DNA and Inheritance
In this lesson you are going to learn about DNA, DNA profiling and inheritance.
What is DNA?
DNA stands for deoxyribose nucleic acid. It is made up of two chains of billions of molecules called nucleotides. These nucleotides are made of of a "nitrogenous base", a "phosphate" and a "deoxyribose sugar, as you can see in the left diagram below.
In DNA there are four different nucleotides. The only difference between the nucleotides is the nitrogenous base. There are four different nitrogenous bases. The four different bases are called Thymine, Guanine, Adenine and Cytosine. These are represented in the diagram to the right by the different shapes and colours.
Bllions of the nucleotides join together. In the diagram below the red line represents the bond that forms between nucleotides that holds them together. When billions of these nucleotides join together they eventually form a long strand. 2 strands are formed and they are joined together with a bond that forms between the nitrogenous bases, represented by the green arrows below. This bond can only form between complimentary bases. Only thymine can form a bond with adenine and only cytosine can form a bond with guanine. The two strands joined together form the double helix, as seen to the right.
Genes
The order of the 4 bases in the DNA is random, to a certain extent. A gene is a section of DNA that determines inherited characteristics. Genes contain the information which codes for protiens. Protiens are made up of amino acids and the order that the amino acids are joined together determine which protien it is. The order of the bases in the gene will tell the cell which order to put the amino acids of the protien in to make it. So we can say that DNA contains the information which codes for protiens, or that DNA contains the genetic code. Below is a diagram of DNA and it shows that a gene is a section of it. Different genes have different numbers of nucleotides and the order of the bases are different in different genes so different genes code for different protiens. Protiens make up many things. Enzymes which control processes in the cell are made of protiens. Hair, blood cells, hormones, and so many other things contain or are made up of different protiens. An allele is a form of a gene. Alleles give rise to variation in inherited characteristics. This will be discussed later on.
Chromosomes
DNA is found in the nucleus of the cell. However, as DNA is very long, in order to fit into the nucleus it is compressed into chromosomes by forming coils around protiens called histones. We can define a chromosome as a strand of DNA or as a linear arrangement of genes. At GCSE level it is not vital to fully understand the diagram below, but as it explains the basics of how chromosomes are formed it is worth looking at in order to help you understand what a chromosome is.
DNA Profiling (genetic profiling)
There is no two people on earth (with the exception of identical twins) who share the exact same DNA, our DNA is unique to us. This means that by looking at someone's DNA we can tell who that individual person is. But how can we look at someone's DNA? We can do this through the proces of genetic profiling. This involves extracting someone's DNA, cutting it into short pieces and using a machine which will seperate those short pieces into bands. The bands created from the DNA of an idividual will have a different pattern to the bands created from someone else'e DNA, so we can see from the pattern of the bands who is who.
This is useful in criminal investigations. If some evidence from the scene of the crime is recovered and it contains some of the criminal's DNA, the bands created from that piece of DNA may match up with the bands created by the DNA of any suspects, comfirming who commited the crime. With today's technology it is possible to extract DNA from a tiny sample such as a single hair, a drop of blood, saliva, or some skin cells from the criminal left on fabric or on a victim.
It can also be used in paternity testing. A child's DNA will never be exactly the same as it's parents' DNA, but it will have some similarities. If there is debate over which of two men is the father to a child, DNA samples can be taken from both men and the child, and the DNA that has the most bands in the same position and of the same thickness as the DNA from the child will be from the father, so we can tell which of the two men is the father. Look at the example below. There are more bars in the child's DNA profile that are in the same position as the bars in the DNA profile of man B than there are in the same position for man A. We can also take a DNA profile of the mother's DNA as some bars in the child will also match those of the mother, so we can eliminate these bars from the child's profile as we know those bars come from the mother and not the father, making it easier to compare the two potential fathers.
DNA profiling is also used in classification of living organisms. The more bands from one species that match the bands from another species, the closer related they are. Having this information is useful when looking at evoloution.
DNA analysis can help us to predict if someone has a particular gene that may cause a disease or pass on a disease to their offspring. By knowing if you might pass on a disease to your offspring, it helps to make the decision on whether to have children or not. It is sometimes also possible to test a fetus to see whether it has the disease causing form of the gene However, as you will see when we discuss inheritance, the likelyhood of passing your genetic order on to your child is not 100% gaurunteed, they may be born without that form of the gene and will not have the disease.
Gametes
Gametes are sex cells. In animals the male gamete is the sperm cell and in the female the gamete is the egg cell. During sexual reproduction the egg and sperm cells fuse together in a process called fertilisation. Gametes are formed differently from body cells. They will end up with half the number of chromosomes of a body cell. Humans who have 46 chromosomes in their body cells will have 23 chromosomes in their gametes. The chromosomes in gametes have one set of the pair of chromosomes a body cell has. During fertilisation the full number of gametes is restored. The nucleus of the sperm and the nucleus of the egg fuse together and the chromosomes pair up with each other to restore the full number of chromosomes.
Note that the top diagram shows only 4 of the chromosomes in the process, the exact same thing will be happening to all 46 chromosomes in real life. The bottom diagram showing what occurs during fertilisation shows one chromosome in the male gamete forming a pair with one chromsome in the female gamete. All 23 chromosomes in the sperm will form pairs with all 23 chromosomes in the egg and the resulting zygote will have 46 chromosomes arranged in 23 pairs.
Allele
Genes will come in different forms. These different forms are called alleles. These alleles determine the charracteritic displayed. For example, there will be genes that control our eye colour is. The type of gene, the allele, will determine what that colour is. The diagram below shows two chromosomes. The coloured in part represents where there is a gene. These genes are in the exact same place, they are the same gene, but they are different types of this gene, this is shown by the different colours.
Sex inheritance
One pair of chromosomes that we have will determine what sex we are. During reproduction, the female gamete will always have one X chromosome. A sperm will either have an X chromosome or a Y chromosome. When the sperm and egg fuse together these chromosomes make up a pair. If the sperm had an X chromosome, when paired with the female X chromosome they will make a pair of XX chromosomes. If the sperm had a Y chromosome, when paired with the female X chromosome they will make a XY pair of chromosomes. If the individual has XX they are female. If the individual has XY they are male.
Punnett squares
In order to predict the possible characteristics of the offspring of a male and female organism we can use punnett suares, if we know what chromosomes or alleles they have in their body cells. To explain how we can do this we will look at a punnett square showing the probability of what sex a baby will be. In the diagram below we start by writing out what sex chromosomes the male has (XY) and what sex chromosomes the female has (XX). During the formation of the gametes the pair of chromomes become separated. We show this where the arrows lead to the individual X's and Y's. All female gametes will have X's because the original pair consists of both X's, but some male gametes will have X's and other male gametes will have Y's because the original pair of chromosomes that determines someone is a male was XY. We then draw a table with 3 rows and 3 columns. In the 1st column in the 2nd and 3rd rows we put the letter representing the chromosomes from the female (X and X), and in the 1st row in the 2nd and 3rd columns we put the letters representing the chromosomes from the male (X and Y). Now the table is set up we can fill it in. This will be shown in red on the diagram. Begin by looking at the letter in the 2nd row, 1st column. It is an X. Go across the table and write an X in each column on that 2nd row. Then look at the 3rd row, 1st column and look at the letter, it is an X. Go across the table and write an X in each column on that 3rd row. Now look at the 1st row, 2nd column. The letter there is an X. Go down the table and write X in each row in the 2nd column. Finally look at the 1st row, 3rd column. The letter there is a Y. Go down the table and write a Y in each row in the 2nd column.
What you have written in your table now is the possibilities of the pair of chromosomes that the offspring could inherit. It may inherit an XX pair of chromosomes, so will be a female, or it may inherit the XY pair of chromosomes. It depends whether the sperm that fertilises the egg is the one with the X chromosomes or the one with the Y chromosome. In the tablle there are 4 boxes which we filled in containing the possibilites. 2 of the 4 contain the letters XX. 2 of the 4 contain the letters XY. We can write this as 2/4, which can be simplified to 1/2. 1/2 written as a percentage is 50%. There will be a 50% chance the offspring will have XX and a 50% chance the offspring will have XY.
Mendel and alleles
Gregor Mendel (20/07/1822-06/01/1884) was a scientist who studied genetics. He looked at inheritence in plants, in particular pea plants.
Chromosomes are found in pairs, apart from in gametes which will be discussed later on. In a human body cells there will be 23 pairs of chromosomes, 46 individual chromosomes. The diagram below represents all of the chromosomes found in human body cells.
The diagram to the left shows what happens to chromosomes when human gametes are being made. The original cell at the start (the parent cell) will be an ordinary body cell from the testes in a male or in the ovaries in a female. As it is a normal body cell it begins with 46 chromosomes (23 pairs). The chromosomes will begin by creating identical copies of themselves. 46 x 2 = 92. There will be 92 individual chromosomes (46 pairs). Similar chromosomes will then pair up. They may swap genetic material with each other, one part of one chromosome will swap with a part on another chromosome. The cell will then split into 2. Each cell produced will have 46 chromosomes in it now. As the chromosomes swapped genetic material with each other before this, these 46 chromosomes at this stage are different to the 46 chromosomes in the original body cell. These two cells with 46 chromosomes will both split again. From these two cells, four cells known as "daughter" cells are produced. These four daughter cells will each have 23 chromosomes. These 23 chromosomes will not be in pairs. These daughter cells are gametes, they will either be a sperm or an egg depending on whether the original cell came from testes or ovaries.
During fertilisation the full number of chromosomes (46) must be restored. The nucleus of the sperm will fuse with the nucleus of the egg. The 23 chromosomes in the sperm will form pairs with the 23 chromosomes in the egg. One cell with 46 chromosomes (23 pairs) is produced. This cell is known as a zygote. This will develop into a fetus. This fetus will have half of it's chromosomes from the mother and half of it's chromosomes from the father.
