DNA: Structure and Function Unit 7

Recall: DNA is a nucleic acid and made of nucleotides Nucleotides contain a sugar, phosphate, and a base In DNA, the bases are adenine, cytosine, guanine, and thymine

What is DNA? Deoxyribonucleic acid The “blueprint” of the cell Stores genetic information Instructions for making proteins

How did we discover DNA? Frederick Griffith experiment

How did we discover DNA? Avery, MacLeod, and McCarty experiment

How did we discover DNA? Hershey and Chase experiment – 1952

What is the structure of DNA? Shape = double helix (“twisted ladder”) The “backbone” is made of the sugar and phosphate groups The “rungs” of the ladder are the bases

What is the structure of DNA? The sugar-phosphate backbone is polar, which means it has a positive and negative end It is also hydrophilic Bases attach 5’ to 3’ Each strand runs in the opposite direction = antiparallel

What is the structure of DNA? Between the sugars and phosphates in the back bone, there are strong covalent bonds Between the bases in the middle, there are weak hydrogen bonds that are easily broken

How did we discover the shape of DNA? James Watson and Francis Crick – 1953 Used a photograph from Rosalind Franklin, given to them by her assistant, Maurice Wilkins Because she died of cancer, she was not awarded the Nobel prize in 1962

Chargaff’s Rule Adenine always pairs with thymine Cytosine always pairs with guanine Question: If a strand of DNA is 15% adenine, how much thymine, cytosine, and guanine does it contain?

Structure of DNA – Levels of Complexity Nucleotides = building blocks Strand = covalently linked nucleotides in a linear fashion Double helix = two strands twisted together Chromosome = association of DNA with proteins to organize strands into a compact structure Genome = complete complement of an organism’s genetic material

How does DNA replicate? DNA replicates semi-conservatively The two strands unwind and one “parent” strand joins with one “daughter” strand to make two new double helixes

How does DNA replicate? Strands are unwound and separated by DNA helicase Sites of separation and replication = replication forks

How does DNA replicate? New nucleotides are brought in and attached to the parent strand  A pairs with T, C pairs with G The new nucleotides are joined to the parent strand by DNA polymerase Usually, there are no mistakes, but sometimes they do occur Mutation = change in a DNA sequence that affects genetic information

What does DNA do? Instructions for making proteins DNA is too big to leave nucleus  it copies the information into RNA, which can leave the nucleus Transcription = copying the information from DNA to RNA Translation = using the information in the RNA to make proteins Where do you think translation happens? Ribosomes

Recall: Amino acids are the building blocks of proteins During protein synthesis, amino acids are joined together in long polypeptide chains, which are folded into proteins

DNA vs. RNA DNA = double stranded, RNA = single stranded DNA = uses deoxyribose as sugar, RNA = uses ribose as sugar DNA = uses A, T, C, G as bases, RNA = uses A, C, G, and U (uracil) as bases

Types of RNA mRNA = messenger RNA, carries information from DNA to ribosome tRNA = transfer RNA, carries anti-codons and amino acids to ribosome to be assembled into proteins rRNA = ribosomal RNA, makes up the ribosomes, formed in nucleolus

Codon vs. anticodon Codon = String of three nucleotides on mRNA that codes for a specific amino acid Anticodon = Group of three bases on tRNA that are complementary to an mRNA codon

Steps of protein synthesis 1. DNA is transcribed into RNA. 2. RNA leaves the nucleus and goes to the ribosome. 3. RNA passes through the ribosome while tRNA brings anti-codons with amino acids to the matching codons. 4. The amino acids are joined together in the ribosome into a long polypeptide chain  making proteins. eature=relatedhttp:// eature=related

1. DNA is transcribed into RNA RNA polymerase separates DNA and binds to a single strand It uses the strand as a template to assemble nucleotides into a strand of mRNA How does RNA polymerase “know” where to start and stop? Promoters = specific base sequences that signal where to start transcription There are also signals in the DNA to stop transcription

2. RNA leaves the nucleus and goes to the ribosome. mRNA leaves through the pores in the nucleus Sometimes, it needs to be “edited” Intron = sequence of nucleotides that does not code for proteins, “junk” DNA Exon = sequence of nucleotides that does code for proteins Introns get cut out, exons get spliced together

3. RNA passes through the ribosome while tRNA brings anti-codons with amino acids to the matching codons.

4. The amino acids are joined together in the ribosome into a long polypeptide chain  making proteins.

How does a codon determine what amino acid is used?

What if something goes wrong? Two kinds of mutations – gene mutations and chromosomal mutations Chromosomal mutations are much more severe because they involve changes in the number or structure of entire chromosomes  more genetic information Gene mutations can still be very significant  two main kinds: point mutations and frameshift mutations

What if something goes wrong? Point mutations = occur at a single point in DNA sequence, one base is substituted for another Ex. The quick brawn fox jumps over the lazy dog Frameshift mutations = a base is inserted or deleted  much more severe change, because it affects every codon that follows Ex. The quick brwnf oxj umpso vert hel azyd og

What causes mutations? Chromosomal mutations occur during early embryonic development Gene mutations can occur during early embryonic development or later in life during cell replication Mutations can be caused by alcohol, drugs, smoking, exposure to radiation (tanning = UV radiation exposure), etc.