Genotype to Phenotype: The Role of Proteins

Genotype to Phenotype: Proteins

Learning Objectives

  • Describe the general arrangement of genes on a chromosome, including coding and non-coding parts.

  • Distinguish between genotypes and phenotypes in multiple case examples.

  • Describe the processes by which information in the genotype contributes to the production of proteins by a cell.

  • Describe the two general types of gene products.

  • Explain how variation in coding DNA between alleles could cause variation in the structure and function of the protein versions encoded by the alleles.

Genetic Differences

  • Genetic differences can be explained by differences in protein-coding and non-protein-coding DNA sequences.

  • Focus: How differences between alleles in protein-coding DNA sequences explain phenotype differences.

Chromosomes and Genes

  • All cells have DNA in chromosomes.

  • Diploid organisms have chromosome pairs (one from each parent).

  • Chromosomes contain genes (DNA segments).

  • Chromosome pairs contain the same genes but may have different versions (alleles).

    • Example: Sam inherits different alleles of gene 2 from his father (green) and mother (orange).

Genotype vs. Phenotype

  • Genotype: An individual's DNA sequence on all chromosome pairs.

    • For a specific gene, it's the two alleles an individual carries.

  • Phenotype: Observable characteristics or traits of an organism or cell.

    • Millions of possible traits.

    • Examples: hair color, eye color, earlobe shape (observable through senses).

Examples of Genotypes vs. Phenotypes

  • Phenotypes

    • Involuntary movements in neurologic disorders.

    • Number of instances of a certain amino acid in a protein

    • Age of onset of disease.

    • Mental states of patients.

    • Brain volume.

  • Genotypes

    • Number of instances of the sequence CAG in the DNA sequence of a gene.

  • Genotypes can be observed using modern technology (DNA sequencing).

  • Genotypic and phenotypic patterns are correlated.

  • Scientists study causal models of how genotypes influence phenotypes.

  • How genotypic information is used to produce phenotypes is a process.

Key Molecules: mRNA and Proteins

  • mRNA and proteins are key links between genotypes and phenotypes.

  • They are gene products.

mRNA (Messenger RNA)

  • A sequence of nucleotides.

  • Contains instructions for assembling amino acids into a protein.

Proteins

  • Large molecules impacting cell structure and function.

  • Examples:

    • Enzymes

    • Antibodies

    • Contractile proteins

    • Completion proteins

    • Structural proteins

    • Hormones

    • Transport proteins

    • Tumor suppressor proteins

    • Regulatory proteins

  • Proteins have a significant impact on the form and function of individuals.

Protein Composition: Amino Acids

  • Proteins are composed of amino acid sequences.

  • Protein Coding DNA (Coding DNA): Contains information to encode specific amino acids via a triplet code (codons).

    • Each triplet of nucleotides encodes a specific amino acid.

    • Example: CCA encodes proline.

    • Stop codons signal the end of protein production.

  • There are 20 amino acids in proteins.

  • Amino acid properties affect protein structure and function.

    • Cysteine: Forms strong disulfide bonds.

    • Proline: Hydrophobic (moves away from water).

    • Glutamate: Negatively charged (attracted to positive charges).

  • Amino acids are connected by peptide bonds.

  • Proteins vary in the number of amino acids, shapes, and sizes.

    • Short proteins are called peptides.

Examples of Proteins

  • Oxytocin:

    • A short protein (9 amino acids).

    • Produced by the hypothalamus.

    • Roles: uterine contractions during childbirth, milk secretion during breastfeeding.

  • Titin:

    • The largest human protein (~38,000 amino acids).

    • Found in skeletal and cardiac muscle cells.

    • Function: molecular spring for muscle elasticity and contraction.

  • Protein amount is quantified by the number of molecules produced, not size.

Processes Connecting Genotypes to Phenotypes

1. Genotype to mRNA: Transcription

  • Transcription: The process of copying DNA sequence of a gene into an mRNA molecule.

Reminders:

  • Cells contain pairs of chromosomes with DNA (one from each parent).

  • Focus on one chromosome (represented by a line).

  • Yellow box: A gene's DNA sequence.

  • Blue box: Coding region of the gene (codes for a protein).

  • Noncoding DNA: sequences to the left and right of the coding region. These do not code for proteins but are necessary for the gene to be transcribed.

  • Introns also do not code of proteins, and are thus noncoding DNA.

The Process of Transcription:

  • RNA polymerase makes an RNA copy of the coding DNA by complementary base pairing.

    • A binds to T (in DNA) or U (in RNA).

    • C binds to G.

  • The RNA copy is called messenger RNA (mRNA) or a transcript.

  • Higher transcription rates mean more mRNA and protein production.

  • Transcription is controlled by the non-coding part of the gene.

  • In mRNA, uracil (U) replaces thymine (T) and pairs with adenine (A).

  • A gene comprises coding and noncoding regions.

  • Noncoding regions regulates the cell to transcribe the protein in the correct patters and cells.

2. mRNA to Protein: Translation

  • Misconception: Genes, DNA, and mRNA contain information but don't physically make proteins.

Role of Ribosomes:

  • Ribosomes make proteins through translation.

  • mRNA transmits information, and ribosomes read it to construct proteins.

  • Ribosomes assemble amino acids into a sequence (polypeptide).

  • Analogy: mRNA is a recipe, ribosomes are the chef.

  • The ribosomes read codons and creates the corresponding proteins.

*To summarize: transcription connects genotype to mRNA, translation connects mRNA to protein.

3. Protein to Phenotype: Folding

  • Amino acid properties cause different folding patterns in proteins.

  • Folding patterns impact protein shape and how it interacts within the cell.

  • Transcription, translation, and folding are interconnected processes.

Key Points to Remember

  • Organisms have two alleles of each gene (one from each parent).

  • Both alleles are typically transcribed into mRNA and translated into protein.

  • If alleles differ in the coding sequence, different protein versions are produced in the same individual.

Lecture Summary

  • Genes are information; most encode proteins (protein-coding genes).

  • Protein-coding genes have a sequence that is transcribed into mRNA.

  • mRNA is translated by ribosomes into a string of amino acids, which folds into a protein.

  • mRNA and proteins are gene products.

  • All protein-coding genes have noncoding regions that regulate the timing, location, and amount of protein production.

Genotype to Phenotype: Proteins

Learning Objectives

  • Describe the general arrangement of genes on a chromosome, including coding and non-coding parts, and understand their respective roles in gene expression.

  • Distinguish between genotypes and phenotypes in multiple case examples, detailing how specific genetic variations manifest as observable traits.

  • Describe the processes by which information in the genotype contributes to the production of proteins by a cell, including transcription, translation, and post-translational modifications.

  • Describe the two general types of gene products: proteins and functional RNA molecules (e.g., tRNA, rRNA, microRNA).

  • Explain how variation in coding DNA between alleles could cause variation in the structure and function of the protein versions encoded by the alleles, impacting phenotype.

Genetic Differences

  • Genetic differences can be explained by differences in protein-coding and non-protein-coding DNA sequences. Understand the functional consequences of variations in these regions.

  • Focus: How differences between alleles in protein-coding DNA sequences explain phenotype differences, with emphasis on specific examples (e.g., point mutations, frameshift mutations).

Chromosomes and Genes

  • All cells have DNA in chromosomes. Describe the structure and organization of chromosomes within the cell nucleus.

  • Diploid organisms have chromosome pairs (one from each parent). Explain the significance of diploidy in genetic inheritance and variation.

  • Chromosomes contain genes (DNA segments). Define a gene and its role as a unit of heredity.

  • Chromosome pairs contain the same genes but may have different versions (alleles). Explain the concept of alleles and their contribution to genetic diversity.

    • Example: Sam inherits different alleles of gene 2 from his father (green) and mother (orange).

Genotype vs. Phenotype

  • Genotype: An individual's DNA sequence on all chromosome pairs. Describe how genotypes are determined through DNA sequencing technologies.

    • For a specific gene, it's the two alleles an individual carries. Explain how different allelic combinations (homozygous vs. heterozygous) affect phenotype.

  • Phenotype: Observable characteristics or traits of an organism or cell. Explain how phenotypes are influenced by both genetic and environmental factors.

    • Millions of possible traits.

    • Examples: hair color, eye color, earlobe shape (observable through senses). Provide additional examples like height, weight, and disease susceptibility.

Examples of Genotypes vs. Phenotypes

  • Phenotypes

    • Involuntary movements in neurologic disorders. Relate these movements to specific genetic mutations and affected neural pathways.

    • Number of instances of a certain amino acid in a protein: Discuss the impact of amino acid repeats on protein structure and function (e.g., Huntington's disease).

    • Age of onset of disease. Explain how genetic predispositions influence the timing of disease manifestation.

    • Mental states of patients. Discuss the genetic basis of psychiatric disorders and their impact on cognitive and emotional functions.

    • Brain volume. Correlate specific genes with variations in brain structure and their functional implications.

  • Genotypes

    • Number of instances of the sequence CAG in the DNA sequence of a gene. Explain how CAG repeats in the Huntingtin gene cause Huntington's disease.

  • Genotypes can be observed using modern technology (DNA sequencing). Describe various DNA sequencing methods (e.g., Sanger sequencing, next-generation sequencing).

  • Genotypic and phenotypic patterns are correlated. Explain how statistical analyses (e.g., GWAS) are used to identify these correlations.

  • Scientists study causal models of how genotypes influence phenotypes. Discuss experimental approaches to validate genotype-phenotype relationships (e.g., gene knockout studies, CRISPR-Cas9).

  • How genotypic information is used to produce phenotypes is a process that involves multiple steps, including transcription, translation, and protein folding.

Key Molecules: mRNA and Proteins

  • mRNA and proteins are key links between genotypes and phenotypes. Elaborate on their roles as intermediaries in gene expression.

  • They are gene products: Explain the central dogma of molecular biology (DNA -> RNA -> Protein).

mRNA (Messenger RNA)

  • A sequence of nucleotides. Describe its synthesis during transcription and processing (e.g., splicing, capping, polyadenylation).

  • Contains instructions for assembling amino acids into a protein. Explain how mRNA codons specify the amino acid sequence of a protein.

Proteins

  • Large molecules impacting cell structure and function. Discuss their diverse roles in cellular processes and organismal physiology.

  • Examples:

    • Enzymes: Catalyze biochemical reactions in cells.

    • Antibodies: Recognize and neutralize foreign pathogens.

    • Contractile proteins: Mediate muscle contraction and cell movement.

    • Completion proteins: Involved in DNA processes.

    • Structural proteins: Provide support and shape to cells and tissues.

    • Hormones: Act as chemical messengers between cells.

    • Transport proteins: Carry molecules across cell membranes.

    • Tumor suppressor proteins: Regulate cell growth and prevent cancer.

    • Regulatory proteins: Control gene expression and cellular differentiation.

  • Proteins have a significant impact on the form and function of individuals. Discuss how protein dysfunction leads to various diseases.

Protein Composition: Amino Acids

  • Proteins are composed of amino acid sequences. Describe the chemical structure of amino acids and the properties of their side chains.

  • Protein Coding DNA (Coding DNA): Contains information to encode specific amino acids via a triplet code (codons). Explain how the genetic code translates codons into amino acids.

    • Each triplet of nucleotides encodes a specific amino acid.

    • Example: CCA encodes proline. Show how different codons encode for different amino acids.

    • Stop codons signal the end of protein production. List the different stop codons and their significance.

  • There are 20 amino acids in proteins. Discuss their classification based on their chemical properties (e.g., polar, nonpolar, acidic, basic).

  • Amino acid properties affect protein structure and function. Explain how amino acid side chains influence protein folding and stability.

    • Cysteine: Forms strong disulfide bonds, stabilizing protein structures.

    • Proline: Hydrophobic (moves away from water), introduces kinks in the polypeptide chain.

    • Glutamate: Negatively charged (attracted to positive charges), forms ionic bonds and salt bridges.

  • Amino acids are connected by peptide bonds. Describe the formation of peptide bonds and their role in creating the protein backbone.

  • Proteins vary in the number of amino acids, shapes, and sizes. Discuss the different levels of protein structure (primary, secondary, tertiary, quaternary).

    • Short proteins are called peptides. Provide examples of biologically active peptides (e.g., insulin, glucagon).

Examples of Proteins

  • Oxytocin:

    • A short protein (9 amino acids).

    • Produced by the hypothalamus. Briefly describe the role of the hypothalamus in hormone regulation.

    • Roles: uterine contractions during childbirth, milk secretion during breastfeeding. Explain the mechanism of action of oxytocin in these processes.

  • Titin:

    • The largest human protein (~38,000 amino acids).

    • Found in skeletal and cardiac muscle cells. Describe the structure and function of muscle cells.

    • Function: molecular spring for muscle elasticity and contraction. Explain how Titin contributes to muscle mechanics.

  • Protein amount is quantified by the number of molecules produced, not size. Discuss factors influencing protein expression levels (e.g., transcription factors, mRNA stability).

Processes Connecting Genotypes to Phenotypes
1. Genotype to mRNA: Transcription

  • Transcription: The process of copying DNA sequence of a gene into an mRNA molecule. Describe the role of RNA polymerase and transcription factors.

Reminders:

  • Cells contain pairs of chromosomes with DNA (one from each parent). Briefly review chromosome structure and DNA organization.

  • Focus on one chromosome (represented by a line).

  • Yellow box: A gene's DNA sequence. Define the components of a gene (promoter, coding region, terminator).

  • Blue box: Coding region of the gene (codes for a protein). Explain the concept of open reading frames (ORFs).

  • Noncoding DNA: sequences to the left and right of the coding region. These do not code for proteins but are necessary for the gene to be transcribed. Describe the roles of promoters, enhancers, and silencers.

  • Introns also do not code of proteins and are thus noncoding DNA. Explain the process of RNA splicing and its significance.

The Process of Transcription:

  • RNA polymerase makes an RNA copy of the coding DNA by complementary base pairing. Describe the steps of transcription (initiation, elongation, termination).

    • A binds to T (in DNA) or U (in RNA).

    • C binds to G.

  • The RNA copy is called messenger RNA (mRNA) or a transcript. Discuss the different types of RNA (mRNA, tRNA, rRNA) and their functions.

  • Higher transcription rates mean more mRNA and protein production. Explain how transcription rates are regulated by various factors.

  • Transcription is controlled by the non-coding part of the gene. Describe the role of transcription factors and regulatory elements.

  • In mRNA, uracil (U) replaces thymine (T) and pairs with adenine (A). Explain the chemical differences between DNA and RNA.

  • A gene comprises coding and noncoding regions. Emphasize the importance of both regions in gene expression.

  • Noncoding regions regulates the cell to transcribe the protein in the correct patters and cells. Give examples of how mutations in noncoding regions can affect gene expression.

2. mRNA to Protein: Translation

  • Misconception: Genes, DNA, and mRNA contain information but don't physically make proteins. Clarify the roles of different molecules in protein synthesis.

Role of Ribosomes:

  • Ribosomes make proteins through translation. Describe the structure of ribosomes (large and small subunits) and their composition (rRNA and proteins).

  • mRNA transmits information, and ribosomes read it to construct proteins. Explain the role of tRNA in delivering amino acids to the ribosome.

  • Ribosomes assemble amino acids into a sequence (polypeptide). Describe the formation of peptide bonds between amino acids.

  • Analogy: mRNA is a recipe, ribosomes are the chef. Extend the analogy to include tRNA as the sous-chef.

  • The ribosomes read codons and creates the corresponding proteins. Discuss the role of start and stop codons in translation.

*To summarize: transcription connects genotype to mRNA, translation connects mRNA to protein.

3. Protein to Phenotype: Folding

  • Amino acid properties cause different folding patterns in proteins. Explain how hydrophobic and hydrophilic interactions drive protein folding.

  • Folding patterns impact protein shape and how it interacts within the cell. Discuss the role of chaperones in assisting protein folding and preventing aggregation.

  • Transcription, translation, and folding are interconnected processes. Explain how errors in these processes can lead to protein misfolding and disease.

Key Points to Remember

  • Organisms have two alleles of each gene (one from each parent). Explain the implications of heterozygosity and homozygosity.

  • Both alleles are typically transcribed into mRNA and translated into protein. Discuss the concept of allelic dominance and recessiveness.

  • If alleles differ in the coding sequence, different protein versions are produced in the same individual. Explain how different protein variants can lead to phenotypic variation.

Lecture Summary

  • Genes are information; most encode proteins (protein-coding genes). Review the central dogma of molecular biology.

  • Protein-coding genes have a sequence that is transcribed into mRNA. Summarize the steps of transcription and RNA processing.

  • mRNA is translated by ribosomes into a string of amino acids, which folds into a protein. Outline the process of translation and the role of ribosomes and tRNA.

  • mRNA and proteins are gene products. Emphasize their roles as intermediaries between genotype and phenotype.

  • All protein-coding genes have noncoding regions that regulate the timing, location, and amount of protein production. Highlight the importance of regulatory elements in gene expression.