Gene Expression - Transcription & Translation

Introduction to Gene Expression

Gene expression is the process by which information encoded in DNA is used to produce functional products, typically proteins. It involves two key steps: transcription and translation.

Types of RNA Involved in Gene Expression

1. Messenger RNA (mRNA)

mRNA carries a copy of the genetic code from the nucleus to the ribosome, where protein synthesis (translation) occurs.

2. Transfer RNA (tRNA)

tRNA molecules are responsible for carrying specific amino acids to the ribosome during translation. Each tRNA molecule folds into a unique three-dimensional shape due to complementary base pairing.

3. Ribosomal RNA (rRNA)

rRNA, along with proteins, forms the structure of ribosomes, where translation takes place. Ribosomes facilitate the assembly of amino acids into proteins.

Selective Gene Expression

In a cell, not all genes are actively expressed at the same time. Cells regulate gene expression to produce the specific proteins they need for their function and stage of development.

RNA Structure

Unlike DNA, which is double-stranded, RNA is single-stranded. It is composed of nucleotides, each containing ribose sugar, phosphate, and one of four bases: cytosine (C), guanine (G), adenine (A), and uracil (U).

Transcription and Translation

Transcription

Transcription is the first step of gene expression. It involves the synthesis of mRNA from a DNA template. Transcription occurs in the nucleus.

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Gene Expression - Transcription & Translation

Human Biology

Translation

Translation is the second step of gene expression, where the information carried by mRNA is used to build a protein. Translation occurs in the cytoplasm at ribosomes.

Role of mRNA in Gene Expression

mRNA carries a copy of the genetic code from DNA. It serves as a messenger that delivers the instructions for protein synthesis to the ribosome.

tRNA in Gene Expression

Amino Acid Transport

tRNA molecules are responsible for transporting specific amino acids to the ribosome. Each tRNA molecule is bound to a particular amino acid.

Anticodon

At one end of the tRNA molecule, there is an exposed triplet of bases called an anticodon. The anticodon is complementary to a specific codon on the mRNA.

rRNA and Ribosomes

rRNA, along with proteins, forms the structure of ribosomes. Ribosomes consist of large and small subunits, each containing rRNA molecules. They provide a platform for protein synthesis.

The Genetic Code

Codon Definition

The genetic code is represented by a sequence of three bases on mRNA, known as a codon. Each codon codes for a specific amino acid.

Amino Acid Identification

The genetic code is degenerate, meaning that multiple codons can code for the same amino acid. For example, different codons may all code for the amino acid alanine.

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Gene Expression - Transcription & Translation

Process of Protein Synthesis

1. Initiation

Protein synthesis begins with the binding of the small ribosomal subunit to the mRNA. The first tRNA molecule, carrying the amino acid methionine, attaches to the start codon on the mRNA.

2. Elongation

During elongation, tRNA molecules carrying their specific amino acids bind to the mRNA codons in a complementary manner. The ribosome helps form peptide bonds between adjacent amino acids, creating a growing polypeptide chain.

3. Termination

Protein synthesis continues until a stop codon on the mRNA is reached. There are three stop codons (UAA, UAG, and UGA). When a stop codon is encountered, protein synthesis terminates, and the polypeptide chain is released.

Summary

In summary, gene expression involves the transcription of DNA into mRNA in the nucleus and its translation into proteins at ribosomes in the cytoplasm. Three types of RNA are essential in this process: mRNA carries the genetic code, tRNA transports specific amino acids, and rRNA, along with proteins, forms the ribosome structure. Only a fraction of genes in a cell are expressed, allowing cells to regulate their protein production. The genetic code is represented by codons on mRNA, each coding for a specific amino acid. Protein synthesis involves initiation, elongation, and termination stages. Understanding gene expression is crucial for comprehending how genetic information is used to produce the functional proteins necessary for an organism's life processes.