Biology Ch. 11

1. Biology Ch. 11 DNA and Genes

2. 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

3. 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

4. 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

5. 11.1 Nitrogenous Bases • The 4 nitrogenous bases in DNA are adenine, guanine, cytosine and thymine

6. 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

7. 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

8. 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

9. 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

10. 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. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 11.3 Mutations • Mutation- any change in the DNA sequence • This can be caused by errors in replication, transcription, cell division, or outside factors

19. 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

20. 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

21. 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

22. 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

23. 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

24. 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