Every organism must reproduce (create offspring) in order to pass a part of themselves on into future generations. Depending on the organism, it can either reproduce using sexual or asexual reproduction, which both involve cell division. The parental traits of sexually reproducing organisms will also mix to form a unique combination in the offspring, and an asexually reproduced organism will inherit all of its genetic material from the parents. 

Reproduction is the process by which organisms create offspring and therefore replicate themselves in future generations. It can happen both sexually, with two parents contributing genetic material, and asexually, or ‘without sex’- one individual passing all their genetic material on to a child. Unless spontaneous changes during reproduction, referred to as mutations, occur, the organism will receive all of its traits from its parents.1 Thus, an organism is always the offspring of at least one parent, and inherited all of its genetic material from those who created it through the process of reproduction.

Sexual reproduction involves intercourse, a process in which one organism (usually a male) inserts their own genetic material into another organism (usually a female). The  genetic material from both parents combines and eventually results in the formation of offspring. As this offspring grows, their development in the womb is referred to as pregnancy. 

Mendelian Genetics:

When two organisms reproduce through sexual reproduction, the offspring display traits from both parents. For example, one’s hair color is often similar to their parents’ or similar to another relative. This is because of inheritance, or the passing on of traits, which was discovered by Gregor Mendel in the 1800s.2 Before Mendel’s realization, farmers used these principles without realizing it in a process called artificial selection, or human-caused changes in a population.3 They selected plants with desirable traits and bred them together to create offspring with a mix of those same favorable attributes. 

In fact, Mendel worked with pea plants to come up with his laws of inheritance. Mendel chose plants with differences in a trait such as stem length, pea shape, or flower color. He noticed that if he mixed two plants with opposite traits, such as wrinkled and round pea shape, then all of their offspring would be round.2 However, when he bred this offspring with itself, he noticed that on average 1/4 of the second generation of plants expressed the “wrinkled” trait, which had supposedly disappeared during the previous generation. From this observation, he realized that genes—or the units of heredity—are made up of different combinations of the parents’ genes.2 

 

The Proliferation of Traits:

There exists both a dominant, or trait that is expressed (visible) regardless of the other trait, and a recessive trait, which is hidden unless present in both sets of genes. These traits are expressed in combinations of either homozygous(both) dominant, heterozygous(other), or homozygous recessive. Homozygous means that the traits are either both of the dominant variety, or both of the recessive variety. In either combination, that trait will be fully expressed as both copies are the same. Heterozygous means that the traits are different—for example, one dominant and one recessive, in which case the dominant will be expressed in the organism. Generally, the dominant trait is expressed whenever it is present in an organism (it will show up whether the organism is homozygous dominant or heterozygous) and the recessive trait is only displayed when the organism is homozygous recessive.2 While it may seem as though an organism would be unable to pass a trait that they do not possess onto their offspring, it can actually be “hidden” in a heterozygous combination and therefore show up in a later generation. 

What are Genes Made of?

Thus, during sexual reproduction, the parents’ traits mix to form a unique combination in their child. A substance present in the parent’s cells mixes to create the genes that are eventually expressed in the offspring. This substance exists inside every part of an organism’s body and is referred to as deoxyribonucleic acid(DNA).4 The DNA gives the cell information and contains the instructions to make the proteins that allow the body to function. DNA is a hereditary material, being passed down from previous generations, and therefore a child will receive pieces of it from both parents. The DNA is twisted into a shape called a double helix, which looks similar to a ladder.4 These long strands of DNA form objects called chromosomes.5 Chromosomes consist of  segments of DNA, each of which encode different traits for the organism. The chromosome itself consists of DNA coiled around proteins known as histones. However, chromosomes are usually extremely loosely coiled and therefore are almost impossible to see until they condense, or wrap tightly around the histones. This condensation in shape occurs during cell replication.5 

 

How do Individual Cells Reproduce?

Organisms are made up of small units called cells, which are able to create copies of themselves to allow the organism to do things such as grow or repair itself. Reproduction of an organism occurs through a process of cell replication that is either called mitosis or meiosis. Meiosis occurs for the purpose of sexual reproduction (which requires input from two individuals), and creates cells called gametes(the egg and the sperm). The egg is the female reproductive cell, and it combines with the sperm (male reproductive cell) to form a combination of the parent cells that will later become an organism.6 During meiosis, one cell divides into four daughter cells. Each of these daughter cells contains half the chromosomes of the original cell.6

There are 9 stages of meiosis, which occurs in two sets of cell division being denoted by either a I or a II.

  • Meiosis I is the first set of cell division that happens during the creation of sex cells.
    • Interphase
      • The chromosomes’ DNA is copied, so that there ends up being two of each chromosome.
    • Prophase I
      • The newly copied chromosomes condense into an “X” shape, with each half of the chromosome containing the same genes as the other half. Then they pair up and swap genes with the other through a process called recombination. Then the covering around the chromosomes disappears, and they are freed into the body of the cell. 
    • Metaphase I
      • The released chromosomes line up in the middle of the cell, and fibers attach to each of them. 
    • Anaphase I
      • The pairs of individual Xs are pulled apart by these fibers, but the individual X-shaped chromosome stays intact(unlike in the second set of meiosis). 
    • Telophase I and Cytokinesis
      • The chromosomes are pulled to opposite ends of the cell, with one of each chromosome ending up on each half of the cell. Then each full set of chromosomes is enclosed by the cell. The cell is then split into two pieces in a process called cytokinesis, with one set of 23 chromosomes within each new cell. 
  • Meiosis II is the second set of cell division that occurs in the creation of sex cells.
    • Prophase II
      • Both new cells only have 23 chromosomes, which are released into the body of the new cell. 
    • Metaphase II
      • In each cell, the chromosomes line up through the middle of the cell, and fibers attach to each half of the X-shaped chromosome. 
    • Anaphase II
      • Each half of the X is pulled to one end of the cell, turning them into individual chromosomes. 
    • Telophase II and Cytokinesis
      • The chromosomes move to opposite ends of the cell, and each set of 23 is then packaged. The cell then undergoes cytokinesis again, resulting in four cells with 23 chromosomes each.6

Humans usually have 46 chromosomes in each cell, so those created through meiosis are unique in that they only have half a set. This is because when parents reproduce to create offspring, those offspring receive half a set from each parent which combine with the other. A cell with 23 chromosomes is called haploid, while one containing 46 is called diploid. Humans’ body cells are normally diploid, and the sex cells are unique in that they are haploid.6

Fertilization:

The process of dividing a cell into 4 haploid sex cells through meiosis is called gametogenesis. This is because the cells created through meiosis are called gametes.7 The male sex cell is called a sperm, and the female sex cell is called an egg. These are both referred to as gametes, or haploid sex cells.7 In a process called fertilization, the sperm fuses with the egg and the combination of the two creates a diploid cell with chromosomes from each parent. This fusion of gamete cells is called a zygote.8 The zygote will then begin to divide, eventually producing a cluster of cells called a blastocyst.8 The cells will continue to divide through mitosis and eventually result in an organism that possesses a unique combination of traits from both parents. 

Asexual Reproduction:

Asexual reproduction does not involve meiosis, instead, it involves mitosis, the type of cell division that all non-gamete body cells go through. Mitosis involves only one set of cell division, and produces diploid cells identical to the parent cell. Additionally, because there is no new genetic material that appears in the offspring, asexual reproduction essentially produces clones of the parent. The cells that are produced in asexual reproduction are diploid, and contain two sets of the original organism’s chromosomes. This type of reproduction commonly occurs in plants and invertebrates. It can reproduce an entirely new, unique individual, or work to help an organism grow. Instead of an egg being fertilized by the sperm, asexual reproduction can occur through a process called budding, in which the offspring forms out of the parent’s body. This process allows the parent to reproduce many times with little effort and greater speed. Because these offspring are genetically identical, these populations may have greater susceptibility to disease.Asexual reproduction functions to produce offspring that are genetically identical to the parent, and thus does not provide the variety and change in population that sexual reproduction through meiosis does.

References

  1. “Mutations.” Understanding Evolution. University of California Museum of Paleontology. 20 April 2016.
  2. "Mendelian Genetics." knowgenetics.org. Genetics Generation, 2015. 16 April 2016.
  3. “Artificial Selection.” Understanding Evolution. University of California Museum of Paleontology. 20 April 2016.
  4. "What Is DNA?" Genetics Home Reference. US National Library of Medicine. 16 Apr. 2016.
  5. "What Is a Chromosome?" Genetics Home Reference. US National Library of Medicine. 16 April 2016.
  6. “What is meiosis?” yourgenome.org. Wellcome Genome Campus. 20 April 2016.
  7. “Gamete.” biology-online.org. Biology Online. 20 April 2016.
  8. “Fetal Development.” University of Maryland Medical Center, 2016. 20 April 2016.
  9. “Asexual Reproduction.” Understanding Evolution. University of California Museum of Paleontology. 20 April 2016.

Last Updated 8 May 2016.

Category: