1. Genetic Code

Concept:

The genetic code is the sequence of nucleotides (nu) in a gene that dictates the sequence of amino acids (a.a) in a polypeptide chain.

Characteristics of the genetic code:

• Triplet code: Each triplet of nucleotides (codon) codes for one amino acid.
• Degeneracy: Multiple codons can code for the same amino acid.
• Specificity: Each codon codes for only one amino acid.
• Universality: Almost all living organisms share the same genetic code, with a few exceptions.

Note:

• Degeneracy helps protect genetic information from point mutations.
• Universality suggests a common origin of life on Earth.

Non-degenerate codons:

• AUG: Codes for methionine (Met) in prokaryotes (SVNS) and formyl methionine (fMet) in eukaryotes (SVNT).
• UGG: Codes for tryptophan (Trp).

2. DNA Replication

Principles of DNA replication:

• Template: The parental DNA strand serves as a template for the synthesis of a new strand.
• Antiparallel: The new strand is synthesized in the opposite direction to the template strand (5′-3′).
• Semi-conservative: Each daughter DNA contains one strand from the parent DNA and one newly synthesized strand.
• Complementary: The nucleotides on the new strand pair with the nucleotides on the template strand according to the A-T, G-C pairing rule.

Mechanism of DNA replication:

1. Step 1: Unwinding the DNA, exposing two single strands (template strands).
   • The 3′ strand is synthesized continuously.
   • The 5′ strand is synthesized discontinuously (forming Okazaki fragments), requiring primers to initiate synthesis.
2. Step 2: Synthesizing the new strand in the 5′-3′ direction based on the complementary base pairing rule.
3. Step 3: From one parental DNA, two identical daughter DNAs are formed.

Number of nucleotides provided by the environment for replication:

N(2^x – 1), where x is the number of replication cycles.

Note:

• Primers are synthesized by RNA polymerase.
• Okazaki fragments are joined together by ligase.

Replication in Prokaryotes (SVNS):

• Has one origin of replication.
• Uses fewer enzymes than eukaryotes.
• Does not occur in the S phase of the cell cycle.

Replication in Eukaryotes (SVNT):

• Has multiple origins of replication.
• Uses more enzymes than prokaryotes.
• Occurs in the S phase of the cell cycle.

Note:

• DNA polymerase plays a key role in assembling nucleotides into the new strand.
• DNA polymerase slides along the template strand in the 3′-5′ direction, while simultaneously synthesizing the new strand in the 5′-3′ direction.

Strand synthesized discontinuously:

• The strand synthesized discontinuously is the 5′ strand.
• This strand is synthesized in the 5′-3′ direction, but it is opposite the direction of the unwinding enzyme.

Number of primers:

• In each replication cycle: Number of primers = Number of Okazaki fragments + 1.
• In one origin of replication: Number of primers = Number of Okazaki fragments + 2.

Note:

• Primers are removed after the daughter DNA is fully synthesized.
• Okazaki fragments are joined together to form a continuous DNA strand.

Culturing in a medium:

• Only 2 daughter DNAs contain the original mt (mitochondrial DNA).
• There are 2^x – 2 daughter DNAs containing the new mt.

Note:

• The original mt is the mt found in the parental DNA.
• The new mt is the mt provided by the culture medium.

RNA polymerase:

• Synthesizes RNA primers.

DNA polymerase:

• Assembles nucleotides to bind to the nucleotides on the template strand.
• Moves in opposite directions on the same template (template strand in the 3′-5′ direction).

Note:

• DNA polymerase has 5′-3′ exonuclease activity, which helps repair errors during replication.
• DNA polymerase has 3′-5′ exonuclease activity, which helps remove primers.

Strand synthesized discontinuously:

• The strand synthesized discontinuously is the 5′ strand.
• This strand is synthesized in the 5′-3′ direction, but it is opposite the direction of the unwinding enzyme.

Note:

• The strand synthesized discontinuously must be joined together to form a continuous strand.
• Okazaki fragments are joined together by ligase.

Summary

The genetic code and DNA replication are two crucial processes in cellular activity. The genetic code ensures the accurate transmission of genetic information from one generation to the next. DNA replication ensures that each daughter cell receives a complete copy of the genetic information from the parent cell.