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Gene Expression

Gene Expression. Transcription and Translation. WHAT IS A GENE?. Gene : 1) A unit of hereditary information which can be passed on to future generations. 2) A segment of DNA on a chromosome which holds the code or “recipe” for a protein. Genes hold the code for synthesis of

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Gene Expression

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  1. Gene Expression Transcription and Translation

  2. WHAT IS A GENE? • Gene: • 1) A unit of hereditary information which can be passed on to future generations. • 2) A segment of DNA on a chromosome which holds the code or “recipe” for a protein. • Genes hold the code for synthesis of • proteins : A functional group of many amino acids. • polypeptides : A smaller, non-functional group of amino acids • traits or parts of a trait: any physical, physiological or behavioral trait.

  3. Types of Traits • PHYSICAL • BEHAVIORAL • PHYSIOLOGICAL

  4. Eye Color Hair Color

  5. Review of the amino acid structure of proteins: Circle R groups and show aa’s. Identify as polypeptide. • How many different TYPES of amino acids could be used to form a single protein or polypeptide? _20_

  6. Proteins come in many forms. Due to the variety of different proteins, they serve many different functions that contribute to the growth, maintenance and replacement of cells.

  7. PROTEINS • In other words, we need proteins to carry out EVERY SINGLE trait. • Movement • Cell respiration • Photosynthesis • Hydrolysis • Dehydration Synthesis • Cardio/Respiratory/Digestive functions • Eye color • Hair color • Skin color • Physical Features • EVERYTHING………..

  8. Eye Color Hair Color

  9. The DNA Code • A gene holds the code for a functional protein or polypeptide. The code is found in thesequence of nucleotides on one strand of the DNA at a specific location on a chromosome. • In 1961, Francis Crick and coworkers proposed a triplet code for the formation of amino acids from DNA. • A set of 3 nucleotides codes for 1 amino acid • For example: the amino acid methionine is coded for by the DNA nucleotide triplet TAC. • The sequence of triplet codes then codes for a protein of polypeptide.

  10. Section of a gene which codes for a polypeptide: • Notice that three bases in the DNA code for a single amino acid. Identify PO4, sugars, N bases, and aa’s. Gene is read from one of the two DNA strands.

  11. If a protein is made of 100 amino acids, how many bases in the DNA are needed to code for the protein? • more than 300 nitrogen bases • (303 with stop code and >303 with introns) • Universality of the genetic code: • ALL organisms use the same triplet code to represent the same amino acids. • ex. whales, spiders, and bacteria-but they don’t necessarily make the same proteins

  12. Protein Synthesis or thebuilding of proteins. • Proteins are synthesized by ribosomes in the cytoplasm and on the rough ER. • Where is DNA located? nucleus of a eukaryotic cell • Additional molecules will be needed to carry the DNA code to the cytoplasm and form proteins. • This molecule is called RNA (Ribonucleic acid) • RNA nucleotides are composed of : • 1) ribose sugar • 2) PO4 • 3) and one of four nitrogen bases: adenine, guanine, cytosine and URACIL (takes the place of thymine in RNA) .

  13. Differences between DNA and RNA:

  14. 3 types of RNA necessary for synthesis of proteins: • m RNA-messenger RNA: carries an RNA “copy” of the DNA gene code from the nucleus to the cytoplasm. • t RNA-transfer RNA: transports amino acids from the cytoplasm to the ribosome so they can be joined to make a protein. • r RNA-ribosomal RNA __molecules of rRNA make up the ribosomes 2 units plus associated proteins.

  15. GENE EXPRESSION • GENE EXPRESSION - How the genes are expressed is a function of the protein produced. • DNA (gene)  m RNA  Protein  Trait (expression)

  16. Steps in the Synthesis of a Protein: • 1) Transcription: mRNA strand is built from the code on the DNA strand (gene) • 2) Translation: mRNA strand is used at the ribosome to help join amino acids to form a protein

  17. Draw in diagram from DNA  Secreted Protein

  18. I. Transcription: • Requirements for Transcription: • a gene segment on the DNA • many free floating RNA nucleotidesand • the enzyme RNA polymerase

  19. Steps in transcription: • DNA double helix unwinds at the area of the DNA where the gene is located. • Weak hydrogen bonds break between the 2 DNA strands. • Free floating RNA nucleotides bond with complimentary DNA nucleotides to form a strand of mRNA. These stages are catalyzed by the enzyme RNA polymerase. • When the mRNA is completely formed, it breaks away from the DNA. The DNA strands reconnect and the mRNA is free to travel to the cytoplasm.

  20. Remember that in transcription, the complimentary base pair for adenine is uracil NOT thymine!

  21. Each triplet _code (3 nucleotides) on a DNA molecule is transcribed into a triplet __codon_ on the mRNA molecule. • If the DNA codes for a polypeptide is • T—A—C—C—C—G—T—A—G—C—T—T—A—C—T • What would the codons on the complimentary strand of mRNA codons look like? AUG – GGC – AUC – GAA - UGA • Example: • DNA codes: • T A C C A T C CC A AAA C T • mRNA codons: AUG – GUA – GGG – UUU - UGA

  22. What does each letter in the code or codon stand for represent? • a nucleotide • What do the particular codes and codons represent? • amino acids • Because each base triplet on the mRNA stands for an amino acid, each mRNA molecule must contain • 3X as many nucleotides as amino acids

  23. 1) An mRNA strand with 66 nucleotides codes for a polypeptide 22 amino acids in length. • 2) A protein 300 amino acid units in length was synthesized from an mRNA strand900nucleotides in length. • A DNA strand 700 nucleotides in length will be transcribed into an mRNA strand700 nucleotides • There are 4 different bases in DNA (A, C, G, C) which can combine in different triplets to form 64 possible triplet codes (43). When these codes are transcribed into mRNA, there are 64 triplet codons which can be formed. Only 20 amino acids are used so some codons represent more than one amino acid.

  24. There are 4 different bases in DNA (A, C, G, C) which can combine in different triplets to form 64 possible triplet codes (43). When these codes are transcribed into mRNA, there are 64 triplet codons which can be formed. Only 20 amino acids are used so some codons represent more than one amino acid.

  25. mRNA Codon Chart

  26. How many different mRNA codons are there on the chart? • 43 = 4 x 4 x 4 = 64 different codons (using A, G, C and U in triplets) • How many different amino acids are there? Only 20 • With the exception of Methionine and Tryptophan, more than one nucleotide triplet codes for each amino acid. • Give an example of an amino acid which has more than one codon. • valine, alanine, isoleucine, threonine, and more • What do you notice about the codons that code for the same amino acid? • They are very similar and usually only the last nitrogen base is different.

  27. Some codons have special functions: • Start Codon: • AUG-codes for the amino acid methionine. • It signals the ribosome that it is the beginning of the mRNA sequence. • Stop Codons:UAA, UAG, UGA • Don’t code for an amino acid!!!! • Signal the ribosome that it is the end of the mRNA sequence and triggers the release of the mRNA from the ribosome • What is the DNA code for the mRNA start codon, methionine? TAC • What are the DNA codes for the stop codons UAA, UAG and UGA? ATT, ATC, ACT

  28. After transcription has taken place, the mRNA moves out of the nucleus to link with a ribosome in the cytoplasm. Translation will take place here.

  29. II. Translation: • Process of using mRNA codons to build proteins at the site of the ribosomes in the cytoplasm or on the rough endoplasmic reticulum. • Requirements for translation: • m RNA __carries the DNA code as “codons” • ribosomes __2 rRNA+ protein subunits--site of protein synthesis • tRNA __carry the amino acids from the cytoplasm to the ribosome to be joined to form the protein • Abundant amino acids in the cytoplasm and all necessary enzymes.

  30. Steps in Translation: • mRNA joins with the __ribosome (in cytoplasm or near the Rough Endoplasmic Reticulum) • tRNA (with an amino acid attached) _joins the mRNA codon to ANTI-codon • (The anticodon on the tRNA is complimentary to the codon on the mRNA.) • a second tRNA with attached amino acid _joins the mRNA • a peptide bond forms between the first two amino acids

  31. Steps in Translation: • the first tRNA is released from the mRNA _amino acids stay bonded to each other • the mRNA slides through the ribosome _to expose the next codon_ • the next tRNA with amino acid _joins the mRNA • Sequence is repeated Repeat steps 4-8 until a stop codon is reached. • mRNA and protein is released from the ribosome.

  32. The Final Products of Protein Synthesis: • protein or polypeptide: __functional or non-functional sequence of aa’s • Many times a protein must be modified after it is translated. This is done in the _cytoplasm, ER, Golgi____ • Some proteins are coded for by a single gene • ex. Insulin (later broken into two segments) • Sometimes proteins are composed of different gene • hemoglobin (2 genes/4 segments)

  33. GENE MUTATIONS • GENE MUTATIONS: Any change in the normal sequence of nucleotides in a gene sequence. • There are different variations. • can be harmful / helpful / have no effect • can occur spontaneously during DNA replication • (despite enzymes that help to check the DNA)

  34. Many of the mutations in DNA are caused by mutagens. Some of these mutagens are: • (1) ultraviolet light (can cause skin cancer) • (2) chemicals (like those found in cigarette smoke) • (3) viruses (like HPV: (human papilloma virus) • (4) X-rays, gamma rays (high energy-given off in atomic blast- formerly used safely in medical procedures)

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