DNA carries the instructions for life, but it cannot leave the nucleus. RNA acts as the messenger that carries these instructions to the cytoplasm, where proteins are made. Before RNA can be used, it goes through several processing steps. Protein synthesis then translates the RNA message into functional proteins. Throughout these processes, the cell regulates genes to ensure the right proteins are made at the right time.
RNA Processing
After transcription, the newly formed RNA (called pre-mRNA) must be modified before it becomes mature mRNA.
1. Addition of 5′ Cap
A special cap is added to the 5′ end of the RNA. This cap:
- Protects RNA from digestion
- Helps ribosomes attach for protein synthesis
2. Addition of Poly-A Tail
The 3′ end of the RNA receives a chain of adenine nucleotides called the poly-A tail.
- Increases RNA stability
- Helps with export from the nucleus
3. RNA Splicing
Genes contain coding regions (exons) and non-coding regions (introns). Introns are removed, and exons are joined together.
Sometimes, different combinations of exons are joined—this is called alternative splicing and allows one gene to produce many proteins.
Protein Synthesis (Translation)
Protein synthesis converts the mRNA message into a chain of amino acids that form a protein. This process takes place in the cytoplasm on ribosomes.
Main Components
- mRNA – carries the genetic code
- tRNA – delivers amino acids
- Ribosomes – join amino acids together
Steps in Translation
1. Initiation
The ribosome binds to the mRNA near the start codon (AUG). The first tRNA carries the amino acid methionine and attaches to this codon.
2. Elongation
More tRNAs bring amino acids. The ribosome joins them to form a growing chain.
3. Termination
When a stop codon (UAA, UAG, UGA) is reached, the protein chain is released.
4. Post-Translational Modifications
After synthesis, proteins may undergo modification such as:
- Folding into the correct shape
- Adding chemical groups (phosphate, sugar, etc.)
- Transport to organelles (ER, Golgi)
Gene Regulation
Cells control gene expression so that proteins are produced only when needed. This saves energy and ensures proper function.
Levels of Gene Regulation
1. Transcriptional Control
Determines whether a gene’s transcription starts. Transcription factors, promoters, enhancers, and silencers play key roles.
2. Post-Transcriptional Control
Controls mRNA processing and stability. Includes alternative splicing and RNA degradation.
3. Translational Control
Regulates how efficiently mRNA is used to make proteins.
4. Post-Translational Control
Involves regulating protein folding, modification, and degradation.
Importance of Gene Regulation
- Ensures proteins are made only when required
- Helps cells adapt to changes
- Controls growth and development
- Prevents abnormal protein production
Examples of Gene Regulation
- Insulin production increases after eating
- Stress activates genes for adrenaline production
- Stem cells switch genes on/off during differentiation
Detailed Notes:
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