Gene Expression And Protein Synthesis Regulation Of Gene Expression

Genes are transcribed into complementary single-stranded molecules of genetic material composed of RNA (Figure 4.1). The primary differences between DNA and RNA are the presence of a hydroxyl group at the 2 -position of the ribose sugar and the use of uracil (U) to replace thymine as the base complementary to adenine. Messenger RNA is then translated into the amino acid sequence of a protein at a cellular structure called the ribosome.

Subsets of RNA molecules serve as the end-product of gene expression. One subset, the rRNA genes, expresses only RNA which combines with proteins to form ribo-somes that participate in protein translation of messenger RNA (mRNA). The other subset is tRNA genes whose products participate in protein synthesis by donating amino acids to a growing protein polypeptide chain. Only a small fraction of each RNA transcript is translated.

Most genes are organized into two main regions, the promoter and the coding regions. The promoter region lies immediately upstream of the coding region. A specific sequence of nucleotides in the promoter region interacts with protein complexes termed transcription factors in a dynamic manner to determine each gene's unique expression pattern (i.e. timing, quantity). In addition, transcription factors provide genes with the ability to interact with and respond to changes in the cellular environment.

The coding region is composed of exons (i.e. the portions of genes that are included in the mature mRNA) and introns (i.e. the portions of genes that are initially transcribed but are later processed, or spliced, out of the mature mRNA). The outer ends of genes are often untranslated and are termed the 5 - (the upstream or beginning) and 3 - (the downstream or end) regions

Sense strand

Theonine Arginine

Tyrosine Threonine

Theonine Arginine

Tyrosine Threonine mRNA

Figure 4.1 The structure of DNA contains information that is transcribed into RNA and subsequently translated into proteins. (a) The DNA molecule consists of two long strands of nucleotides that are complementary and which coil into a double helix for stability. The two strands (i.e. sense, antisense) are held together by hydrogen bonds between complementary nucleic acid bases: A with T, G with C. The double helix opens and one side is transcribed into a single complementary strand of mRNA. (b) The resulting mRNA is then translated into amino acids (read in units of three adjacent nucleotides, or codons) and a peptide chain is formed at a cell structure called the ribosome. The amino acid chain may be further processed to form a mature protein. (a) Reprinted from National Human Genome Research Institute. Online Education Kit: Bioinformatics: Finding Genes. Bethesda, MD, USA: National Human Genome Research Institute, last updated: April 27, 2007. Available from: www.genome.gov/25020001. (b) Reprinted from National Institute of General Medical Sciences. The New Genetics. Bethesda, MD, USA: National Institute of General Medical Sciences, 2006: 13. Available from: http://publications.nigms.nih.gov/thenewgenetics/index.html.

mRNA

Figure 4.1 The structure of DNA contains information that is transcribed into RNA and subsequently translated into proteins. (a) The DNA molecule consists of two long strands of nucleotides that are complementary and which coil into a double helix for stability. The two strands (i.e. sense, antisense) are held together by hydrogen bonds between complementary nucleic acid bases: A with T, G with C. The double helix opens and one side is transcribed into a single complementary strand of mRNA. (b) The resulting mRNA is then translated into amino acids (read in units of three adjacent nucleotides, or codons) and a peptide chain is formed at a cell structure called the ribosome. The amino acid chain may be further processed to form a mature protein. (a) Reprinted from National Human Genome Research Institute. Online Education Kit: Bioinformatics: Finding Genes. Bethesda, MD, USA: National Human Genome Research Institute, last updated: April 27, 2007. Available from: www.genome.gov/25020001. (b) Reprinted from National Institute of General Medical Sciences. The New Genetics. Bethesda, MD, USA: National Institute of General Medical Sciences, 2006: 13. Available from: http://publications.nigms.nih.gov/thenewgenetics/index.html.

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