Molecular Genetics (Biology): An Overview

Whether you're taking general biological sciences, cell biology or molecular biology courses, genetics will be a major part of your study.

Genetics determines who we are, what we are and how we act at both the human level and the cellular level.

When you're learning about molecular genetics, it's best to start with the basics. What exactly is your genetic material, anyway?

The tl;dr is that DNA is deoxyribonucleic acid: a double helix-shaped molecule made up of two complementary strands of DNA. DNA is one of the two major types of nucleic acids that exist in nature (the other is RNA). Nucleic acids are made of subunits called nucleotides. Each nucleotide is made of a 5 carbon ribose sugar, a nitrogenous base and a phosphate molecule.

Four types nitrogenous bases make up the nucleotides of nucleic acids – adenine, thymine, guanine and cytosine – that make up your genetic code. Your genetic material undergoes DNA replication each time your cell divides, so that (virtually) every cell in your body has a complete set of genes.

In eukaryotes, the DNA is packaged into large chromosomes. And for humans, most cells contain two sets of 23 chromosomes, for 46 chromosomes, total. Two of those chromosomes – the X and Y chromosome – are called sex chromosomes. They determine your sex and also code for specific traits, called sex-linked traits.

The genetic code is separated into two basic categories. One category is exons, which are the coding regions that make up genes. These get transcribed and translated to create proteins, which allow your cells to function.

The other category of genetic code is introns, which are non-coding regions. Because they're non-coding, they don't create proteins. However, introns play an important role in your DNA function, because they influence gene activity – in other words, how much the gene is expressed.

While your DNA might be the blueprint of life, RNA – also called ribonucleic acid – is just as important to molecular genetics. Like DNA, RNA is made up of nucleic acids, though it contains uracil instead of thymine. Unlike DNA, though, it's a single-stranded molecule, and it doesn't have the same double-helix structure as your DNA.

There are several types of RNA in your cells, and each carry out distinct roles. Messenger RNA, or mRNA, serves as a blueprint for protein production. Ribosomal RNA (rRNA) and transport RNA (tRNA) also play a key role in protein synthesis. And other types of RNA, like microRNA (miRNA) affect how active your genes are.

Just as important as the content of your genes is how active (or inactive) they are – which is why gene expression is important. Genes are expressed when they're transcribed and translated into proteins.

The concept of gene expression traces back to the central dogma of molecular genetics: that the flow of genetic information moves from DNA to RNA and, finally, to protein.

So how does it work? The first step in the process is _transcription_. During transcription, your cells use your DNA as a blueprint to create a complementary strand of messenger RNA (mRNA). From there, the mRNA goes through a few chemical modifications – like removing the introns – so that it's ready to serve as a blueprint for protein synthesis.

The next step in the process is _translation_. During translation, your cells "read" the mRNA template and use it as a guide to create a polypeptide – a strand of amino acids that'll eventually become a functional protein. Translation relies on a triplet code, where three nucleic acids in the mRNA strand correspond to one amino acid. By reading each triplet code (also called a codon), your cells can be sure to add the right amino acid at the right time to create a functional protein.

You already know genes are passed on from parents to their offspring and shared among family members – but how exactly does it work?

Part of it comes down to genes and alleles. While all humans share the same set of genes – so, for instance, everyone has genes that code for hair color or eye color – the content of those genes is different, which is why some people have blue eyes and some people have brown ones.

Different variations on the same genes are called alleles. Different alleles code for slightly different proteins, which lead to different observable traits, which are called phenotypes.

So how do different alleles lead to different observable traits? Some of it comes down to whether an allele is dominant or recessive. Dominant alleles take center stage – if you have even one dominant allele, you'll develop the phenotype associated with it. Recessive alleles don't lead to a phenotype as easily – generally, you'll need two copies of a recessive allele to see the associated phenotype.

So why are dominance and recessiveness important to understand? For one thing, they help you predict the phenotype – the observable traits – you'll see in the next generation. What's more, you can use probabilities to figure out both the genetic information and the phenotype of the next generation of offspring, using a simple tool called a Punnet square.

As for who figured out the basics of dominant and recessive genes? You can thank Gregor Mendel, a geneticist who carried out experiments in the mid-1800s. Observing how traits were passed down from pea plants generation over generation, he worked out the theory of dominant and recessive traits – and essentially created the science of genetics.

Most of the content of your genes is passed down from your parents, but you can also develop genetic mutations throughout your life. Genetic mutations can impact your overall health if they end up impacting translation and changing the amino acid sequence of the resulting protein.

Some genetic mutations, called point mutations, might only affect a single amino acid. Others can affect large regions of your DNA.

Some genetic abnormalities affect very large regions of DNA – a part of a chromosome or even an entire chromosome. Chromosomal deletions lead to offspring missing an entire chromosome, while other abnormalities can mean inheriting too many copies of chromosomes.

So now you understand the basics of molecular genetics – now, how does it apply to science today?

The truth is, scientists have more tools than ever to both study and manipulate DNA. And if you're planning on taking science at a university, you'll get to try out some genetic experiments yourself.

So how do all those genetic tools affect the real world? One of the biggest impacts of advances is genetics is the impact on human health.

Thanks to the Human Genome Project, we now know the sequence of human DNA. And follow-up studies have given scientists the opportunity to study genetic variation and trace patterns of inheritance to understand human history.

Of course, genetic engineering and genetic modification is also important for the agricultural industry – and unless you've been living under a rock, you've heard at least some of the controversy surrounding genetically modified organisms, or GMOs.

Genetic modification can make crops easier to grow, and you'll find GMOs in (almost) any packaged food you eat.

As you may have guessed, the advances in molecular biology and genetic engineering come with ethical concerns. Can corporations "own" a patent to a human gene? Are there ethical issues to creating and using genetically modified crops, especially without labelling them at the grocery store?

Could voluntary genetic testing, like ancestry tests, put your privacy at risk?