Molecular Biology: DNA and RNA Structure

Study Notes

Structure of Genetic Material

DNA (deoxyribonucleic acid) consists of 2 long anti-parallel chains of nucleotides, forming a double-stranded structure.
A nucleotide consists of a phosphate group, a pentose sugar (deoxyribose), and a nitrogenous base (purines: Adenine and Guanine, pyrimidine: Cytosine and Thymine).
Complementary base pairing occurs between Adenine and Thymine, and Guanine and Cytosine.

RNA (Ribonucleic Acid)

RNA is single-stranded and contains a pentose sugar (ribose) and Uracil instead of Thymine.
There are 3 types of RNA: mRNA (messenger), rRNA (ribosomal), and tRNA (transfer).

DNA Replication

DNA replication occurs in the cytoplasm (prokaryotes) and nucleus (eukaryotes).
The process is semi-conservative, resulting in one old strand and one new strand.
The process involves:
- Unwinding the double helix using enzymes gyrase and helicase.
- Proteins stabilizing the unwound DNA.
- Leading strand extending continuously.
- RNA primers being synthesized and removed by DNA polymerase.
- Discontinuous fragments being sealed by DNA ligase.

Protein Synthesis

The central dogma of molecular biology states that DNA → mRNA → protein.
Transcription involves making a mRNA copy of DNA.
Translation involves mRNA → protein.

Transcription

Transcription occurs in the cytoplasm (prokaryotes) and nucleus (eukaryotes).
The process involves:
- RNA polymerase binding to the promoter site.
- Unwinding the DNA and synthesizing complementary mRNA.
- RNA polymerase continuing to move down the DNA until it reaches the terminator.
mRNA processing in eukaryotes includes:
- Removing introns (intervening, non-coding regions) and joining exons (expressed, coding regions).
- Using small nuclear ribonucleoproteins (snRNPs).

Translation

Translation occurs in the cytoplasm (both prokaryotes and eukaryotes).
The process involves:
- mRNA being sandwiched by ribosomal subunits.
- tRNA carrying amino acids and binding to the mRNA.
- Amino acids forming peptide bonds and becoming a strand.
- The process repeating until the stop codon.

Genetic Code

The genetic code is read in codons, which are groups of 3 nucleotides (A,C,G,U).
Each codon codes for:
- An amino acid (sense).
- A stop signal (non-sense).
The genetic code is degenerate, meaning multiple codes can code for one amino acid.
There are 64 possible codons but only 20 amino acids.
The start codon is AUG (methionine).

Description

Learn about the structure and composition of DNA and RNA, including nucleotides, nitrogenous bases, and complementary base pairing. Understand the differences between DNA and RNA, and the three types of RNA.