Structure of DNA

Almost every cell has a nucleus. In each nucleus there are chromosomes. In human cells there are 46 chromosomes (diploid number) except for the gametes in DNAwhich there are 23 chromosomes (haploid number). In each chromosome there are long strands of DNA double helix (meaning two backbones connected by the bases). This DNA is short for deoxyribonucliec acid. Its structure is like a spiral ladder.

The backbone of the DNA strand is made of sugar and phosphate. The rungs of the ladder has bases in it. There are four bases – adenine, thymine, cytosine, guanine. The sequence these bases are merged together in, makes up the genetic code. Each section of the DNA strand is called gene. Each gene has instructions and information for producing proteins like enzymes.

DNA replication

DNA is capable of copying itself. The details of the process is not necessary for O-level Cambridge students to know, other than that the process is more complicated. In meiosis cell division, the DNA replicates itself before cell division. Soon the the chromosomes have double the DNA. At the end of the process when there are four daughter cells the DNA number and chromosome number becomes half (haploid number formed) of the original.


Different forms of a gene are called alleles. Alleles also have genetic code passed on from parents of an organism. To understand this you must know that an Allele 2organism has always had a pair of chromosomes called homologous chromosomes. In a homologous chromosome either chromosome can be paternal or maternal (from father or mother). They may have genes at the same loci. They may also have different alleles. An allele has different genetic information and are at the same positions in a pair of homologous chromosomes, this same positioning is called a Gene Loci.

Mendelian Genetics

Alleles play a very important part in making an organism’s personality and its appearance. When alleles of the homologous chromosomes are compared, one of them can either be dominant over the other which will be recessive. The allele dominant whether from the maternal chromosome or the paternal chromosome, will show the features of either the mother or father depending on the chromosome. The terms used for the case of alleles are:

  • Geno-type means the genetic make up of organism.
  • Phenotype means the appearance of an organism.
  • Homozygous means that the alleles are similar.
  • Heterozygous means that the alleles are different.

To calculate the probability of which genotype or phenotype the organism may adopt we need to use alphabets to define each genotype.

An example is eye color. If any one of your parents have a blue eye color  and the other has brown color, we will let brown be B and let blue be b.

B        b

B     BB      Bb

b     Bb       bb

You will notice that the chances you have a blue eye color is 1/3 or 3:1. This is because allele that has the information for your eye color being brown is dominant over the allele having the information for blue eye color.

  • The BB is homozygous dominant.
  • The Bb are heterozygous.
  • The bb is homozygous recessive.

Diagrams for genetic crosses

F1 regenerationThis is rather more complicated. There are two diagrams. The first generation is called F1 and the second generation is called F2. F2 is used to describe how the next generation will be different due to the heterozygous organism.


Many organisms have alleles that are co-dominate. Meaning they show no dominance over each other. They have greater probability of genetic variation. If the allele of the paternal and maternal chromosomes are co-dominate then the organism will show a different phenotype and geno-type. For example, a pollen from a red plant is pollinated to a white plant and fertilization occurs. Soon the plant grows but the color of the flower is rather questionable. It can be red or white or pink. We can use the alphabet R for red and W for white. Using a probability grid we can see the probability of the color of the plant in F1 (first generation).

          W              W

R       RW           RW

R       RW         RW

Did you notice that the plant has the probability of becoming pink rather than white or red.

When the Pink plants self pollinates you will notice that its probability of having the same genotype and phenotype in F2 (second generation) decreases:

      R           W

R           RR         RW

W          RW        WW

Further reproduction causes the plant to have a probability of growing a red or a white flower. The ratio is 1:2:1.

With this I end my article. If there is any error you might have noticed, feel free to comment. This article marks the start of the genetics part of biology which is compulsory for O level Cambridge students.


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s