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Reproduction, Continuity and Variation: CSEC Biology

Updated: Apr 19, 2020

Reproduction is the process by which living organisms produce new individual organisms or offspring. Reproduction is divided into two categories: sexual and asexual.

The most basic difference between the two types is that sexual reproduction requires the combination of reproductive cells from two individuals to form a third unique offspring. The new offspring has a corresponding new arrangement of genes. This variability in genetic information between parent organisms and their offspring in sexual reproduction is called genetic variation.

By comparison, asexual reproduction requires only one organism which creates a genetically identical 'clone' of itself. This means that asexual reproduction is conservative, producing no variation.

To help understand reproduction more, we will start to cover continuity and variation (Section C in the CSEC syllabus).


First of all, we must distinguish among several terms that will be thrown around a lot while covering this topic.

DNA- (stands for Deoxyribonuceic Acid) DNA is a molecule containing the unique genetic code of an organism. DNA is a two stranded molecule that forms a distinct double helix shape which looks sort of like a twisted ladder.

Each strand comprises long sequences of paired bases, or base pairs, which are formed when the bases (Adenine, Thymine, Guanine and Cytosine) on one strand of the DNA molecule pair with complementary bases on the other strand. These base pairs are joined by hydrogen bonds and always in the same way, i.e. Adenine (A) with Thymine (T) and Guanine (G) with Cytosine (C).


Gene- A portion of DNA within the genome that carries the information to make a molecule, usually a protein. Each gene usually corresponds to a certain characteristic or trait, like eye colour.


Chromosome- A threadlike structure in cells, made of a long DNA molecule, wrapped tightly around proteins known as histones. Humans have 23 pairs of chromosomes.

In the picture, you can see the central constriction point known as the centromere, which divides the chromosome into two arms, the shorter 'p' arm and the longer 'q' arm.

This gives the chromosome its characteristic shape.

This is a good opportunity to mention haploid cells, which contain only one set of chromosomes (known as n), or half the usual amount. In humans, these are gametes, which contain 23 chromosomes rather than 23 pairs (46). Diploid cells have two sets of chromosomes (2n), one set from each parent paired together. In humans, every cell except for the gametes are diploid and have 23 pairs of chromosomes (46).


Allele- An allele is a variant form of a certain gene. Every gene is found at a certain location on a chromosome, known as a locus. There are two copies, one copy of the gene inherited from each parent. These copies aren't always identical, and when they are different, they are known as alleles. Alleles can sometimes result in different physical traits (known as phenotypes) with certain alleles being dominant and others being recessive.


Now that those terms have been discussed in detail, we can begin to talk about mitosis, which is the main mechanism in asexual reproduction. Mitosis is a form of cell division where a single parent cell divides into two genetically identical daughter cells. Mitosis' main purpose is in growth and the replacement of worn out cells. Mitosis itself is really a part of an overall cell cycle, which is the life cycle of the cell. The phases leading up to mitosis are known as the interphase, (which comprises the G1 phase, S phase and G2 phase).


Mitosis has 5 phases:

Interphase*

Prophase

Metaphase

Anaphase

Telophase


*It is important to note that interphase consists of several phases that occur between mitotic divisions.

Interphase- The DNA in the cell is copied to prepare for the division of the cell, forming two identical full sets of chromosomes.

Prophase- The chromosomes condense into compact X-shaped structures. Each chromosome consists of two identical sister chromatids.

The chromosomes pair up so that both copies of each chromosome are together.

The membrane around the nucleus then dissolves, releasing the chromosomes.

The mitotic spindle, consisting of the microtubules and other proteins, extends across the cell between the centrioles as they move to opposite poles of the cell.

Metaphase- The chromosomes align themeselves along the metaphase plate (the equator of the cell).

Anaphase- The sister chromatids are disjointed by the mitotic spindle which pulls them opposite poles.

Telophase- At each pole of the cell a full set of chromosomes gather together. A new nuclear membrane forms around each set of chromosomes grouped at either pole of the cell to create two new nuclei.

The chromosomes uncoil and the spindle fibres disappear. The single cell then pinches in the middle to form two separate daughter cells each containing a full set of chromosomes within a nucleus. This process is known as cytokinesis.


The best way (probably) to understand the stages of mitosis is through a diagram.


Diagram of the stages of mitosis
Mitosis Diagram

(You can go to this website (yourgenome.org) and search up any terms that you are unfamiliar with. It also has a few activities and videos that can help you understand as well.)


On the syllabus, CSEC requires that you know the role of mitosis in asexual reproduction. Mitosis is the main process which allows the multiplication of the cells of asexually reproducing organisms. By repeated mitotic division of cells, the cells increase in number. The cells eventually differentiate into different kinds of tissues for the body of those organisms.

The increased number cells by repeated mitotic divisions facilitate the growth of asexually reproducing organisms.

Some plants also undergo asexual reproduction (naturally). These plants have different methods of asexual reproduction, including . budding, vegetative propagation, fragmentation and spore formation. (The next section is based on this post)

In budding, an entirely new plant is produced from a special growth known as a bud.

For example, if you keep a potato for a long time, you can notice a number of small outgrowths, which are commonly referred to as ‘eyes’.

Each of them can be planted which will grow up like a clone of an original potato plant.



Vegetative propagation is where a new plant is produced by the vegetative parts of a 'parent' plant, like the stems, roots and leaves. This can occur naturally, but it is also done artificially by horticulturalists.


Stems – Runners are the stems which usually grow in a horizontal form above the ground. They have the nodes where the buds are formed. These buds usually grow into a new plant.

Roots – A new plant is developed from around, inflamed, modified roots called tubers. Example: Sweet Potato






Leaves – In some plants, leaves detached from the parent plant can be used to grow a new plant. They demonstrate growth of small plants, called plantlets, on the edge of their leaves. Example: Bryophyllum.

Image: bryophyllum pinnatum, commonly known as 'Leaf of Life'




FAQ: Why does asexual reproduction produce genetically identical offspring?

This is simply because the offspring are literally 'clones' of the parent organism. The parent organism creates a copy of its genetic material, then, usually through mitosis, it splits itself into two organisms of the same genotype (genetic makeup).


Leading into sexual reproduction, we must first understand meiosis, which usually occurs in the formation of gametes. A gamete is a male or female reproductive cell that contains half the genetic information for an organism. In humans the gametes are the sperm and egg cells.

Said simply, meiosis is a process where a diploid cell divides twice to produce four daughter cells that have half the original number of chromosomes (such as the gametes). These cells are haploid (having one set of chromosomes).

Meiosis has 9 stages. Yep, 9 of them- divided into meiosis I and meiosis II for each of the two times that the cell divides:

Meiosis I:

Interphase

Prophase I

Metaphase I

Anaphase I

Telophase I (and cytokinesis)

Meiosis II:

Prophase II

Metaphase II

Anaphase II

Telophase II (and cytokinesis)

(Here is a useful