- Introduction to DNA
- What is DNA?
- Where is DNA found?
- What does DNA look like?
- Where does DNA come from?
- Why do cells divide?
- How do cells divide?
- How does DNA control cellular functions?
- DNA abnormalities
- Genetic disease
- Take home messages
Humans are made up of billions and billions of cells, all these cells working together in order maintain life. All our organs are made up of different types of cells, for example our largest organ, the skin, is made up of skins cells that are all tightly adhered together to function as a protective barrier for the rest of our body. The cells of the skin have different functions and characteristics than cells that make up our heart, or the ones that line our stomach. It is the specific functions of cells that allow us to live.
So how do these cells perform certain functions? How does a new cell “know” that it is a skin cell and not a brain cell? And once it knows it is a skin cell how does it know how to perform functions that skin cells need to do? And how does it know if it has been damaged by the sun and needs to be replaced by another skin cell?
Well cells do not have a brain, so they do not “know” anything, they instead are controlled by a very important chemical; deoxyribonucleic acid, or as it is better known, DNA. Every characteristic, quality, function, appearance and location of a cell is determined by DNA, which means that every characteristic, quality, function and appearance of our body is also determined by our unique DNA.
DNA is very complex and very tightly regulated chemical sequence that contains all the information our cells require to grow, perform functions and replicate. The information is contained in gene sequences, which are particular stretches of chemical patterns within the DNA that make up our genes.
So firstly, what are genes? Genes carry our hereditary characteristics. One gene is responsible for one function in one type of cell. For example one gene is responsible for our eye colour. This eye colour gene sits on the same part of DNA in every cell in the body however it is only the in the cells that make up the iris that that gene will be expressed. The same way that the gene that controls your heart beat will only be expressed in the heart, although the cells in the iris will also carry these genes.
All the genes and genetic information required for life contained in a cell is called the cell’s genome. So you can imagine if each function of our cells and body is controlled by a separate gene then each of us needs a lot of genes. The Human Genome Project set out to determine just how many genes there are in our DNA and what these genes are responsible for. In 2001 the first draft of the Human Genome Project was released; 30,000 genes were identified! The Human Genome Project or “mapping of human DNA” has been one of the most informative biological investigations. Knowing where genes exist within the millions of chemicals that make up DNA has helped to characterise genetic defects and diseases and how these diseases can be treated or cured.
The genome is important in every aspect of cellular function, including cell structure, growth, movement and division. Though we all have a very similar genetic make up that functions in similar ways, there are lots of very small differences in our DNA and this is what makes each and every one of us unique.
The same set of DNA is found in the nucleus of every cell in our body. The DNA is so tightly coiled and packed that it is estimated the nucleus of each human cell can hold about 2 meters of DNA. The DNA exists in 46 separate segments within the nucleus; these segments are known as chromosomes.
Each chromosome has a partner that contains the same genes in almost the same sequence of DNA, one member of the chromosome pair or homolog came from the father and one came from the mother during fertilisation. The chromosomes are what carry genes. An example of the genes carried by chromosomes are the sex genes, how the chromosome combination determines which gender you will be will be discussed later on.
The shape of DNA at the molecular level is thought to look like a gently twisting ladder. Each of the rungs on the ladder represents a chemical bond between the chemicals that make up the DNA molecule. These chemicals are called nucleotides and include:
- Adenine (A);
- Thymine (T);
- Cytosine (C); and
- Guanine (G).
DNA is said to be “double-stranded” because it is made up of two sequences of nucleotides that are tightly connected together by chemical bonds. The chemical bond between the nucleotides always exists between A and T and G is always bound to C. For example;
- Strand 1: C – A – G – C – A – T – T – G
- Strand 2: G – T – C – G – T – A – A – C
Chemical bonds exist between strand 1 and strand 2 that connect the nucleotide pairs The strands are complimentary to one another, by knowing the sequence to one you can figure out the other because of the specific chemical bonds. The very long DNA strands are coiled very tightly into chromosomes.
The sperm contains half the amount of chromosomes that exist in other cells of the body and so does the egg. Because there are only 23 chromosomes in the sperm and the egg, they are known as haploid cells whereas every other cell in the body contains 46 chromosomes and are known as diploid cells.
When the sperm fertilises the egg half of our chromosomes from our mother, maternal chromosomes, and half from our father, paternal chromosomes, are combined in the one cell (the egg) this is why we share certain traits with either parent. For example, you may have blue eyes like your father, but have your mother’s blonde hair.
23 chromosomes from the father have a chromosome pair with each of the 23 chromosomes from the mother. These pairs are referred to as homologs. The homologs contain a very similar DNA sequence and therefore they contain the same set of genes. However the paternal chromosome that contains the hair structure gene will contain an alternative version of this gene than the maternal chromosome; one may be a curly hair gene whereas the other chromosome may contain a straight hair gene. An allele is the name given to one of these alternative sets of genes.
Our DNA is even further mixed up by a process called ‘independent assortment’ or ‘chromosomal crossover’ which change our genetic make up slightly and explain why siblings do not have the same genetic make-up, despite sharing the same parents. Cross-over occurs where segments of the one chromosome are replaced with the corresponding segment from its homolog and vice versa. Once all 46 chromosomes have paired up and cross-over has occurred then the cell divides.
Cells divide for a number of reasons. Firstly cells must divide in order to create life. We all start out as a single cell, this cell must divide, then divide again then divide again and again, to become the millions of cells that make up the human body. As cells continue to divide they become the different cells in our body through a process called cell differentiation which will be discussed later on.
Even after we have been born, cells in our body are continually dividing, and hence we grow and change. For example our skin cells have quite a rapid life cycle and are continuously being replaced by new cells. Cells need to be continually replaced because all cells have a life cycle and they will eventually die. Cells die either because they only have a short life cycle and are programmed to die or because they have become damaged and need to be replaced in order to avoid cancer formation.
When cells divide they undergo a process called mitosis. Before mitosis can occur however all the DNA needs to be accurately replicated. Each chromosome is duplicated so that the cell contains two lots of DNA. This DNA replication is necessary so that when the cell divides both cells will have identical chromosomes and genetic information.
DNA replication is the duplication of all of a cell’s genetic material. The double-stranded DNA is ”un-zipped’ by an enzyme that exists in the cell nucleus called DNA helicase. The helicase breaks the chemical bonds between the nucleotides and the two complementary strands are separated, both these strands act as a “template” for replication. These strands are then duplicated nucleotide for nucleotide by another enzyme, DNA polymerase. The polymerase binds to the template and chemically attaches nucleotides to it in order to create a new complimentary strand to form an identical copy of the original DNA molecule. As each template strand now has a new complimentary DNA strand attached to it, the cell contains two copies of the DNA. Every chromosome is replicated in this manner so the cell will contain double copies of all of its DNA.
Mitosis is the division of a cell to form two identical daughter cells.
There are seven phases of mitosis that are outlined in the figure below:
Cells are continuously replicating and dividing. The daughter cells formed from mitosis go on to replicate their DNA and divide into two more daughter cells and so on.
It is not the DNA itself that controls cellular functions, it is the proteins that are coded by the DNA. The nucleotide sequences that make up DNA are a “code” for the cell to make hundreds of different types of proteins; it is these proteins that function to control and regulate cell growth, division, communication with other cells and most other cellular functions. This is why DNA is said to “carry” or “store” information in the form of nucleotide sequences.
The sequences need to be “decoded” and then translated in order to form the protein. This process is called protein synthesis.
Transcription is the first stage of protein synthesis. It is the process of reading the DNA, this is carried out by enzymes within the nucleus. The “transcript” from this process is another chemical molecule called messenger ribonucleic acid or mRNA. RNA is very similar to DNA in structure except it has only one strand compared to DNA which is double-stranded.
Translation is the second stage of protein synthesis where the mRNA is translated into a protein. This involves another RNA molecule, transfer RNA (tRNA). tRNA matches three nucleotides at a time from the mRNA. These 3-nucleotide sequence combinations are called codons. There are 64 possible combinations; for example
- G - A – C
- C – C – G
- G – G – G and so on.
Each of these codons code for a specific amino acid that is floating around freely within the nucleus. Amino acids are the very small molecules that make up proteins. It is the tRNA molecule that binds these amino acids together as they are matched to the codon. Note there are only 20 amino acids and 64 codons so more than one codon matches each amino acid.
Once this process is completed a functional protein is released to perform cellular functions. When a cell grows, communicates with other cells, excretes waste, absorbs water, moves, sticks to other cells or dies (and much more) this is all due to the work of these proteins.
As we know, different cells have different functions so not all cells will have the same proteins, yet all cells have the same DNA. This leaves the question; how does one type of cell make different proteins from another type of cell?
In order to answer this question it is helpful to think of DNA as a huge “instruction booklet”. For different cells, different parts of the booklet are read and this is what differentiates one type of cell from another. For example our blood cells will read one chapter of this huge book. The next chapter may be read by kidney cells and the next by liver cells and so on. So every cell type will transcribe and translate different genes that are contained in the DNA. A blood cell will “ignore” the section of DNA that codes for the eye colour gene because blood cells do not need the proteins that are involved in eye colour and vice versa.
Genetic disorders or diseases are caused by particular genes or chromosomes abnormalities, sometimes these are inherited but they can also occur spontaneously during reproduction.
When meiosis occurs, the stage where the chromosome pairs separate and migrate to opposite sides of the cell ready for cell division is a process called disjunction. Disjunction ensures each daughter cell receives its own copy of that chromosome. Non-disjunction occurs when the chromosomes fail to seperate resulting in one daughter cell with one too many of the chromosome and one daughter cell with one too little. This can happen with the sex chromosomes or any of the other chromosomes
As we discussed earlier, the sex of a growing embryo is determined by the combination of genes passed on from our parents. When the egg divides one X chromosome is given to each of the daughter cells. The sperm then enters the egg carrying either an X or a Y chromosome. If the sperm is carrying an X, the cell will contain two X chromosomes and will become a female. If the sperm is carrying a Y, the cell will have an X and a Y and will develop into a male. When this process is disturbed it can lead to a genetic abnormality.
Sometimes during cell division a sex chromosome will fail to separate form its homolog and therefore when the cell divides one daughter cell will have no sex chromosome and the other will have an extra chromosome, this dysfunctional step during meiosis is called non-disjunction. This causes genetic abnormalities and can cause the growing foetus to die.
Sometimes the DNA nucleotide sequence is altered and this is known as a DNA mutation. DNA mutations can occur sponateously when mistakes are made in the following cellular processes:
- Errors during replication; for example the DNA polymerase could read an A instead of a C and hence add a G instead of a T.
- Errors during transcription; or
- Errors during translation.
DNA mutations can also be caused by certain agents, these agents are called mutagens. Examples of mutagens include:
- Cigarette smoke; and
- Some viruses.
- Down syndrome: occurs when an extra chromosome 21 is passed on to a daughter cell.
- Cystic fibrosis: caused by genetic defects in a specific gene called CFTR.
- Turner syndrome: occurs when a second sex chromosome is missing, leaving only one X chromosome. Girls with Turner syndrome often have a webbed neck and the ovaries are nearly absent. If the sperm brings a Y chromosome the cell will die.
- Klinefelter syndrome: occurs there is an extra X chromosome in the egg and the sperm brings a Y chromosome. The person will be a male with undeveloped testes and unusually large arms and legs.
- Triplo-X syndrome: occurs there is an extra X chromosome in the egg and the sperm brings a X chromosome. The person will be an infertile female with some mild cognitive impairment.
For more information on testing for genetic diseases see genetic testing for hereditary diseases.
Cancer is a disease that arises from mutations in genes.
Special areas of our DNA regulate how quickly our cells divide to make new ones as we grow. If the DNA is mutated in some of these regulatory areas this can form the basis of a tumour, where cells begin to divide too quickly. For example there is one protein called the p53 tumour suppressor gene. p53 is activated when DNA is damaged by a mutagen and it fixes the damages. If a mutation occurs when p53 is being transcribed or translated it will not be able to function properly and hence when the DNA is damaged (from the sun for example) it will not be fixed. This is one instance when cancer can develop.
For more information on cancer cells see What Is Cancer?
DNA, genes and chromosomes are very complicated and can be hard to understand when first reading about it. The main points to remember are:
- DNA carries our genetic information
- Half our DNA comes from our mother and half from our father
- DNA is packed into 46 chromosomes and the same 46 chromosomes are found in every cell in the body
- DNA needs to be transcribed and translated in order to form proteins which carry out cell functions
- Different cells will make different proteins and this is what differentiates one cell from another
- When DNA is replicated or transcribed mistakes can be made and these are usually fixed up, however when they aren’t this can cause diseases such as cancer.
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