Gene

Genes are special pieces of material inside every living thing that help decide what that thing will be like. They are made of tiny building blocks called nucleotides, arranged in long chains. These chains are found in DNA, which is like a recipe book for life.

There are two main ways scientists think about genes. One way is about how traits are passed from parents to children, called the Mendelian gene. The other way looks at genes as specific pieces of DNA that can make molecules, either directly or by making proteins that do the work in our cells.

Genes help decide many things about an organism, like its shape, color, or even how it grows. Sometimes, small changes called mutations can happen in genes. These changes create different versions of a gene, called alleles, which can give people or other living things slightly different traits. Over time, these differences help populations change and evolve through natural selection and genetic drift.

Chromosome (107 - 1010 bp) DNA Gene (103 - 106 bp ) Function A chromosome and its packaged long strand of DNA unraveled. The DNA's base pairs encode genes, which provide functions. A human DNA can have up to 500 million base pairs with thousands of genes.

Definitions

The word "gene" can mean two different things depending on how we think about it.

The Mendelian gene is the classic idea of a gene as something that passes traits from parents to children. It helps explain how traits like eye colour or height are inherited.

The molecular gene is the idea most scientists use today. It describes a piece of DNA that makes a special kind of molecule called RNA. Some of these genes make proteins, which are tiny workers inside our cells. Others make RNA molecules that help in important jobs but do not make proteins.

Scientists have many ways to describe what a gene is, and no single definition fits every situation perfectly. But all agree that a gene is a piece of DNA that makes something useful for the cell.

History

Main article: History of genetics

Discovery of discrete inherited units

A scientist named Gregor Mendel discovered that traits are passed from parents to their children in specific ways. He studied pea plants and noticed that certain features, like flower color or seed shape, followed patterns. Even though he didn't use the word "gene," Mendel showed that these traits come from small units that stay the same through generations.

Before Mendel, people thought traits from parents just mixed together. But Mendel's work showed that traits can be strong or hidden and that they don't always blend. Later, other scientists built on his ideas, and in 1909, a term "gene" was created to describe these tiny units of heredity.

Discovery of DNA

In the 1900s, scientists learned that a substance called DNA holds the instructions for these genes. They figured out DNA's shape and how it copies itself. Experiments showed that genes are like small pieces of DNA, each with a specific job.

Modern synthesis and its successors

Main article: Modern synthesis (20th century)

Later scientists combined Mendel's ideas with how animals and plants change over time. They showed that genes are important in how species evolve and develop. New ways to study DNA helped us understand how life has changed over millions of years.

Molecular basis

Main article: DNA

Most living things store their genes in long strands of DNA, a special kind of molecule. DNA is made of smaller parts called nucleotides. These nucleotides have a sugar, a phosphate group, and one of four bases: adenine, cytosine, guanine, and thymine.

DNA strands twist together to form a shape like a spiral staircase, called a double helix. The bases point inward and pair up in a special way: adenine pairs with thymine, and guanine pairs with cytosine. This helps DNA copy itself accurately.

Genes stored in DNA can be read to make RNA, another type of molecule. Some genes help build proteins, which are tiny workers inside cells. Each protein is made from a group of three nucleotides called a codon. The same set of rules, called the genetic code, is used by almost all living things to decide which protein each codon makes.

Genes are found on structures called chromosomes. A chromosome is a very long piece of DNA that can hold thousands of genes. In most cells, chromosomes are found inside a special area called the nucleus. They are wrapped around proteins called histones, which help control which genes are active at any time.

Some simple cells have their genes on a single round chromosome. More complex cells, like those in humans, usually have many chromosomes. Even parts of cells, such as tiny structures called organelles, can have their own small chromosomes.

Recently, scientists have learned that much of the DNA in complex cells does not make proteins but may still play important roles. This used to be called “junk DNA,” but we now know it might be doing important jobs we are still learning about.

DNA

Chromosomes

Structure and function

Genes are the basic units that help decide how living things look and work. They are made of special molecules called DNA.

Some genes make proteins, which are like tiny workers inside cells. Others make types of RNA that help in different ways. All genes have parts that control when and how they work, like switches that turn them on or off. These parts help the cell read the gene correctly to make what it needs.

Gene expression

All living things need to read the information stored in DNA to make proteins. This happens in two main steps. First, the DNA is copied into a type of RNA called messenger RNA (mRNA). Second, the mRNA is used to build the protein. This whole process is called gene expression.

The order of nucleotides in DNA tells cells which building blocks, called amino acids, to use when making a protein. Special groups of three nucleotides, called codons, match up with specific amino acids. There are also special start and stop signals that tell the cell when to begin and end making a protein.

During the first step, called transcription, an enzyme makes a matching RNA copy of the DNA. In most cells, this happens inside a special part of the cell where the DNA is kept. The RNA copy then gets changed a little before it moves out to where proteins are built.

The second step, called translation, uses the RNA copy as a guide to link together amino acids into a chain, forming a protein. Special helpers called ribosomes and another type of RNA called transfer RNA work together to match the correct amino acids to the instructions in the RNA. After being made, proteins usually need to fold into the right shape before they can do their jobs in the cell.

Cells can control when and how much of each protein they make. This helps them use energy wisely and respond to changes inside and outside the cell. For example, bacteria can turn on certain genes when they need to break down a specific food source.

Inheritance

Main articles: Mendelian inheritance and Heredity

Living things get their genes from their parents. For example, plants that can only make one type of flower will pass that trait to their offspring. Animals that can make two types of flowers, one from each parent, will have a mix of traits.

Genes are like instructions that tell our bodies how to grow and look. Some genes are strong and always show up, while others are hidden unless both parents pass the same gene to their child. This is why some children look more like one parent and sometimes get traits from both.

Genome

The genome is all the genetic material in an organism. It includes genes and other parts that do not code for proteins. The size of a genome and the number of genes it has can vary a lot between different living things. Very small genomes are found in viruses and viroids, while plants like rice can have very large genomes with thousands of genes.

Scientists have been trying to figure out how many genes humans have. Early guesses were around 30,000, but newer studies show that humans likely have about 19,000 genes that make proteins. There are also many genes that do not make proteins, but the exact number is still being studied. Some genes are very important for an organism to survive, and these are called essential genes. For example, in simple bacteria, only a few hundred genes are essential, while in yeast and animals, thousands of genes are needed for survival.

Genetic engineering

Genetic engineering is a way to change the genes inside living things. Scientists use special tools to add, remove, or change genes in plants, animals, and even tiny cells like bacteria. These tools help them fix broken genes or add new ones to make new things.

We use genetic engineering in many ways. It helps farmers grow better crops, lets scientists study how genes work, and can even help doctors treat sicknesses caused by gene problems. It’s a very important tool in science today!