Sarah Barker

Genetics and Deoxyribonucleic acid or DNA is a long molecule made up of monomers called nucleotides which contains your unique genetic code.

DNA is made up of four building blocks, or nitrogenous bases. These are called Adenine, Cytosine, Guanine and Thymine – usually shortened to A, C, G and T.

DNA Code

It holds the instructions your cells need to make all the proteins in your body.

Human DNA code is made of more than 3.2 billion nitrogenous bases of DNA.

One full set of chromosomes from an organism is called their genome. 

Our Genome

Your genome is the complete set of instructions your body needs to function.

  • The human genome is made of 3.2 billion bases of DNA but other organisms have different genome sizes.
  • If printed out the 3.2 billion letters in your genome would:
    • Fill a stack of paperback books 200 feet (61 m) high
    • Fill 200 telephone directories, each with 500 pages.
    • Take a century to recite, if we recited at one letter per second for 24 hours a day
    • Extend 3,000 km (1,864 miles) – approximately the distance from London to the Canary Islands, Washington to Guatemala, or from New Delhi to Hanoi.

The nitrogenous bases can be arranged in lots of different orders, known as the DNA sequence.

The order of these bases forms specific instructions for your cells to follow – just like the order of letters form specific words and sentences.

Other organisms have different genome sizes, with some many times smaller, and some more than ten times bigger than ours.

DNA and RNA are well known as the macromolecules of the genetic code. 

RNA is a nucleic acid similar to DNA, but with only a single, helical strand of bases. It plays a key role in turning DNA instructions into functional proteins.

  • Ribonucleic acid, or RNA, plays a key role in turning the instructions held in the DNA of your genome into functional proteins in your cells.
  • RNA is closely related to DNA, but has one different building block. A base called uracil (U).

RNA

RNA is closely related to DNA, but it contains a different sugar – ribose – and the base uracil (U) replaces thymine (T). The other bases, adenine (A), cytosine (C) and guanine (G), are common in both molecules.

  • RNA makes it possible for DNA to act as genetic instructions, in spite of being trapped in the nucleus of the cell.
  • RNA has three main roles in the cell:
    It carries the instructions from the DNA in the nucleus to the ribosomes where proteins are made in the cytoplasm of the cell.
  • RNA picks up specific amino acids from the cytoplasm of the cell and delivers them to the ribosomes where protein synthesis takes place.
  • It makes up around 50% of the structure of the ribosomes.

The structure of DNA: Double Helix

DNA’s double helix structure was first discovered in 1953 by Francis Crick and James Watson, with the help of Rosalind Franklin and Maurice Wilkins.

  • A DNA molecule is made up of two long strands of bases that wind around each other into a spiral shape.
  • This is called the double helix and looks like a twisted ladder.
  • The bases on one strand of the DNA molecule pair with bases on the opposite strand, coming together as a base pair. They form the ‘rungs’ of the ladder, joined together by hydrogen bonds.
  • Base pairs are complementary and always pair in the same way: A with T, and C with G.

Sense and anti-sense strands of DNA

  • Each strand of DNA has a beginning and an end. The beginning is called 5’ (pronounced “five prime”), and the end is called 3’ (pronounced “three prime”).
  • The two strands run in the opposite direction, or antiparallel, to each other. One runs from 5’ to 3’, known as the sense direction. The other runs 3’ to 5’, known as the antisense direction.
  • This gives the strands their names: the sense strand and the antisense strand, respectively.
  • This structure is important during DNA replication, when the strands separate from each other and the DNA is copied.

Genes are small sections of DNA that code for a functional product, like a protein.

  • Our DNA is organised into small sections, called genes.
  • Each gene contains the instructions for making a specific product – usually a protein.
  • Our proteins serve many functions to keep our cells functioning, such as protecting against disease or absorbing nutrients from food.

What is a gene?

  • Our DNA stores the information our bodies need to function.
  • The DNA is organised  into sections called genes.
  • Each gene contains the instructions a cell needs to make a specific molecule – usually a protein.
  • Proteins do nearly all the tasks that keep our cells operating and our bodies alive.
  • Variations in our genes and their encoded proteins also affect our individual characteristics – such as eye and hair colour. Some characteristics – like height – are affected by both our genes and our environment.
  • The human genome contains approximately 20,600 protein-coding genes. Other organisms have different numbers of genes.

How do our genes affect our characteristics?

  • Genes come in different forms, or variants, known as ‘alleles’ (pronounced “ah-lee-els”).
  • As humans, each of our genes comes as a pair – meaning we have two alleles for each gene. The combination of our alleles determines some of our physical characteristics.
  • Scientists used to think that some of our characteristics, like our eye colour, were controlled by one single gene. However, we now know that eye colour is controlled by at least eight different genes.
  • Of these genes, different alleles in a gene that codes for a pigment, called melanin, can affect eye colour:
    • If a person has two identical ‘high pigment’ alleles, they may have dark brown eyes.
    • If they have two identical ‘low pigment’ alleles, they may have blue eyes.
    • If they have two different alleles – such as one ‘high’ and one ‘low’ pigment allele – they might have light brown, green or hazel eyes.
DNA and Genes

Genes are small sections of DNA, and theses sections are organised into larger structures called Chromosomes. 

Chromosomes are tightly coiled bundles of DNA found in the nucleus of almost every cell in our body.

  • In plant and animal cells, DNA is tightly packaged into thread-like structures called chromosomes.
  • Humans have 23 pairs of chromosomes.
  • Other living things have a different number of chromosomes.

What is a chromosome?

  • A chromosome is a tightly wound bundle of DNA. It’s the way DNA is packaged in animal and plant cells – contrasting with bacteria where DNA floats freely around the cell.
  • The DNA making up each of our chromosomes contains thousands of genes.

How many chromosomes do we have?

  • Humans have 23 pairs of chromosomes (46 in total): one set comes from each biological parent
  • One pair are the sex chromosomes, called the X and Y chromosomes. People with XX are usually assigned female at birth while people with XY are usually assigned male at birth. Though rare, other combinations of sex chromosomes are possible.
  • The other 22 pairs are autosomes (non-sex chromosomes).

Human chromosome karyotype. Image credit: Doc. RNDr. Josef Reischig / via Wikimedia Commons

What is the structure of a chromosome?

  • A single length of DNA is wrapped many times around lots of proteins called histones, to form structures called nucleosomes.
  • These nucleosomes then coil up tightly to create chromatin loops.
  • The chromatin loops are then wrapped around each other to make a full chromosome.
  • Each chromosome has two short arms (p arms), two longer arms (q arms) and a centromere holding it all together at the centre.
  • At the ends of each of our chromosomes are sections of DNA called telomeres. Telomeres protect the ends of the chromosomes during DNA replication by forming a cap – a bit like the plastic tip on a shoelace.

Some organisms have fewer chromsomes than the 23 pairs humans have – like the Koala with only 16 chromosomes in 8 pairs. Others have lots more than we do, such as several species of butterfly, such as the Atlas Blue, whose chromosomes number in the hundreds.