DNA Structure and Gene Expression

Study Notes

Structure of DNA and RNA

DNA has a double-stranded structure with two sugar-phosphate backbones, while RNA has a single backbone.
Base pairing rules differ: DNA pairs adenine (A) with thymine (T) and cytosine (C) with guanine (G); RNA pairs adenine (A) with uracil (U) and cytosine (C) with guanine (G).

Transcription and Translation

DNA transcription involves RNA polymerase unzipping DNA, using it as a template to synthesize messenger RNA (mRNA).
mRNA exits the nucleus and enters the cytoplasm for translation.
In translation, mRNA binds to a ribosome, which locates the start codon AUG to begin protein synthesis.
The ribosome continues scanning the mRNA until it encounters a stop codon, completing protein synthesis.

Amino Acids and Genetic Code

Each codon consists of three nucleotide bases on mRNA; corresponds to specific amino acids according to the genetic code.
For example, a 3-letter codon can be used to find and define the specific amino acid sequence.

Mutation and Sickle Cell Disease

A mutation is defined as an error in the DNA sequence that leads to variations; it can alter amino acids in proteins.
In sickle cell disease, the mutation process includes several steps:
- DNA replication errors lead to base substitutions.
- Mutated DNA results in altered mRNA.
- Ribosome translates the mutated mRNA, producing an abnormal protein.
- This abnormal protein leads to the formation of sickle-shaped red blood cells, causing the disease.

Natural Selection

Natural selection favors organisms with advantageous traits that enhance survival and reproduction, allowing those traits to be passed to their offspring.
Sickle cell disease is prevalent in individuals of Afro-Caribbean descent due to historical exposure to malaria; the sickle cell trait offers protection against malaria, leading to a higher frequency of the trait.

Yeast Cell Model

Model 2 accurately demonstrates the process in yeast cells, showing how RNA polymerase transcribes spider DNA into mRNA, which ribosomes then translate into proteins.
Model 1 is incorrect as it claims yeast cells cannot read spider genes; all DNA can be transcribed and translated universally across species.

Description

Understand the structure of DNA, including its sugar-phosphate backbone and base pairing rules. Learn about the process of transcription and translation, and how RNA polymerase plays a crucial role in gene expression.