Usually, in everyday speech, we may use certain words interchangeably but in science words must have precise meaning in order to be scientifically useful. Two such words I’m referring to are “replication” and “reproduction” and at first these words may mean alike, that is both deal with increase in numbers.
However, in biology, these words refer to two different processes that occur in different levels. Indeed these terms do refer to increase in numbers but when you focus on a feature of life such as a cell, replication and reproduction are actually two different processes in that one occurs at the molecular level which is replication, and reproduction occurs at the level of the cell, whether it is an unicellular organism such as a bacterium or amoeba or a whole, multicellular organism such as a cat.
Now that we have a feel for the differences, let use look into detail of the two different processes of replication and reproduction.
All life on earth share a common biochemistry such as the twenty amino acids which form into proteins which can perform a variety of biochemical processes needed for the survival of life forms and a genetic code where the sequences of DNA specify an amino acid needed to form a protein. To survive, the instruction for building a cell must be passed on to the next generation of life forms, and this is true for all species of life, small and large.
Ever since 1953, with the discovery of the double helix of DNA, it was now possible to explain how traits for survival could be passed on and if we focus on the molecular level, that is the level of DNA, can we then see the difference between replication and replication.
What exactly is replication and what does it have to do with molecular systems such as DNA?
Replication involves making an exact copy or replica of a pre-existing template. This requires the original template plus components needed to be arranged to take an exact copy of whatever the template is.
Of all the biological molecules, it is the DNA or deoxyribonucleic acid that has this capability. DNA consists of nucleotides and these are the building blocks of DNA. A nucleotide consists of three parts; a phosphate group bonded to a ribose, a five carbon sugar, which is bonded to one of four nitrogen containing bases called adenine, thymine, cytosine, and guanine.
A nucleotide can bind to another nucleotide in what is called a phosphodiester bond and so along one side of a DNA molecule, is a uniform sequence of phosphate and riboses but a base can form a bond called a hydrogen bond to another base but each base can pair with a specific base. An adenine, which is one of the nitrogen containing bases that I’ve mention in the previous paragraph, can bond to a thymine, and a guanine can bond to a cytosine.
When one polynucleotide or the many nucleotides in one strand of DNA meets another polynucleotide, the two strands, along with the specific base pairs, will wind around each forming the double helix.
The fact that DNA is a double helix allows it to replicate and in the sequence of bases which is the part of the DNA molecule that is variable, contains all the information for the synthesis off all the various kinds of proteins including the proteins that are needed to help the double helix make more double helices.
To do that, we must understand how this is done, even though in reality it is a complex process, but I will only present a simplified version of how DNA replicates and to show why replication is something that occurs at the molecular level.
Recall that to make a replica requires a template and a set of components. For DNA, the template is the sequence of bases present in the original DNA molecules and since the molecule is occurring in the basic fundamental unit of life, the cell, there are plenty of nucleotides available.
Here is where replications comes into play. To replicate a DNA molecule, first a set of enzymes or protein molecules that speed up biochemical reactions are needed. One such enzyme involved in DNA replication is aptly named the DNA polymerase and it’s function is to help DNA replicate into two DNA molecules.
Once the DNA polymerase is attached to the DNA, it begins to unravel apart the polynucleotides and carefully the enzyme takes in surrounding nucleotides and with each nucleotide, the enzyme carefully pairs the nucleotide along with each base to one of the bases in the template strand. For example, if the DNA polymerase encounters a thymine, it will select an adenine and pair and if it finds a cytosine then it will pair it with a guanine.
The DNA polymer continues in this fashion until another DNA molecule with the sequence from the original template strand is completed.
Thus we see that replication such as the DNA polymerases and DNA is something that occurs at the molecular level and this fulfills the definition of replication. The template in this case is the sequence of the parent DNA and the components are the available nucleotides. Indeed this process of molecular replication can continue indefinitely, even though some errors in base pairs do occur, but the DNA polymerase is very efficient that in a billion replications only one DNA molecule may have a base pair slightly different than the original.
Reproduction, generally defined, involves producing likeness from the original, just like the sequence of nucleotides from the original polynucleotide sequence but this is something that occurs in a whole and complex system and it is much more involved than molecular replication.
Reproduction then occurs at the fundamental unit of life and what exactly the fundamental unit of life for reproduction to occur? That would be the cells and all processes of life such as reproduction occur at the level of the cell.
A cell is enclosed by a thin plasma membrane and inside the cell is the cytoplasm, a jelly like water based substance with all the molecules of life such as amino acids for proteins, nucleotides for DNA and a similar nucleic acid, RNA, as well as glucose for sugars and fatty acids for fats.
Cells come in two main forms. There are the prokaryotic cells and these are cells that only have a cell membrane and another outer covering called a cell wall that is made out of a protein carbohydrate structure and inside a prokaryotic cells are all the molecules of life and the DNA is arranged in a localized region of the cell in the form of chromosomes. The other kind of cell are the eukaryotic cells and these are more complex than prokaryotes in that a eukaryotic cell consists of a nucleus which is the region of the eukaryotic cell that houses the cell’s genetic information and the chromosomes are bound by proteins called histones. The inside of the eukaryotic cell has a variety of subcellular structures called organelles which carry out various kinds of biochemical functions. There are the mitochondria which are found in plant and animal cells and are both eukaryotes which release biochemical energy in a process called metabolism. There are the lysosomes which house enzymes to degrading parts of the cell, the Golgi bodies which package proteins for transport out of the cell, as examples.
Reproduction then occurs at the cellular level for all the biological molecules of many kinds making up the cell are involved. To make an two cells, for example, requires the cell membrane to pinch into two and at the same time both of the cells receives an equal amount of cytoplasm and an equal amount of all the molecules.
The new cells then are alike from the original cells but like replication it is not a perfect process and indeed there can be a new cell that is slightly different than the original but it will in some respects resemble the original cell.
Since there are two main kinds of cells, the process of reproduction will be different. In prokaryotes, they reproduce by binary fission and this is a “simple” form of cellular reproduction where both the cell membrane and cell wall split in two while at the same time the cytoplasm along with all the molecules and the chromosomes are parceled into two new cells, resulting in two cells, identical to the original more or less.
In eukaryotes a much complicated form of cellular reproduction called mitosis is where cell divide in a controlled step by step fashion, In mitosis, the chromosomes in the nucleus begin to replicate (at the molecular level of DNA) and condense while the nuclear membrane breaks apart. There are specialized organelles called centrosomes that produce protein strand that attach to each chromosome and there are two sets of centrosomes at both ends.
An eukaryotic chromosome consists of two pairs of chromosomes and each centrosomes attaches its protein strand to each end of each chromosome. The whole cell then divides at the cell membrane and as this division continues two new cells with identical chromosomes as well as all the molecules and organelles are partitioned into two daughter cells.
As you can see, there is indeed a difference between replication which only occurs at the level of molecules while reproductions requires the whole cell.
Also in eukaryotic cells, reproduction is taken to another level and there is another form of cell division called meiosis. Like mitosis, this involves cell division but there is a huge and crucial difference.
In meiosis, a cell has a pair of chromosomes called the maternal and paternal chromosomes and in the process of meiosis, each pair of chromosomes pairs with another pair and undergoes what is called recombination where parts of one of the chromosomes exchange another part from the other pair resulting in a different combination. These chromosome hybrids are then distributed into other cells but the number of chromosomes is halved, unlike in mitosis where the exact number of chromosomes is identical to the parent cell.
The result is cells called gametes which have half the number of chromosomes and only in meiosis is where recombination and halving of chromosomes and this kind of cellular division is only present in multicellular eukaryotic organisms.
I’ve mentioned that in both reproduction and replication, these are imperfect processes and a different kind of resultant cell can form. This kind of change is known in biology as a mutation and a mutant cell, or multicellular organism may be different than its parents and if this difference, affects the survival of the organism in such a way that it would have an advantage in comparison to those offspring that are identical, it may be able to survive in a varying environment and will end passing its new survival characteristics to its offspring.
This is the basis of Darwinian evolution and this depends on the reproductive process being imperfect.
Replication occurs at the molecular level while reproductions involves the organism as a whole. Of course in a reproducing cell, the molecules have to replicate whether in binary fission or mitosis in order for the two cells to receive (almost) identical copies of molecules. Replication and reproduction, both different ways of increasing numbers, are involved in allowing life to propagate, survive, and adapt.
Fleishaker, G.R (1994).Self-Production of Supramolecular Structures (eds.). The Netherlands: Kluwer Academic Publishers.