Recent several months, I have encountered and read papers, news, posts about advancements in drug discovery, synthetic biology, CRIPS, protein folding (AlphaFold) etc. These are all fascinating improvements that might make world a better place to live. While reading the details, I noticed, I am lacking basic, fundamental biology knowledge in order to grasp the details. Therefore, I decided to invest some of my time to learn very basic components in genetics, and biology and share it with the people who are in the similar position. Since this post is 101 and also I dont know more yet, I will try to keep the post as simple as I can.

What is Genetics?

Genetics is the field of science that examines how traits are passed from one generation to the next.

What is Gene?

The Genes are the segments of DNA(Deoxyribonucleic acid) that are the fundamental units of heredity.

Genetics is generally divided into four major subdivisions:

  • Classical or Mendelian genetics: A discipline that describes how physical characteristics (traits, a.k.a phenotypes) are passed along from one generation to another.
  • Molecular genetics: The study of the chemical and physical structures of DNA and its close cousin RNA (ribonucleic acid) and proteins. Molecular genetics also covers how genes do their jobs.
  • Population genetics: A division of the genetics that looks at the genetic makeup of larger groups. Basically, it’s a search for patterns that help describe the genetic signature of a particular group, such as consequences of migration, isolation from other populations, and mating choices.
    • Evaluationary genetics: this is a type of population genetics that involves studying how traits change over time.
  • Quantative genetics: A highly mathematical field that examines the statistical relationships between genes and the traits with which they are associated. One application of quantative genetics is determining how heritable a particular trait is. This measure allows scientists to make predictions about how offspring will turn out based on the characteristics of the parent organisms.

Basic Cell Biology

There are two basic types of organisms distinguished by whether or not they have a nucleus. All living things fall into these two groups.

  • Prokaryotes: Organisms whose cells lack a nuclesus and therefore have DNA floating loosely in the liquid center of the cell.
  • Eukaryotes: Organisms that have a well-defined nucleus to house and protect DNA.

Cells without a nucleus

Organisms without nuclei(plural of nucleus) are classified as prokaryotes which means “before nucleus”. Prokaryotes are most common forms of life on earth. Bacteria is the most famous one. Most of the bacteria in our body are harmless, however some species of bacteria can be deadly, causing rapidly transmitting diseases such as cholera.

All bacteria are simple one-celled prokaryotic organisms. Exterior of a prokaryotic cell is covered by a cell wall that serves as the bacteria’s only protection from the outside world. A plasma membrane regulates the exchange of nutrients, water and gases nourish the bacretial cell. DNA floats around inside the cell. The liquid interior of the cell is called the cytoplasm.

Cells with a nucleus

Cells with nuclei is called eukaryotes, which means “true nucleus”. Eukaryotes can be one-celled or multicellular organisms. Like prokaryotes, eukaryotes cells are held together by a plasma membrane. Most important part of eukaryotes cell is the nucleus, which houses the DNA that is divided into one or more chromosomes. Eukaryotes usually have much larger amounts of DNA than prokaryotes. Unlike prokaryotes, eukaryotes have all sort of cell parts called organelles. These organelles float aroung in the cytoplasm outside of nucleus. Two important organelles are:

  • Mitochondria: The powerhouses of the eukaryotes cell. Mitochondria pump out energy by converting glucose to ATP(adenosine triphosphate). ATP acts like a battery of sorts, storing energy until it needed. All eukaryotes have mitochondria
  • Chloroplasts: These organelles are unique to plants and algae. They process the energy from sunlight into sugars needed to nourish the plant cells.

In most multicellular eukaryotes, cells come in two basic types: body cells (called somatic cells) or sex cells.

Somatic cells

Somatic cells are produced by simple cell division called mitosis. Somatic cells have different types, for instance, skin cells are different than muscle cells. Some other cell types are intestinal cells for digestion, fat cells for energy storage, blood cells for transporting oxygen to your tissues. Changes that occur only in the DNA of somatic cells during person’s lifetime cannot be passed from parent to child

Sex cells

Sex cells are specialized cells used for reproduction. Only eukaryotes organisms engage in sexual reproduction. In humans, two types of sex cells are egg and sperm. Unlike somatic cells, changes in the DNA of sex cells can be passed from parent to child.

What is in a nucleus?

The nucleus of a eukaryotes organism is home to most of its DNA (The mitochondria and chloroplasts also have DNA). Each of our cells contain two copies of our genome, that is, all the DNA we inheritted from our mother, and all the DNA we inheritted from our father. DNA is generally measured in the base pairs. Our genome has approximately 3 billion base pairs. In order to organize all that information and to fit it into each of our cells, DNA is packaged into chromosomes. Our genome is contained in 23 chromosomes, and each cell contains two sets of those 23 chromosomes (total of 46 chromosomes in EVERY cell).

Basics of chromosomes

Chromosomes are threadlike strands composed of DNA. To pass genetic traits from one generation to the next, the chromosomes must be copied, and then the copies must be divvied up.

Each eukaryotic organism has a specific number of chromosomes per cell, however it is diffirent for each organism. For example, the human genome is contained into 23 chromosomes, while the fruit fly is contained in only 4 chromosomes. Chromosomes come in two varieties:

  • Sex chromosomes: These chromosomes define sex, male or female. Human cells contain two sex chromosomes. Females should have two X chromosomes, while males should have an X chromosome and Y chromosome.

  • Autosomal chromosomes: Autosomal simply refers to non-sex chromosomes. Sticking with human example, if you do the math, you can see that humans have 44 autosomal chromosomes.

In humans, chromosomes come in pairs, and individual chromosomes in each pair are considered homologous, meaning that the paired chromosomes are identical to each other with respect to which genes they carry. In addition, homologous chromosomes are identical in shape and size. These pairs are sometimes referred as homologs for short.

So Where are my genes?

The diffirences in shapes and sizes of the chromosomes are easy to see, but the most important diffirences between chromosomes are hidden deep inside the DNA. Chromosomes carry genes - sections of DNA that provide building plans for specific proteins and that are associated with various traits. The genes tell the body how, when and where to make all the structures that are necessary for the processes of living. Our genome contains approximately 22000 genes, with each chromosome containing a varying number of genes, from fewer than 300 genes on the smallest chromosome (The Y chromosome), to more than 3000 genes on the largest (chromosome 1). The smallest human genes are only few hundred base pairs in length. The largest in the gene that encodes the muscle protein dystrophin, which has 2.4 million base pairs.

You have two copies of each gene, with exception of genes on the sex chromosomes in males. We inherit one copy of each gene from each parent. Each pair of homologous chromosomes carries the same - but not necessarily identical genes. For example, both chromosomes of a particular homologous pair may contain a gene that controls hair color, but one can be a “brown hair” version of the gene and the other can be a “blond hair” version of a gene – alternative versions of genes are called alleles. One of these alleles would e from your mother and the other from your father.

Any given gene can have one or more alleles. Alleles are associated with different physical traits (phenotypes) you see in organisms, like hair color or flower shape.

Each point along the chromosome is called locus (Latin for place). The plural of locus is loci. Most of the phenotypes that you see are produced by multiple genes occuring in different loci and often on different chromosomes acting together. For instance, human eye color is determined by at least three genes that reside on two diffirent chromosomes.

If you have reached to this point, thanks for taking you time to read it, and hopefully this post gave a very high level overview of cell biology. In the next post, planning to give more details about DNA.

Resources: Genetics for dummies, 3rd Edition