Gene Expression & Phenotype (College Board AP® Biology): Study Guide

Gene expression & phenotype

• The phenotype of a cell or organism depends on which genes are expressed and at what levels

• Although all cells in a multicellular organism contain the same DNA, different sets of genes are active in different cells

• Active genes are transcribed into mRNA and translated into proteins, which determine the cell’s structure, function, and behaviour

• Regulation of gene expression is central to cell differentiation and development

Cell differentiation

• Stem cells contain the full genome, but only certain genes are expressed during differentiation

• This selective gene expression produces proteins unique to a cell type, giving rise to specialised structures and functions

• This is the process by which cells become specialized

  • Under certain conditions, some genes in a stem cell are activated, whilst others are inactivated

  • mRNA is transcribed from active genes only

  • This mRNA is then translated to form proteins

  • These proteins will be specific to the tissue in which the cell is found; they determine the structure of the cell and the processes that occur within it

Development

• Development is the process by which a fertilized egg develops into a multicellular organism

• It requires expression of the right genes, at the right time and place, and in the right sequence

• Transcription factors coordinate this process, turning gene sets on or off in sequence

  • For example, Hox genes encode transcription factors that control body plan development

Small RNAs and regulation

• Small RNA molecules (e.g. microRNAs, siRNAs) regulate gene expression post-transcriptionally

• They bind to complementary mRNA sequences, leading to:

  • mRNA degradation (preventing translation)

  • blocking ribosome binding, reducing protein synthesis

• This adds another layer of control over how much protein is produced, further influencing phenotype