Protein Synthesis Notes

How Cells Grow, Specialize, and Die: Protein Synthesis

Specific Learning Objectives

• Describe the structure and functions of proteins.
• Describe the structure of nucleic acids, differentiating between DNA and RNA.
• Define the components of a nucleotide.
• Differentiate between the nucleotide bases of DNA and RNA.
• Explain what the genetic code is and what it is coding for.
• Describe the two-step process (transcription & translation) that results in gene expression.
• Explain the role of DNA, rRNA, tRNA, and mRNA in the production of a protein.
• Describe the cell cycle.
• Give the details of DNA replication.
• Explain what occurs during mitosis and cytokinesis.
• Describe how mitosis differs from meiosis.

The Proteome

• Proteome: all the proteins that a cell makes.
• Proteomics: the study of the proteins in a cell.
• Not all cells make all proteins; the proteome varies between cells (e.g., muscle vs. skin cell, melanoma vs. normal melanocyte).
• Cells are protein factories that constantly synthesize many different proteins for cell functions or export.
• The cell’s DNA contains all the instructions for making proteins.

Protein Synthesis – Basic Concepts

• The arrangement of amino acids determines a protein's shape, properties, and functions.
• Gene: a segment of DNA that specifies the structure of a protein.
• Genetic code: arrangement of nucleotides in DNA and RNA that determines amino acid sequence of a specific protein.
• Gene expression: production of proteins from the information stored in DNA.
• Central dogma: a directional flow of information from DNA to RNA to protein.
• Gene expression involves two steps:
  • Transcription
  • Translation

Transcription

• Occurs in the nucleus.
• A copy of a small part of the stored information in DNA (gene) is produced.
• DNA is transcribed into mRNA: DNA \rightarrow mRNA.

Translation

• Occurs in the cytoplasm.
• Copied information is converted into a protein.
• mRNA is translated into protein: mRNA \rightarrow protein.

Transcription

• DNA uncoils and unzips.
• RNA polymerase produces messenger RNA (mRNA).
• mRNA exits the nucleus through nuclear pores.
• Initiated by transcription factors that recruit RNA polymerase enzyme.
• DNA has two strands:
  • Coding strand: contains the gene sequence.
  • Template strand: used to make mRNA.
• The two strands of DNA separate (unzip).
• RNA polymerase binds at a promoter region.
• Genetic information (a gene) is copied from the template strand of DNA to make a strand of ribonucleic acid (RNA) called mRNA (messenger RNA).
• RNA polymerase catalyzes the formation of an mRNA chain following the rules of complimentary base pairing:
  • A with U
  • C with G
• Special sequences/signals in DNA indicate when a gene starts and stops.
• Transcription ends at a terminator sequence.
• mRNA exits the nucleus through nuclear pores into the cytoplasm.

Translation

• mRNA carries genetic information from the nucleus to the ribosomes.
• The sequence is read by translational machinery in the ribosome, in triplets of nucleotides (codon).
• Translation starts at the start codon (AUG) of each gene in the mRNA.
• Each codon codes for a specific amino acid.
• As each codon is read, a tRNA with a complimentary sequence (anticodon) binds to each triplet.
• The tRNA also carries the amino acid specified by the codon.
• Amino acids are joined together by peptide bonds, in the sequence specified by the mRNA, to make a peptide/protein.
• There are 64 possible codons in mRNA and only 20 naturally occurring amino acids (aa).
• Some amino acids are specified by only one codon (e.g., methionine, also the start codon AUG).
• Others are specified by up to six different codons (e.g., Leucine: UUA, UUG, CUU, CUC, CUG, CUA).
• DNA code is "degenerate."
• Three codons do not code for an amino acid but signal termination of the peptide chain (stop codons: UAG, UAA, UGA).

Post-Translational Modification

• The chemical modification of a protein following translation.
• It is one of the last steps in protein synthesis.
• After translation, proteins can be modified by attaching other functional groups, which can change or extend their functions (e.g., lipids to form lipoproteins, carbohydrates to form glycoproteins).
• Amino acids may be cleaved off the end of the protein, or the polypeptide can be cut in half (e.g., insulin).
• Other modifications, such as phosphorylation, are a common way of controlling the behavior of a protein (e.g., activating or inactivating an enzyme).

Compendium Questions

1. Why do we need proteins in the body?
2. What is a nucleic acid?
3. What are the similarities and differences between DNA and RNA?
4. What is a gene?
5. What is the connection between DNA, RNA, and protein synthesis?
6. Describe transcription and translation.
7. What is meant by the term phenotype and genotype?
8. What is the difference between a chromosome and a chromatid?
9. What are the stages of mitosis?
10. What are the stages of meiosis?
11. Which cell types undergo each type of cell division?