Question

Define replication, transcription, and translation.

Short answer

Replication copies DNA; transcription makes mRNA from DNA; translation produces proteins from mRNA.

Step-by-step solution

Define Replication

Replication is the process by which DNA makes a copy of itself during cell division. This is crucial for genetic continuity in living organisms. DNA replication ensures that each new cell receives an exact copy of the DNA.

Define Transcription

Transcription is the process by which the information in a strand of DNA is copied into a new molecule of messenger RNA (mRNA). This occurs in the nucleus of the cell and is the first step of gene expression.

Define Translation

Translation is the process in which cellular ribosomes create proteins using the mRNA produced during transcription. The mRNA is decoded by the ribosome to build a polypeptide chain, which will fold into an active protein.

Key concepts

DNA Replication

DNA replication is a fundamental process occurring in all living organisms to ensure genetic continuity. When a cell divides, it must first duplicate its DNA so that each new daughter cell receives an exact copy. This process starts at specific locations in the DNA molecule called origins of replication.
The two strands of DNA are separated by an enzyme called helicase, creating a 'replication fork'.
Another enzyme, DNA polymerase, then attaches to the separated strands and adds complementary nucleotides to form two new strands:

• The leading strand is synthesized continuously.
• The lagging strand is synthesized in short fragments called Okazaki fragments, which are later joined together by the enzyme ligase.

This semi-conservative method ensures each new DNA molecule consists of one old strand and one new strand.

Transcription

Transcription is the first step of gene expression, where a segment of DNA is copied into RNA, specifically messenger RNA (mRNA). This takes place in the cell nucleus. An enzyme called RNA polymerase binds to a specific region of the DNA, known as the promoter. RNA polymerase then unwinds the DNA and synthesizes a complementary strand of mRNA by adding RNA nucleotides. The mRNA strand grows until a termination signal is reached, indicating the end of the gene. At this point, the mRNA strand detaches from the DNA and undergoes processing before it is transported to the cytoplasm for translation.

Translation

Translation is the process by which the mRNA produced in transcription is decoded by a ribosome to produce a specific protein. This occurs in the cytoplasm. Each set of three nucleotides on the mRNA, called a codon, corresponds to a specific amino acid.
Transfer RNA (tRNA) molecules bring amino acids to the ribosome. Each tRNA has an anticodon that matches the codon on the mRNA. The ribosome moves along the mRNA, reading codons and facilitating the binding of corresponding tRNAs.
As amino acids are brought to the ribosome, they are linked together to form a polypeptide chain. Once a stop codon is reached, the completed polypeptide chain folds into a functional protein.

Gene Expression

Gene expression is the process by which information from a gene is used to synthesize a functional gene product, usually a protein. This process encompasses both transcription and translation. Gene expression is regulated at multiple levels, from DNA packing to RNA processing and degradation.
Gene expression allows cells to respond to their environment and perform specialized functions. For example, different cells in the body express different genes, making them liver cells, skin cells, or muscle cells.

• Transcription factors are proteins that help turn specific genes on or off by binding to nearby DNA.
• Epigenetic modifications, like DNA methylation, can also influence gene expression without changing the underlying DNA sequence.

Understanding gene expression is crucial for studying how cells operate and develop, and for identifying the causes of certain diseases.