32 DNA Replication
Cells divide for a number of different reasons:
- Growth – as a baby grows into an adult, new cells must be created so the organism can increase in size.
- Repair – if you cut yourself, new cells must be creased to heal the wound.
- Replacement – you are constantly shedding skin cells, so new cells must be created to replace the ones that are lost.
- Reproduction – egg cells and sperm cells need to be produced in order for sexual reproduction to take place.
When a cell divides, it is important that each daughter cell receives a copy of the DNA. This is accomplished by the process of DNA replication. The replication of DNA occurs before the cell begins to divide into two separate cells.
The discovery and characterization of the structure of the double helix provided a hint as to how DNA is copied. Recall that adenine nucleotides pair with thymine nucleotides, and cytosine with guanine. This means that the two strands are complementary to each other. For example, a strand of DNA with a nucleotide sequence of AGTCATGA will have a complementary strand with the sequence TCAGTACT (Figure 32.1).
Figure 32.1 Image Description
The image shows a simplified diagram of a flattened DNA double-helix, which is shaped like a ladder. The outside edges of the ladder are made up of 2 blue arrows, one with the arrowhead pointing up and one with the arrowhead pointing down. The flat ends of the arrows are labeled 5′ and the arrowheads are labeled 3′. The “rungs” of the DNA ladder are made up of colored boxes. A red box labeled “A” connects to a yellow box labeled “T”. A blue box labeled “G” connects to a green box labeled “C”. 8 pairs of A-T and C-G bases are shown in the ladder.
Because of the complementarity of the two strands, having one strand means that it is possible to recreate the other strand. This model for replication suggests that the two strands of the double helix separate during replication, and each strand serves as a template from which the new complementary strand is copied (Figure 32.2).
During DNA replication, each of the two strands that make up the double helix serves as a template from which new strands are copied. The new strand will be complementary to the parental or “old” strand. Each new double strand consists of one parental strand and one new daughter strand. This is known as semiconservative replication. When two DNA copies are formed, they have an identical sequence of nucleotide bases.
Video Transcript
It might be hard to believe, but at the very start of your life you were a single, microscopic cell called a zygote. Your body now contains millions of cells, which all came about by the process of cell division.
Cell division happens when one cell divides to form two cells and it is the basis of growth and repair in humans. It is also used for asexual reproduction in organisms like bacteria.
But, a single cell cannot just split itself in half to form two whole new cells – they would essentially only be half a cell. Before the cell divides everything inside needs to be copied – this includes all the parts of the cell like the mitochondria, as well as the chromosomes inside the nucleus.
A human body cell contains 23 pairs of chromosomes – 46 altogether. During cell division, the DNA in the chromosomes is copied, or replicated to form 46 pairs (92), double the number of chromosomes. These pairs split apart when the cell divides to form two new daughter cells, each having the correct number of chromosomes. The splitting apart of the chromosome pairs happens during a stage in cell division called mitosis.
Let’s take a closer look at DNA replication.
DNA is made up of many nucleotides, each containing a base represented by the letters A, T, C, and G.
A piece of DNA contains two strands of nucleotides, twisted together to form a double helix. The strands are complementary, this means that whenever there is an A in one strand, it will be joined to a T in the opposite strand, and whenever there is a C, it will be joined to a G.
If you are not familiar with the structure of DNA then take a look at this video. [What is DNA?]
During DNA replication, the double helix unwinds. Then, an enzyme called DNA helicase unzips the DNA so the two strands are separated. This happens at several points along the DNA.
An enzyme called DNA polymerase attaches itself to the DNA strands and is used to add complementary free nucleotides to the now exposed bases on both strands. A pairs with T, and C pairs with G. This forms two DNA molecules, each of which have one brand new strand, and one from the original DNA.
These two strands twist to form a double helix. The two new DNA molecules are both identical to the original DNA molecule.
In some ways, DNA replication is similar to transcription, a process that happens during protein synthesis.They both use enzymes to join free nucleotides together and they both take place in the nucleus. But there are important differences: In DNA replication, each DNA strand is used as a template to make two new identical DNA molecules. In transcription, the coding strand of the DNA molecule is used to produce a single strand of RNA.
So, to recap. In this video you have learnt why DNA needs to be replicated before a cell divides and how this takes place.
References
Unless otherwise noted, images on this page are licensed under CC-BY 4.0 by OpenStax.
OpenStax, Concepts of Biology. OpenStax CNX. May 18, 2016. http://cnx.org/contents/s8Hh0oOc@9.10:2ousESf0@5/DNA-Replication
