How genes are expressed for a particular trait? – (Explained in Detail)

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How genes are expressed for a particular trait?

The DNA that is present in every cell’s nucleus is like a long double-stranded thread. For example, in each human cell, the DNA is about 2.2 metres long.

This long DNA contains various genes. Genes are DNA sequences that encode proteins to bring out the characteristic change in the body. For example, in humans, there are 20,000 and 25,000 genes inside every cell of the body.

So, the various genes need to be expressed properly in order to bring out the particular traits in the given body.

The genes are expressed by following the process of transcription to mRNA, and then translation from mRNA to the various proteins.

However, it is also to be noted that there are various non-coding genes as well like tRNA and snRNA parts that can’t code for proteins and so they don’t get expressed for a particular trait. But, they do help in the gene expression of other coding genes.

Just like, tRNA molecule having two distinct ends, can bind to a specific amino acid with an end, and the other-end which binds to the corresponding mRNA codon during translation of mRNA to proteins.

So, a gene (the particular sequence of DNA) can be expressed for a particular phenotypic trait during transcription of the DNA to form mRNA. Then, the mRNA transfers the code for the gene to the ribosome with the help of tRNA, and then the protein production begins joining amino acids together which is called translation.

Next, when the proteins are produced the genes can be expressed. So, in simple words, the genotype can be expressed to phenotype only when the proteins are produced from the genes.

For example, in humans, the OCA2 gene (formerly called the P gene) provides instructions for making a protein called the P protein. This protein produces a pigment called melanin, which gives the skin, hair, and eyes the color.

What are the steps of gene expression? Explained in detail how the genes are expressed for a particular trait

There are two main steps in the process of gene expression. These are:

  1. Transcription: This whole step is the series of various sub-steps that lead to the production of mRNA from the DNA segment (gene). It takes place inside the cell’s nucleus.
  2. Translation: This whole step is the series of various sub-steps that lead to the production of proteins from the mRNA segments. It takes place mostly outside the nucleus and in a very few cases inside the nucleus.

Step 1: Transcription

Transcription leads to the production of mRNA (messenger RNA) from the genes along with other non-protein-coding sequences like tRNA (transfer RNA) and rRNA (ribosomal RNA) that will help during the transcription process.

The transcription process includes four steps viz. initiation, elongation, termination, and processing of the genes.

Before transcription starts, the two strands of the DNA unwinds and opens up forming a transcription bubble.

One of the strands will now act as a template to produce a separate mRNA strand.

Initiation: The initiation of transcription is the step when the transcription bubble is formed and the RNA Polymerase enzyme gets attached to the 100-1000 base pairs long promoter of the template strand.

An RNA polymerase (RNAP) is a multi-subunit enzyme that catalyzes the process of transcription where an RNA polymer is synthesized from a DNA template.

RNA polymerase always builds a new RNA strand in the (5’ to 3’) direction. That is, it can only add RNA nucleotides (A, U, C, or G) to the 3′ end of the strand.

Elongation: This is the step when the mRNA strand gets long and long by the addition of nucleotides with the help of RNA polymerase enzyme in the (5’ to 3’) direction respectively.

RNA polymerase will keep transcribing until it gets signals to stop at the STOP codon of the DNA template.

Termination: This is the step in which the transcription process ends once the RNA Polymerase reaches the sequence of DNA known as a terminator (STOP codon). The DNA stop codons are TAG, TAA, and TGA. The corresponding RNA stop codons are UAG, UAA, and UGA.

Next, the processing of the mRNA occurs.

Processing: After transcription, the RNA molecule is processed by keeping the exons (coding parts) and by removing the introns (non-coding parts) through a process called splicing. This produces a mature mRNA molecule consisting of a single protein-coding sequence. The processing of mRNA only happens in eukaryotes.

This is how the first step i.e the transcription of a DNA segment (gene) to mRNA takes place. Next, step is the translation.

Step 2: Translation

The translation is the process of producing proteins by adding together long chains of amino acids from the mRNA strand that was produced during transcription.

In prokaryotes, translation can happen at the same time when the mRNA is being formed during transcription, as no processing of mRNA is required.

However, in eukaryotes, translation only begins when the processing of the mRNA strand has been fully completed.

Translation occurs at particular sites within the cytoplasm. This process is catalyzed by ribosomes. Each ribosome molecule is made up of a large subunit on top and a small subunit at the bottom.

One large subunit sits on top of the Start Codon of mRNA, and the one small subunit sits at the bottom at the same Start Codon position of mRNA.

tRNA also gets attached to the mRNA. Each tRNA molecule has two distinct ends, one of which binds to a specific amino acid, and the other which binds to the corresponding mRNA codon.

In this way, it covers the mRNA strand from top and bottom forming the ribosomal structure. As the ribosome is formed it starts translation that is the synthesis of proteins from the Start Codon end of mRNA to the Stop Codon end of mRNA.

The smaller subunit reads the coding of mRNA, and the larger subunit functions to link the amino acids with peptide bonds to form various protein chains.

tRNA’s function there is to carry amino acids to the ribosome and join with their complementary codons.

Translation has 3 steps viz. Initiation, elongation, and termination.

During initiation, the ribosome assembles around the mRNA to be read and the first tRNA carrying the amino acid methionine matches the amino acid chain with the start codon, AUG.

During elongation, the amino acid chain gets longer. In elongation, the mRNA is read at one codon at a time, and the amino acid matching each codon is added to a growing protein chain.

During termination, the finished protein polypeptide chain is released after reaching any of the stop codons (UAG, UAA, or UGA). This is the final step of translation leading to the production of protein.

How does a cell know which genes to express?

This is one of the most important conceptual questions that will help you know more about the gene expression in detail.

The body has various kinds of cells that act and perform almost differently. This is why each type of cell is different. Just for instance, a brain cell is very different from a skin cell on how it works and acts.

So, every kind of a cell has its own way of knowing which genes it needs to express to maintain its type of functionality.

For a much more common answer, each of the cell’s gene expression initiation, pattern, and timing is determined by the information from both inside and outside the cell.

Each of the cell has its own unique way of gene regulation that also allows cells to react quickly to changes in their environments and keep itself healthy. And, also that different cell types express different sets of genes at the same time or at different time.

However, two different cells of the same type may also have different gene expression patterns depending on their environment and internal state.

Now, in more simple words, the complete set of instructions between the genes, RNA molecules, proteins (including transcription factors), and other components of the expression system determines a cell to know which particular genes to express.

So, the various set of instructions and cellular connections taking place inside the cell also lets the cell determine when and where specific genes are required to be activated and the amount of protein or RNA product needed to be produced eventually.

Now that, some genes always remain on, or more of a time they remain activated to produce proteins involved in basic metabolic functions like cell signaling, cell growth, cell differentiation, cell division, and for the various other cellular activities taking place inside the cell.

For example, if a cell’s mitochondria is not working properly to generate enough ATP for the cells sustainability then, it may initiate various gene expression patterns to cope-up with the needs. However, a different cell of the same type may not do so because it is possibly producing enough ATP for the cells sustainability.

Also, for example, Growth hormone (GH) is secreted episodically from somatotroph cells in the anterior pituitary and these cells have that unique gene expression patterns to produce only GH whenever required.

However, in womans, only estrogens are produced primarily in the ovaries, corpus luteum, and placenta and the ovaries, corpus luteum, and placental cells know when and how to produce estrogen. These cells don’t have the genes to produce GH.

So, depending on the cell types, the gene expression patterns are different and unique. And so, each of the cells better knows what, when, and how to produce based on the cell signaling and gene regulatory methods.

What controls gene expression?

What controls gene expression during transcription?

The gene expression during translation is controlled by the promoter, enhancer, and the termination sequence of the template DNA. The template DNA is that strand of DNA from which mRNA transcript is produced.

The fact is that transcription is carried out by an enzyme called RNA polymerase along with a number of accessory proteins called transcription factors.

Transcription factors can bind to specific template DNA’s regulatory sequences called enhancer and promoter sequences in order to recruit RNA polymerase to an appropriate transcription site.

Together, the transcription factors and RNA polymerase form a complex called the transcription initiation complex.

The beginning of transcription starts at the promoter sequence of the template DNA. Simply, meaning that when RNA polymers bind to the promoter sequence of DNA, transcription starts.

A promoter contains template DNA sequences that let RNA polymerase or the transcription factors to attach to the template DNA and form the transcription bubble to start transcription. So, in simple words, the promoter starts the process of transcription.

Enhancer sequences may be located near or many base pairs far from the promoter sequence of the template DNA. These are regulatory DNA sequences that when bound by transcription factors enhance the transcription of an associated gene.

The termination (stopping) of the transcription process happens when the RNA polymerase has successfully transcribed the terminator sequence of the template DNA. The terminator sequence of template DNA has the stop codons TAG, TAA, and TGA.

After termination of the transcription process, an RNA transcript that is ready to be used in translation is called a messenger RNA (mRNA) after its finally processed.

So, the promoter signals the start of transcription, and the terminator signals the end of transcription. Between the signal of start to termination, the elongation which is the formation of mRNA transcript occurs from the template DNA.

What controls gene expression during translation?

The various codons that include the start codon, the amino acid coding codons, and the 3 stop codons that control the gene expression during translation to determine how, when, and what proteins need to be translated from the mRNA transcript. Also, the tRNA and ribosomes help in the process of controlling gene expression.

There are 61 codons in the mRNA transcript that codes for amino acids. Each of the codons are As, Us, Cs, and Gs read in groups of three.

There is one codon AUG that codes for the amino acid Methionine and initiates translation. And there are three codons UAA, UAG, and UGA that don’t code for any amino acids and act as stop codons to terminate translation.

So, in total there are 64 codons and out of which only 61 codons codes for 20 different amino acids.

Along with that both tRNA and ribosomes work together to form proteins during translation starting from the start codon to any of the stop codons.

Ribosomes catalyze the assembly of amino acids into protein chains. They also bind tRNAs and various accessory molecules necessary for protein synthesis.

Are all traits inherited? Can a gene be a set of instructions for a trait?

No, not all traits are inherited. But yes, the majority of the traits are inherited from parents to offsprings in the form of genes.

Traits those are inherited when the genes are passed from the mother and some from the father may or may not show the characteristic in the offspring.

If only the allele of the gene is dominant then only it will showcase the trait in the offspring, else if recessive, the traits will be hidden.

The physical traits are inherited in the form of genes from birth if the allele of the gene can express itself as being dominant. While most of the behavioral traits aren’t inherited via. genes but learned after birth.

Just like the eye color, the tallness of the body, muscular fitness, etc is all inherited. However, how we sing, talk, laugh aren’t inherited but are supported by those genes that are inherited.

In situations when any type of mutation occurs in the genes, the characters that are newly formed may be inherited or may not be inherited to the next generation.

So, Can a gene be a set of instructions for a trait? Yes, a gene can be a set of instructions for a trait, as the genes carry the set of traits from parents to offsprings generation after generation. The traits are carried in the genotype in the form of nucleotides of DNA that expresses itself as phenotype by forming proteins.

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