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.