dna: structure and function
DESCRIPTION
DNA: Structure and Function. Unit 7. Recall:. DNA is 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 - PowerPoint PPT PresentationTRANSCRIPT
DNA: Structure and Function
Unit 7
Recall:
• DNA is 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 - 1928
How did we discover DNA?
• Avery, MacLeod, and McCarty experiment - 1944
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
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 = repliation 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.
• http://www.youtube.com/watch?v=983lhh20rGY&feature=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.
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.