Genes in our DNA provide instructions for building proteins. 

For IGCSE Biology, you should be able to understand the steps of protein synthesis from DNA.

DNA 

• A DNA molecule is a polymer composed of many nucleotides.
•  Each nucleotide consists of a sugar molecule, phosphate group, and a base.
  • The sugar molecule and phosphate group make up the sugar-phosphate backbone of the DNA strand.
  • There are 4 types of bases: thymine (T), cytosine (C), adenine (A), guanine (G)
  • A base from one strand bonds with a base from another strand and forms complementary base pairing.
    • Thymine (T) can bond with adenine (A).
    • Cytosine (C) can bond with guanine (G).
• A DNA molecule is a double helix where the two strands are twisted around each other.

👆 A diagram of the structure of DNA 👆 

From DNA to mRNA (Transcription)

DNA is located inside the nucleus of cell. Since it is too large to be moved out of the nucleus into ribosomes, where protein is synthesized, a copy of a section of DNA is made to transfer the genetic information. The copy is called mRNA (messenger RNA), and the process is called transcription.

Steps of transcription:

1. The two strands of DNA are zipped open by breaking the bond between base pairing.
2. RNA nucleotides form complementary base pairings with the exposed base from one of the DNA strands.
3. The RNA nucleotides join with each other to form a mRNA strand.
4. The newly formed mRNA strand can now leave the nucleus and move to ribosome.

• Instead of thymine (T) base, RNA nucleotide has uracil (U) base.
• Uracil (U) will pair with adenine (A) from DNA strand.

☝️ Comparison of the structure of DNA and RNA ☝️

From mRNA to Protein (Translation)

Steps of Translation:

1. mRNA attaches to the ribosome.
2. Every 3 bases of mRNA base is called a codon. 
3. A tRNA (transfer RNA) molecule contains a distinct anticodon base sequence, which can form complementary base pairing with the codon on the mRNA.  
4. Each tRNA carries a specific amino acid that corresponds to the anticodon on the tRNA.
5. As the mRNA moves along the ribosome, tRNAs bring more and more amino acid, and the amino acids bond together to become a polypeptide chain. 
6. The polypeptide later folds itself into its unique structure to become a protein.

👇 This is a diagram summarizing the steps of transcription and translation 👇 

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