Biology Chapter 10: Gene Expression

Chapter 10: How Genes Work

• The process by which genes are transcribed into RNA and translated to make proteins is called gene expression
• Nucleotides are the building blocks of proteins leading to traits/phenotypes

Gene Expression Steps

• Transcription: The first step, converting DNA information into RNA information. An mRNA copy of the gene is made in the nucleus.
• Translation: The second step, converting RNA information into amino acid order in a protein. Amino acids are linked together at the ribosome to create the protein encoded by the mRNA.

Transcription

• RNA is produced to create a blueprint for protein production
• RNA differs from DNA:
  • Uses ribose instead of deoxyribose
  • Uses uracil instead of thymine
  • Is single-stranded
• Three major types of RNA are needed for translation:
  • Messenger RNA (mRNA)
  • Transfer RNA (tRNA)
  • Ribosomal RNA (rRNA)
• All RNA is produced via transcription
• Transcription begins when RNA polymerase binds to the promoter
• An mRNA molecule is produced as RNA polymerase moves down the template strand of DNA
• Transcription ends when RNA polymerase reads through the terminator

RNA Produced During Transcription

• Transfer RNA (tRNA): Transports correct amino acids to the ribosome, pairs with mRNA codons revealed in the mRNA
• Messenger RNA (mRNA): Specifies order of amino acids in a protein, using three-base codons
• Ribosomal RNA (rRNA): Main component of ribosomes; assists in making covalent bonds linking amino acids

Translation

• Translation is the process that converts the information in RNA into amino acid sequences
• Three types of RNA have specific roles in translation:
  • Messenger RNA (mRNA) contains the information for amino acid sequence, read in groups of three nucleotides called codons.
  • Transfer RNA (tRNA) has a group of three nucleotides (anticodon) at one end that match with the mRNA codon, and an amino acid attachment site at the other end
  • Ribosomal RNA (rRNA) forms ribosomes and provides a location for amino acid connections
• Translation begins when mRNA binds to a ribosome
• A first tRNA molecule carrying the amino acid methionine binds to the start codon
• As the ribosome moves one codon at a time, tRNA molecules bind to mRNA, allowing the ribosome to link the amino acids in the correct order.
• When the ribosome reaches a stop codon, the mRNA and the completed amino acid chain separate from the ribosome.

The Genetic Code

• Genetic code is the information in DNA
• Nucleotides are read in groups of three (codons)
• Each codon codes for only one specific amino acid
• Organisms on Earth use the same codons to code for the same amino acids
• 61 codons code for 20 amino acids
• Many amino acids are coded for by two or more codons

Sickle Cell Disease

• Sickle cell disease occurs when red blood cells have an abnormally shaped hemoglobin, causing cells to be sickle-shaped
• Red blood cells (RBCs) are responsible for carrying oxygen to the body's organs.
• Hemoglobin is a protein in RBCs that carries oxygen.
• A person deficient in hemoglobin is considered anemic; their RBCs can't carry oxygen efficiently
• Sickle cell disease results from inheriting a gene that codes for abnormal hemoglobin (HbS)
• Normal hemoglobin is called HbA
• The disease occurs because of a change/mutation in only one nucleotide which leads to a change in the amino acid sequence of the protein, resulting in a dysfunctional protein.

Additional Notes

• Proteins are composed of amino acids in a specific sequence. A change in just one amino acid can cause a significant change in the protein.
• Amino acids link together forming a shape that is stable and most important to the chemically stable protein shape, allowing the protein to work correctly. The shape of a protein is crucial for its function.