1. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 3 Cell Structures and Their Functions Dividing Cells

2. Protein Synthesis DNA serves as master blueprint for protein synthesis DNA controls enzyme production and cell activity is regulated by enzymes (Proteins) Genes are segments of DNA carrying instructions for a polypeptide chain Triplets of nucleotide bases form the genetic library Each triplet specifies coding for an amino acid

3. Protein Synthesis Two step process –Transcription cell makes a copy of gene necessary to make a particular protein: messenger RNA (mRNA) mRNA then travels from nucleus to ribosomes where information is translated into a protein –Translation requires both mRNA and transfer RNA (tRNA) tRNA brings amino acids necessary to synthesize the protein to the ribosome

4. Overview of Protein Synthesis 1.DNA contains the information necessary to produce proteins 2.Transcription of one DNA strand results in mRNA, which is a complementary copy of the information in the DNA strand needed to make a protein 3.The mRNA leaves the nucleus and goes to a ribosome 4.Amino acids, the building blocks of proteins, are carried to the ribosome by tRNAs 5.In the process of translation, the information contained in mRNA is used to determine the number, kinds, and arrangement of amino acids in the polypeptide chain Fig. 3.23

5. Overview of Protein Synthesis 1.DNA contains the information necessary to produce proteins 2.Transcription of one DNA strand results in mRNA, which is a complementary copy of the information in the DNA strand needed to make a protein 3.The mRNA leaves the nucleus and goes to a ribosome 4.Amino acids, the building blocks of proteins, are carried to the ribosome by tRNAs 5.In the process of translation, the information contained in mRNA is used to determine the number, kinds, and arrangement of amino acids in the polypeptide chain Fig. 3.23

6. Overview of Protein Synthesis 1.DNA contains the information necessary to produce proteins 2.Transcription of one DNA strand results in mRNA, which is a complementary copy of the information in the DNA strand needed to make a protein 3.The mRNA leaves the nucleus and goes to a ribosome 4.Amino acids, the building blocks of proteins, are carried to the ribosome by tRNAs 5.In the process of translation, the information contained in mRNA is used to determine the number, kinds, and arrangement of amino acids in the polypeptide chain Fig. 3.23

7. Overview of Protein Synthesis 1.DNA contains the information necessary to produce proteins 2.Transcription of one DNA strand results in mRNA, which is a complementary copy of the information in the DNA strand needed to make a protein 3.The mRNA leaves the nucleus and goes to a ribosome 4.Amino acids, the building blocks of proteins, are carried to the ribosome by tRNAs 5.In the process of translation, the information contained in mRNA is used to determine the number, kinds, and arrangement of amino acids in the polypeptide chain Fig. 3.23

8. Overview of Protein Synthesis 1.DNA contains the information necessary to produce proteins 2.Transcription of one DNA strand results in mRNA, which is a complementary copy of the information in the DNA strand needed to make a protein 3.The mRNA leaves the nucleus and goes to a ribosome 4.Amino acids, the building blocks of proteins, are carried to the ribosome by tRNAs 5.In the process of translation, the information contained in mRNA is used to determine the number, kinds, and arrangement of amino acids in the polypeptide chain Fig. 3.23

10. Transcription Synthesis of mRNA, tRNA, and rRNA based on the nucleotide sequence in DNA –Messenger RNA (mRNA) – carries genetic information from DNA in nucleus to ribosomes in the cytoplasm –Transfer RNAs (tRNAs) – bound to amino acids base pair with codons of mRNA at ribosome to begin process of protein synthesis –Ribosomal RNA (rRNA) – a structural component of ribosomes

11. Transcription 1.The strands of the DNA molecule separate from each other. One DNA strand serves as a template for mRNA synthesis 2.Nucleotides that will form mRNA pair with DNA nucleotides according to the base-pair combinations shown in the key at the top of the figure. Thus, the sequence of nucleotides in the template DNA strand (purple) determines the sequence of nucleotides in mRNA (grey). RNA polymerase (the enzyme is not shown) joins the nucleotides of mRNA together 3.As nucleotides are added, an mRNA molecule is formed Fig. 3.24

12. Transcription: RNA Polymerase An enzyme that oversees synthesis of RNA Unwinds the DNA template Adds complementary ribonucleoside triphosphates on the DNA template Joins these RNA nucleotides together Encodes a termination signal to stop transcription

13. Transcription Posttranscriptional processing modifies mRNA before it leaves nucleus by removing introns (non-coding) and then splicing exons (coding) together with enzymes called spliceosomes –Functional mRNA consists only of exons Alternative splicing produces different combination of exons, allowing one gene to produce more than one type of protein

16. Translation Synthesis of proteins in response to codons of mRNA –Codon: set of 3 nucleotides, codes for 1 amino acid during translation –Anticodon: part of tRNA; consists of 3 nucleotides and is complementary to a particular codon of mRNA mRNA moves through nuclear pores to ribosomes tRNA, which carries amino acids, interacts at ribosome with mRNA. anticodons of tRNA bind to codons of mRNA, amino acids are joined to form a protein

17. Translation 1.To start protein synthesis, a ribosome binds to mRNA. The ribosome has two binding sites for tRNA with its amino acid. Note that the first codon to associate with a tRNA is AUG, the start codon, which codes for methionine. The codon of mRNA and the anitcodon of tRNA are aligned and joined. The other tRNA binding site is open Fig. 3.25

18. Translation 1.To start protein synthesis, a ribosome binds to mRNA. The ribosome has two binding sites for tRNA with its amino acid. Note that the first codon to associate with a tRNA is AUG, the start codon, which codes for methionine. The codon of mRNA and the anitcodon of tRNA are aligned and joined. The other tRNA binding site is open 2.By occupying the open tRNA binding site, the next tRNA is properly aligned with mRNA and with the other tRNA Fig. 3.25

19. Translation 1.To start protein synthesis, a ribosome binds to mRNA. The ribosome has two binding sites for tRNA with its amino acid. Note that the first codon to associate with a tRNA is AUG, the start codon, which codes for methionine. The codon of mRNA and the anitcodon of tRNA are aligned and joined. The other tRNA binding site is open 2.By occupying the open tRNA binding site, the next tRNA is properly aligned with mRNA and with the other tRNA 3.An enzyme within the ribosome catalyzes a synthesis reaction to form a peptide bond between the amino acids. Note that the amino acids are now associated with only one of the tRNAs Fig. 3.25

20. Translation 3.An enzyme within the ribosome catalyzes a synthesis reaction to form a peptide bond between the amino acids. Note that the amino acids are now associated with only one of the tRNAs 4.The ribosome shifts position by three nucleotides. The tRNA without the amino acid is released from the ribosome, and the tRNA with the amino acids takes its position. A tRNA binding site is left open by the shift. Additional amino acids can be added by repeating steps 2 through 4 Fig. 3.25

21. Translation 3.An enzyme within the ribosome catalyzes a synthesis reaction to form a peptide bond between the amino acids. Note that the amino acids are now associated with only one of the tRNAs 4.The ribosome shifts position by three nucleotides. The tRNA without the amino acid is released from the ribosome, and the tRNA with the amino acids takes its position. A tRNA binding site is left open by the shift. Additional amino acids can be added by repeating steps 2 through 4 5.Eventually a stop codon in the mRNA, such as UAA, ends the process of translation. At this point, the mRNA and polypeptide chain are released from the ribosome. 6.Multiple ribosomes attach to a single mRNA to form a polyribosome. As the ribosomes move down the mRNA, proteins attached to the ribosomes lengthen and eventually detach from the mRNA Fig. 3.25

22. Translation 1.To start protein synthesis, a ribosome binds to mRNA. The ribosome has two binding sites for tRNA with its amino acid. Note that the first codon to associate with a tRNA is AUG, the start codon, which codes for methionine. The codon of mRNA and the anitcodon of tRNA are aligned and joined. The other tRNA binding site is open 2.By occupying the open tRNA binding site, the next tRNA is properly aligned with mRNA and with the other tRNA 3.An enzyme within the ribosome catalyzes a synthesis reaction to form a peptide bond between the amino acids. Note that the amino acids are now associated with only one of the tRNAs 4.The ribosome shifts position by three nucleotides. The tRNA without the amino acid is released from the ribosome, and the tRNA with the amino acids takes its position. A tRNA binding site is left open by the shift. Additional amino acids can be added by repeating steps 2 through 4 5.Eventually a stop codon in the mRNA, such as UAA, ends the process of translation. At this point, the mRNA and polypeptide chain are released from the ribosome. 6.Multiple ribosomes attach to a single mRNA to form a polyribosome. As the ribosomes move down the mRNA, proteins attached to the ribosomes lengthen and eventually detach from the mRNA Fig. 3.25

23. Information Transfer from DNA to RNA DNA triplets are transcribed into mRNA codons by RNA polymerase Codons base pair with tRNA anticodons at the ribosomes Amino acids are peptide bonded at the ribosomes to form polypeptide chains Start and stop codons are used in initiating and ending translation