L5-the central dogma of molecular biology?

Question 1

What is the central dogma of molecular biology?

Answer 1

The central dogma describes the flow of genetic information inside the cell.

It follows this sequence:

1. DNA is transcribed into mRNA
2. mRNA is translated into protein

This process ensures that the genetic instructions in DNA are ultimately used to build functional proteins.

Only specific DNA sequences (called genes) are used in this process—not the entire DNA molecule.

This flow includes two major steps: transcription and translation.

Question 2

What happens during transcription? (Step 1 of the central dogma)

Answer 2

Transcription occurs in the nucleus and creates a pre-mRNA copy from a DNA gene.

Key components:

1. Promoter and terminator sequences
2. RNA polymerase II (DNA-dependent enzyme)
3. Ribonucleotides: ATP, GTP, CTP, UTP

Transcription has three phases:

1. Initiation: RNA polymerase binds to the promoter
2. Elongation: the RNA strand is synthesized (RNA polymerase moves along the DNA adding ribonucleotides one by one.)
3. Termination: transcription ends at a terminator sequence

The product is a pre-mRNA strand (also called hnRNA).

This pre-mRNA -> undergoes maturation before leaving the nucleus.

Question 3

How is pre-mRNA matured before translation?

Answer 3

Maturation of mRNA happens in the nucleus and includes 3 key modifications:

1. 5’ Capping: adds a special methylated nucleotide to protect the mRNA
2. 3’ Poly-A Tail: adds** hundreds of adenines** for stability and regulation
3. Splicing:
   * introns(non-coding sequence) are removed, and
   * exons(coding squence) are joined

Splicing is performed by a structure called the spliceosome (made of snRNPs-small nuclear ribonucleoproteins).

Once mature ->mRNA exits the nucleus* to be used in* translation.

Different RNA polymerases
->different types of RNA:

1. Polymerase I ➝ rRNA(ribosomal RNA)
2. Polymerase II ➝ mRNA
3. Polymerase III ➝ tRNA(transfer RNA)

Only mature mRNA can be translated into protein.

Question 4

What happens during translation? (Step 2 of the central dogma)

Answer 4

Translation occurs in the cytoplasm at the ribosome.

1. The mRNA codons (triplets of bases) are matched to amino acids using the genetic code.

2.Important genetic code features:

*Start codon = AUG (Methionine)

*Stop codons = UAG, UAA, UGA

3. Redundancy(Some amino acids are coded by multiple codons) of codons
4. Universality(The genetic code is shared by almost all organisms) of codons
5. Each amino acid is delivered by a tRNA molecule, which has:

*An anticodon that matches the mRNA codon

*A CCA tail that binds the correct amino acid

6.Ribosomes guide the process via three key sites:

1.A (Aminoacyl site),
2.P (Peptidyl site ), and
3. E (Exit site)

Question 5

What happens after translation?

Answer 5

Once the amino acid chain is complete ->the protein undergoes maturation, including:

1. Folding into its 3D structure
2. Post-translational modifications (e.g.:
   * phosphorylation-addition a phosphate group **
   * glycosylation-addition sugar chains**)
3. Proteins are then localized to specific areas in the cell using signal sequences.
4. This final protein is now functional and can:

1.Act as an enzyme

2.Form structural components

3.Serve in signaling, transport, or regulation

This completes the flow of information:
DNA ➝ RNA ➝ Protein ➝ Function

The entire central dogma ensures that genes control the traits and functions of living organisms.