REVIEW! What is DNA? • Deoxyribonucleic Acid (DNA) • Monomers made up of nucleotides: • Nucleotides consist of: • A five carbon sugar, deoxyribose • Four in it’s ring, one extending above the ring • Missing one oxygen when compared to ribose • Phosphate group • Is the source of the “acid” in nucleic acid • Nitrogenous base (Adenine, Guanine, Cytosine, Thymine) • A ring consisting of nitrogen and carbon atoms with various functional groups attached • Double ring= purines (A and G) • Single ring= pyrimidines (T and C) • Double helix consists of: • Sugar-phosphate backbone held by covalent bonds • Nitrogen bases are hydrogen bonded together; A pairs with T and C pairs with G
Protein synthesis: overview • DNA inherited by an organism specifies traits by dictating the synthesis of proteins. • However, a gene does not build a protein directly; it dispatches instruction in the form of RNA, which in turn programs protein synthesis. • Message from DNA in the nucleus of the cell is sent on RNA to protein synthesis in the cytoplasm. • Two main stages: • Transcription • Translation
Protein Synthesis: Overview • Two main stages: • Transcription • The transfer of genetic information from DNA into an RNA molecule • Occurs in the eukaryotic cell nucleus • RNA is transcribed from a template DNA strand • Translation • Transfer of the information in RNA into a protein.
Transcription • Details: • 1. Initiation- • Promoter is the nucleotide sequence on DNA that marks where transcription of a gene begins and ends; “start” signal • Promoter serves as a specific binding site for RNA polymerase and determines which of the two strands of the DNA double helix is used as the template.
Transcription • Elongation- • RNA elongates • As RNA synthesis continues, the RNA strand peels away from its DNA template, allowing the two separated DNA strands to come back together in the region already transcribed.
Transcription • 3. Termination- • RNA polymerase reaches a sequence of bases in the DNA template called a terminator. • Signals the end of the gene; at that point, the polymerase molecule detaches from the RNA molecule and the gene. • mRNA (messenger RNA) or “transcript” exits the nucleus via the nuclear pores and enter the cytoplasm
Transcription animation • http://www-class.unl.edu/biochem/gp2/m_biology/animation/gene/gene_a2.html
RNA processing • Before mRNA leaves the nucleus, it is modified or processed. • 1. addition of extra nucleotides to the ends of the transcript • Include addition of a small cap (a single G nucleotide) at one end and a long tail (a chain of 50 to 250 A’s) at the other end • Cap and tail facilitate the export of the mRNA from the nucleus, protecting the transcript from attack by cellular enzymes, and help ribosomes bind to the mRNA • Cap and tail are NOT translated into protein. http://vcell.ndsu.edu/animations/mrnaprocessing/movie.htm
RNA processing • 2. RNA splicing • Cutting-and-pasting process catalyzed by a complex of proteins and small RNA molecules, but sometime the RNA transcript itself catalyzes the process. • Introns • “intervening sequences”; internal noncoding regions • Get removed from transcript before it leaves nucleus • Exons • Coding regions; parts of a gene that are expressed as amino acids • Joined to produce an mRNA molecule with a continuous coding sequence • Cap and tail are considered parts of the first and last exons, although are not translated into proteins. • http://student.ccbcmd.edu/biotutorials/protsyn/exon.html
More animations • http://www.pbs.org/wgbh/aso/tryit/dna/protein.html • http://www.wisc-online.com/objects/index_tj.asp?objID=AP1302
Translation- overview • A typical gene consists or hundreds or thousands of nucleotides in a specific sequence, which get transcribed onto mRNA. • Translation is the conversion of nucleic acid language into polypeptide language • There are 20 different amino acids. • A cell has a supply of amino acids in cytoplasm, either obtained by food or made from other chemicals. • Flow of information from gene to protein is based on a triplet code: genetic instructions for the a.a. sequence of a polypeptide chain are written in DNA and mRNA as a series of three-base pairs, or codons.
Translation- tRNA • To convert the codons of nucleic acids on mRNA to the amino acids of proteins, a cell employs a molecular interpreter, called transfer RNA (tRNA) • tRNA molecules are responsible for matching amino acids to the appropriate codons to form the new polypeptide. • tRNA’s unique structure enables it to be able to: • 1. pick up the appropriate amino acids • 2. recognize the appropriate codons in the mRNA
Translation- tRNA • at one end of the folded molecule contains a special triplet of bases called an anticodon. • Complementary to a codon triplet on mRNA • Anticodon recognizes a particular codon triplet on mRNA • At the other end of the tRNA molecule is a site where an amino acid can attach.
Translation- rRNA • Ribosomal RNA (rRNA) • Organelle in the cytoplasm that coordinates the functioning of mRNA and tRNA and actually makes polypeptides. • Consists of two subunits: large and small
Translation • Initiation- • mRNA arrives at the ribosome • Translation begins at AUG, the start codon. • Each transfer RNA has an anticodon whose bases are complimentary to the bases of a codon on the mRNA strand.
Translation • Elongation- • The ribosome moves along the mRNA binding new tRNA molecules and amino acids • Amino acids form peptide bonds
Translation • Termination • The polypeptide chain continues to grow until a “stop” codon on the mRNA molecule is reached. • The ribsome then releases the mRNA molecule and the newly formed polypeptide chain
Translation Animation • http://www-class.unl.edu/biochem/gp2/m_biology/animation/gene/gene_a3.html
Polysome Several ribosomes can translate an mRNA at the same time, forming what is called a polysome.
Free ribosomes vs. bound ribosomes • Free ribosomes • Found in cytoplasm • Synthesize proteins for use primarily within the cell • Bound ribosomes • Found on rough ER • Synthesize proteins primarily for secretion or for lysosomes
After protein synthesis… • Each polypeptide coils and folds, assuming a 3-D shape, its tertiary structure. • Several polypeptides may come together, forming a protein with quaternary structure. • Overall significance: • Process whereby genes control the structures and activities of cells • The way genotypes determine phenotypes; proteins made from the original DNA nucleotides determine the appearance and capabilities of the cell and organism!
Mutations • Mutation is any change in the nucleotide sequence of DNA. • Can involve large regions of a chromosome or just a single nucleotide pair, as in sickle cell disease • In one of the two kinds of polypeptides in the hemoglobin protein, the sickle-cell individual has a single different amino acid. • This small difference is caused by a change of a single nucleotide in the coding strand of DNA. Only ONE base pair!
Mutations on DNA • Two general categories: • Base substitution • Also known as a point mutation • Replacement of one nucleotide with another. • Depending on how the base substitution is translated, it can result in no change in the protein (due to redundancy of genetic code), an insignficant change, or a change that significantly affects the individual. • Occasionally, it leads to an improved protein that enhances the success of the mutant organism and its descendants. • More frequently, its harmful. • May cause changes in protein that prevent it from functionally normally. • If stop codon is a result of mutation and protein is shortened, it may not function at all.
Mutations on DNA • Base insertions or deletions • Also known as frameshift mutation • Often has a disastrous effect • Adding or subtracting nucleotides may result in an alteration of the reading frame of the message • all the nucleotides that are “downstream” of the insertion or deletion will be regrouped into different codons. • Result will most likely by a nonfunctional polypeptide
Mutations on DNA • What causes mutations? • Mutagenesis, or the production of mutations, can occur in a number of ways. • Spontaneous mutations: errors that occur during DNA replication or recombination are called. • Mutagen, a physical or chemical agent that causes mutations • Physical mutagen: high-energy radiation, such as X-rays and UV light • Chemical mutagen: consists of chemicals that cause incorrect DNA bases pairs, such as asbestos.
Mutations on DNA • Can also be helpful both in nature and in the laboratory. • It is because of mutations that there is such a rich diversity of genes in the living world, that make evolution by natural selection possible. • Also essential tools for geneticists. • Whether naturally occurring or created in the laboratory, mutations create the different alleles needed for genetic research.
Mutations- Chromosome Number • Nondisjunction • Members of a chromosome fail to separate. • Can lead to an abnormal chromosome number in any sexually reproducing diploid organism. • For example, if there is nondisjunction affecting human chromosome 21 during meiosis I, half the resulting gametes will carry an extra chromosome 21. • Then, if one of these gametes unites with a normal gamete, trisomy 21 (Down Syndrome) will result.
Mutations- Chromosome Structure • Abnormalities in chromosome structure: • Breakage of a chromosome can lead to a variety of rearrangements affecting the genes of that chromosome: • 1. deletion: if a fragment of a chromosome is lost. • Usually cause serious physical and mental problems. • Deletion of chromosome 5 causes cri du chat syndrome: child is mentally retarded, has a small head with unusual facial features, and has a cry that sounds like the mewing of a distressed cats. Usually die in infancy or early childhood.
Mutations- Chromosome Structure • 2.duplication: if a fragment from one chromosome joins to a sister chromatid or homologous chromosome. • 3.inversion: if a fragment reattaches to the original chromosome but in the reverse direction. • Less likely than deletions or duplications to produce harmful effects, because all genes are still present in normal number • 4. translocation: moves a segment from one chromosome to another nonhomologous chromosome • Crossing over between nonhomologous chromosomes!
Karyotype • The term karyotype refers to the chromosome complement of a cell or a whole organism. • A karyotype is an ordered display of magnified images of an individual’s chromosomes arranged in pairs, starting with the longest. • In particular, it shows the number, size, and shape of the chromosomes as seen during metaphase of mitosis. • Chromosome numbers vary considerably among organisms and may differ between closely related species.
Karytype • Karyotypes are prepared from the nuclei of cultured white blood cells that are ‘frozen’ at the metaphase stage of mitosis. • Shows the chromosomes condensed and doubled • A photograph of the chromosomes is then cut up and the chromosomes are rearranged on a grid so that the homologous pairs are placed together. • Homologous pairs are identified by their general shape, length, and the pattern of banding produced by a special staining technique.
Karyotype • Male karyotype • Has 44 autosomes, a single X chromosome, and a Y chromosome (written as 44 + XY) • Female karyotype • Shows two X chromosomes (written as 44 + XX)