Biology Ch. 11 DNA and Genes

11.1 DNA  DNA controls the production of proteins Living tissue is made up of protein, so DNA determines an organism’s traits  In 1952, experiments by Alfred Hershey & Martha Chase demonstrated that DNA is genetic material  DNA stands for DeoxyriboNucleic Acid

11.1 The Makeup of DNA  DNA is a polymer made up of repeating nucleotides Nucleotides have 3 parts  A simple sugar The sugar in DNA is deoxyribose  A phosphate group This is a phosphorus atom surrounded by 4 oxygen atoms  A nitrogenous base There are 4 bases in DNA: adenine, cytosine, guanine, and thymine

11.1 DNA chains  In the DNA chain, nucleotides bond between the phosphate group of one nucleotide and the deoxyribose sugar of the next nucleotide The phosphate groups and deoxyribose sugars are called the backbone The nitrogenous bases stick out from the backbone

11.1 Nitrogenous Bases  The 4 nitrogenous bases in DNA are adenine, guanine, cytosine and thymine

11.1 DNA structure  In 1953, Watson & Crick proposed that DNA was made of 2 strands of nucleotides that were joined by the nitrogenous bases These bases paired in a certain way and are held together by hydrogen bonds  Adenine pairs with thymine  Guanine pairs with cytosine These are called complementary base pairs

11.1 DNA 3-D structure Watson & Crick also proposed that the 2 strands were joined, and then twisted to form a shape called a double helix

11.1 Nucleotide Sequencing  The order in which the nucleotides bond determines the characteristics of that organism Evolutionary relationships, whether or not 2 people are related, and identity confirmation of crime victims can all be determined by analyzing this information

11.1 DNA Replication  You have already learned that DNA must replicate before cell division can occur This process is called DNA replication  An enzyme separates the 2 strands of DNA  Free nucleotides bond to the surface of each separated DNA strand using the appropriate base pairing (A-T & G-C)  Another enzyme bonds these free nucleotides together to make a new strand  The result is 2 identical strands of DNA

11.2 RNA vs. DNA  RNA is a nucleic acid similar to DNA, but with a few minor differences: RNA is a single strand instead of a double strand like DNA The sugar in RNA is ribose instead of the deoxyribose in DNA RNA has the nitrogenous base uracil, instead of thymine  Uracil binds to adenine, just like thymine would

11.2 Types of RNA  There are 3 types of RNA and each type of RNA has a different function in the cell: Messenger RNA (mRNA)  Brings instructions from the DNA in the nucleus to a ribosome in the cytoplasm Ribosomal RNA (rRNA)  Binds to the mRNA and uses the instructions to assemble the amino acids in the correct order Transfer RNA (tRNA)  Brings the amino acids to the ribosomes, where they are assembled into a protein

11.2 Transcription  In the nucleus, mRNA makes a copy of the DNA in a process called transcription First, the enzymes unzip the DNA strand Then free RNA nucleotides bind to the complementary nucleotide on one of the DNA strands Another enzyme binds the mRNA nucleotides together The mRNA strand then separates from the DNA and the 2 DNA strands bind back together

11.2 Codons  A codon is a series of 3 nitrogenous bases in mRNA that code for a specific amino acid There are 64 possible codons and 20 possible amino acids  This means that each amino acid has more than one possible 3 letter combination  Table 11.1 on p. 292 shows all the possible codon combinations and the amino acid associated with each codon  There are also start and stop codons that signal the beginning or end of a protein strand

11.2 Translation  Translation is the process of converting the information in a sequence of nitrogenous bases in mRNA into a sequence of amino acids in protein. Translation occurs at the ribosomes in the cytoplasm In the cytoplasm, a ribosome attaches to the strand of mRNA

11.2 transfer RNA  The tRNA transfers the amino acids in the cytoplasm to the ribosomes to be made into proteins There are 20 different amino acids Each tRNA molecule attaches to only one kind of amino acid Each tRNA carries an anticodon that binds to the codon in the mRNA  The anticodon is the complement to the codon on the mRNA strand

11.2 The translation process  a ribosome attaches to the starting end of the mRNA strand  tRNA molecules, carrying specific amino acids move towards the ribosome  The tRNA anticodons pair with the complementary mRNA codon  The ribosome then slides down the mRNA strand to the next codon and the process repeats  The previous tRNA then separates, leaving the amino acid and goes to get another amino acid  Eventually, a stop codon is reached, and the amino acid chain is released from the ribosome

11.2 Proteins  When amino acid chains are separated from the ribosome, they twist and curl into 3D shapes and become proteins Each different protein strand forms the same shape  The process from DNA to mRNA to proteins is called “the central dogma” of biology It is found in all organisms

11.3 Mutations  Mutation- any change in the DNA sequence This can be caused by errors in replication, transcription, cell division, or outside factors

11.3 Cellular Mutations  Mutations occur in 2 types of cells: Reproductive cells- a change in the nucleotide sequence in a sperm or egg cell  These result in changes in the genes of that individuals offspring  It can harm the offspring, even causing the embryo to die, or help the offspring Body cells- changes in the DNA of a non- reproductive cell that are not passed on to offspring  These can show no effect, damage individual cells, or lead to the formation of cancer

11.3 Point Mutations  A point mutation is a change in a single base pair in DNA It involves a switch in a single base pair It results in the creation of a completely different protein because one codon changed, therefore one amino acid was different They are generally less harmful than frameshift mutations

11.3 Frameshift Mutations  A frameshift mutation is the addition or deletion of a single base pair This changes every codon after the change, since the codons shift by one base pair This causes all of the amino acids following the mutation to be different They are generally more harmful than point mutations

11.3 Chromosomal Mutations  Chromosomal mutations are structural changes in chromosomes These mutations occur in all organisms, but are most common in plants These are often not passed on to offspring because the embryo usually dies  When they do, they are unable to reproduce

11.3 Types of Chromosomal Mutations  There are several types of chromosomal mutations:  Nondisjunction- when chromosomes fail to separate correctly during meiosis and results in gametes with the incorrect number of chromosomes  Deletion- when part of a chromosome is left out  Insertion-when part of a chromatid breaks off and attaches to the sister chromatid  Inversion- when part of a chromosome breaks off and reattaches backwards  Translocation- when part of a chromosome breaks off and adds to another chromosome

11.3 Causes of Mutations  Spontaneous mutations, such as errors in base pairing, do not appear to have a specific cause  Other mutations are caused by environmental factors Any agent that causes a change in DNA is called a mutagen Mutagens such as x-rays, UV light, and nuclear radiation often causes deletions Mutagens such as chemicals often cause substitutions in the DNA chain