From our physical appearances to our personalities, we are all so very different form one another. Imagine what the world would be like if every single one of us looked and acted the same?
Our DNA is to thank for our uniqueness. Without DNA, we wouldn’t be able to develop, survive or reproduce.
During Year 10 Science, you will learn all about DNA and genetics, and how certain attributes are passed along generations. It truly is a fascinating topic which will help you understand more about yourselves. To help get you excited about the topic, this article will give you an overview of some of the main concepts you will cover.
DNA: the blueprint of life
Before you begin learning about anything else in the topic of Genetics, it is important that you have a solid understanding of what DNA is and its structure.
DNA (which stands for deoxyribose nucleic acid) is responsible for carrying and transmitting genetic instructions from parents to offspring. The cells in every single living organism contain DNA, which is also essential for synthesising proteins.
DNA is a polymer. This means that it is made up of lots of individual subunits, which we call nucleotides. Nucleotides are composed of three parts:
- Nitrogenous base
- Deoxyribose sugar
- Phosphate group
There are four different nitrogenous bases that make up DNA:
- Guanine
- Adenine
- Thymine
- Cytosine
A DNA molecule is double stranded, and has a structure shaped like a helix. It truly is a fascinating structure, which was discovered in 1953 by James Watson and Francis Crick. However, they shouldn’t get all the credit – their discovery was based on an X-ray diffraction image that was produced by Rosalind Franklin.
The two strands are connected by hydrogen bonds, which occur between the nitrogenous bases on either strand. The nitrogenous bases always pair up in a very specific way – cytosine is always paired with guanine and thymine is always paired with adenine.
You should also know that the two strands in the DNA molecule run in opposite directions to each other (antiparallel). One strand runs from 3’ to 5’, while the other runs from 5’ to 3’. (The symbol ' after the number is said as "prime".)
Genes, genomes, chromosomes: what’s the difference?
There’s a lot of words in this topic that sound very similar but mean different things. There’s also a lot of words that sound different but mean similar things. Therefore, it is important that you really understand the definitions of each term. Need help with this? You can download a free glossary of all the words you need to know for this topic here.
We’ll start with genes, genomes, and chromosomes.
An organism’s complete set of DNA is referred to as its genome. In humans, almost every cell contains the ENTIRE copy of the genome.
A gene is a specific section of DNA that codes for a protein. They vary in length immensely, from 14 base pairs up to 2.4 billion!
Chromosomes are made up of multiple genes that have been connected. They are thread like structures that are found in the nucleus of a cell. DNA is wrapped very tightly around proteins, called histones, which ultimately makes up the ‘X’ shaped structure of the chromosome, which you can see below.
In humans, the chromosomes are paired – one comes from the biological mother, while the other comes from the biological father. We have 23 pairs of chromosomes altogether.
The main function of a chromosome is to transfer genetic information from parents to offspring. They also play an important part in cell division.
Genotypes and phenotypes
There’s a reason why two black dogs can breed to produce a brown dog, or the fact that you might have a different eye colour to both your parents. Although someone may not physically exhibit a certain attribute, they may still carry the gene for it. In this section of the topic, you will get to learn all about how that happens, and perhaps you might understand more about where you inherited certain features from!
Before we get to the exciting bit, we need to understand the difference between a genotype and phenotype.
A genotype refers to the genetic makeup of an individual. A genotype is described in terms of alleles. An allele is a variation of a gene.
For example, we all have a gene for hair colour, but there’s so many different variations of hair colour. Each allele will represent a different colour. However, the alleles we have depend on what our parents have passed down to us. Unfortunately, we have no chance of being born with blue eyes if our parents didn’t carry the allele for it.
We all have two alleles for each gene. If an organism has two copies of the same allele, they are homozygous, but if they have two different ones, they are heterozygous. When we write down a genotype, we write it using two letters, each letter representing an allele.
- Dominant traits are represented using capital letters
- Recessive traits are expressed using lowercase letters
Dominant traits will always be expressed, while recessive traits will only be expressed if the organism is homozygous for that allele (has two of the recessive alleles).
A phenotype is a physical/observable trait that an organism may exhibit. The phenotype always depends on an organism’s genotype and their environment.
Hopefully this article got you excited about the topic of Genetics. There are still so many fascinating things that you will cover, and hopefully by the end of your studies, you will gain a deeper appreciation for your DNA and all that it does!
We've included a free downloadable resource on this topic HERE. Or you can find the whole Year 10 Science course covered in our complete course notes.
