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.
(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 University of Mississippi article on meiosis that look at if you want more detail)
Meiosis I:
Interphase- The DNA is copied so that there are two identical full sets of chromosomes and microtubules extend from the centrosomes.
Prophase I- The copied chromosomes condense into X-shaped structures. Each chromosome now consists of two identical sister chromatids. The homologous chromosomes pair up so that both copies of each chromosome are paired together (this may seem the same as mitosis, but this is where it becomes a little different).
The pairs of chromosomes will now exchange bits of DNA in a process called recombination or crossing over to form recombinant chromosomes.
The membrane around the nucleus dissolves and releases the chromosomes. The meiotic spindle (a structure of microtubules and proteins) extends across the cell.
Metaphase I- The chromosomes line up along the equator (metaphase plate) of the cell. The meiotic spindle fibres attach to one chromosome of each pair. The arrangement of the paired chromosomes is random along the metaphase plate.
Anaphase I- The pair of chromosomes are pulled apart by the meiotic spindle fibres and pulled to opposite poles of the cell. (Notice here that the sister chromatids are not pulled apart).
Telophase I (and cytokinesis)- At each pole, a full set of chromosomes gather together. A new nuclear membrane forms around each full chromosome set to form two new nuclei. The single cell then pinches in the middle to form two separate daughter cells each containing a full set of chromosomes within a nucleus (cytokinesis).
Meiosis II:
Prophase II- Since there are now two daughter cells with 23 chromosomes (and therefore, 23 pairs of sister chromatids), the chromosomes in each now condense into compact X-shaped structures (again). The nuclear membrane dissolves, releasing the chromosome and the meiotic spindle forms again. Notice that the chromosomes are not copied before this, unlike in Prophase I.
Metaphase II- Now the chromosomes line up end-to-end along the equators of the two cells. Meiotic spindle fibres at each pole of the cell attach to each of the sister chromatids.
Anaphase II- The sister chromatids are pulled to opposite ends of the cell by the meiotic spindle. The separated chromatids are now individual chromosomes.
Telophase II (and cytokinesis)- The chromosomes complete their move to the opposite poles of the cell. At each pole of the cell a full set of chromosomes gather together.
A membrane forms around each set of chromosomes to create two new cell nuclei. Once cytokinesis is complete there are four granddaughter cells, each with half a set of chromosomes (haploid). In males, these a four sperm cells, while in females, there is one ovum (egg cell) and three polar bodies.
A diagram is useful in understanding meiosis:
FAQ: What is the importance of the halving of chromosomes in the formation of gametes?
The chromosomes must be halved so that only one set of chromosomes is in the gamete. The gamete must merge with another gamete from a male or female parent so that two sets of chromosomes (one from each of the haploid gametes from each parent) can join to form a diploid zygote with the full set of chromosomes. In short, the chromosomes must be halved so that the haploid gamete can be formed with one set of chromosomes and fulfill its function of combining with another gamete in sexual reproduction.
Meiosis also plays an important role in increasing genetic variation. To understand its role here, we must first understand the processes within meiosis that cause this variation.
It all comes down to independent/random assortment and crossing over (recombination). Another factor is random fertilization, which isn't directly related to meiosis.
Independent or random assortment is the process during (Metaphase I of) meiosis where the chromosomes move randomly to separate poles of the cell. Each gamete does end up with 23 chromosomes (in humans) after meiosis, but the reshuffling of the chromosomes that occurs in independent assortment creates unique combinations of chromosomes in the gametes, and ultimately, greater genetic variation in the offspring. In humans, there are over 8 million possible combinations of chromosomes in independent assortment.
In the above diagram, you can see the chromosomes at either pole during metaphase I in two possible configurations. (Please note that this diagram does not show the full 23 chromosomes from either parent)
Crossing over or recombination is the process during (Prophase I) meiosis where genetic material is exchanged between non-sister chromatids of homologous chromosomes*. When the homologous chromosomes line up in pairs, the genetic material from two chromatids (that aren't attached) intertwine around one another and some material from them 'switch chromosomes,' that is, the material breaks off and reattaches at the same position on the homologous chromosome.
(*Homologous chromosomes are pairs of chromosomes which carry the same genes, one from each parent. )
The diagram above shows how the recombination occurs. The following video provides a pretty entertaining animation of the chromosomes' crossover.
Random fertilization also increases genetic variation. Sexual reproduction is the random fertilization of a gamete from the female using a gamete from the male. So, considering that there are over 8 million possible combinations/assortments for both the male and female gametes (in humans), when an ovum is fertilized by a sperm cell, there are over 64 trillion unique combinations.
This immense genetic variation is the cause of most of the benefits of sexual reproduction.
The benefits of sexual reproduction include:
It produces genetic variation in the offspring.
The species can adapt to new conditions because of the genetic variation, giving it a survival advantage.
A disease is less likely to affect the entirety of a population.
Moving into the next section, where we will deal more with genetic traits and phenotypes, we must first define some related terms.
Dominant trait- Traits occur due to a corresponding allele (version of a gene). Dominant alleles show their effect even if the individual only has one copy of the allele (also known as being heterozygous). For example, the allele for brown eyes is dominant, therefore you only need one copy of the 'brown eye' allele to have brown eyes.
Recessive trait- Recessive traits only show their effect if the individual has two copies of the corresponding allele (also known as being homozygous). For example, the allele for blue eyes is recessive, therefore to have blue eyes you need to have two copies of the 'blue eye' allele.
Co-dominance- This is when both alleles are dominant, so they are expressed equally. An example of this is a person of blood type AB, which is the result of co-dominance of the A and B dominant alleles.
Genotype- This is the genetic makeup of an organism or cell. The term can also be used to refer to the alleles, or variant forms of a gene, that are carried by an organism.
Humans are diploid organisms, which means that they have two alleles at each genetic position, or locus, with one allele inherited from each parent. Each pair of alleles represents the genotype of a specific gene. A particular genotype is described as homozygous if it features two identical alleles and as heterozygous if the two alleles differ.
Phenotype- This is the observable physical appearance of an organism. The phenotype of an organism is defined by its genotype. Phenotypes include height, hair colour and eye colour.
Homozygous- This means having two of the same allele. (Homo-:same -zygous: related to the zygote, i.e. the combination of two parental gametes)
Heterozygous- This means having two different alleles. (Hetero-: different)
Understanding these concepts allows us to now discuss the inheritance of traits.
As a result of meiosis, the gametes contain only one chromosome from each homologous pair. Hence, they can only contain one allele from each pair, and therefore only one allele from each of the organism's parents.
The traits inherited therefore depend on which alleles are dominant or recessive.
For example, if we consider the allele activating melanin production (N), which is a dominant allele, and the allele for albinism (n), which is a recessive allele, we know that there are only three possible combinations, Nn (heterozygous)*, NN (homozygous dominant) and nn (homozygous recessive). We also know that in the heterozygous (Nn) and homozygous (NN) dominant, we would observe normal melanin production and hence, normally pigmented skin, hair and eyes. In the homozygous recessive (nn) however, the phenotype observed would be albino (with pale skin, light hair and pale blue eyes).
However, if you remember the concept of co-dominance previously mentioned where both alleles are dominant, you will realize that there are still three possible combinations, These would be the homozygous dominant, the heterozygous dominant, and the homozygous dominant again.
*Please note that if an organism is heterozygous, it is said to be normal, but a carrier of a recessive allele.
An example of this is in sickle cell anemia. In this disease, a person contains abnormal haemoglobin S instead of the normal haemoglobin A. Haemoglobin A production is stimulated by the normal allele HbA while Haemoglobin S production is stimulated by the abnormal allele HbS. These alleles exhibit co-dominance, since neither dominates the other allowing both to have an influence on the organism.
There are therefore three possible combinations again:
HbA+HbA: Here, there is 100% Haemoglobin A produced, and the phenotype is normal.
HbA+HbS: Here, 55-65% Haemoglobin A and 35-45% Haemoglobin S is produced, and the phenotype exhibits sickle cell trait. In sickle cell trait, no symptoms are observable until low oxygen concentrations occur (such as during strenuous physical exericise), at which point symptoms of sickle cell anemia may develop.
HbS+HbS: There is 100% Haemoglobin S produced, The phenotype is sickle cell anemia. The symptoms include anemia and increased susceptibility to infections and jaundice.