Unit 6: DNA & Protein SynthesisCh. 28: DNA—Life’s Code

DNA = Deoxyribonucleic Acid

Describing DNA

• Who first described DNA?– James Watson &

Francis Crick

• What shape did they use to describe DNA?– double helix

DNA is a Polymer…

• What is the monomer that repeats to make DNA?–nucleotide

What are the parts of a nucleotide?• phosphate group

• sugar (deoxyribose)

• nitrogenous base– 4 kinds:

• Adenine (A)• Thymine (T)• Guanine (G)• Cytosine (C)

– base gives name to nucleotide

adenine, thymine, guanine, or cytosine

A Model of DNA• Is a DNA molecule single-

stranded or double stranded?– double-stranded

• What parts make up the strands?– “backbone”/sides

• sugars & phosphates

– “rungs”/steps• nitrogenous bases

– always paired as:» A - T

» G - C

DNA Structure Video

• Strands are complementary (A-T & G-C) & antiparallel (opposite directions/upside down)

Where is DNA found in eukaryotic cells?

• DNA is contained in chromosomes (chromatin) within the nucleus

sugar-phosphate “backbone” & nitrogenous base “rungs”

DNA Replication• What is another word for replication?

– copying

• Why must DNA replicate?– so when cell divides, each new

cell gets a copy of DNA

http://www.stolaf.edu/people/giannini/flashanimat/molgenetics/dna-rna2.swf

DNA Replication• When does DNA replicate?

– before the cell divides (before mitosis or meiosis)

• Where does DNA replicate?– inside the nucleus

• 1. DNA molecule unzips between bases forming two “parent strands”

What are the steps in DNA replication?

• 2. The “parent” strands act as templates (guides)…

– “new” nucleotides are paired up with the complementary base on the “parent” strand

• forming a complementary daughter strand– ex. “G” pairs with “C”

What are the steps in DNA replication?

T

C

• 3. Now, there are 2 exact copies of the original DNA molecule.– & when the cell divides, each

“new” daughter cell gets a copy

What are the steps in DNA replication?

DNA Replication Video

T

C

What are the steps in DNA replication?

Let’s Practice… Predict the next base…

Let’s Practice… Finish the replications…

Let’s Practice… Finish the replications…

Deoxyribonucleic Acid vs. Ribonucleic Acid

DNA RNA

• sugar = deoxyribose

• sugar = ribose

• double stranded

• single stranded

•CANNOT leave nucleus

•Can move between nucleus & cytoplasm

• N bases = adenine, thymine, guanine, cytosine

• N bases = adenine, uracil, guanine, cytosine

The DNA Code & Protein Synthesis• What is a gene?

– section of DNA that codes for synthesis of a specific protein

• Which part of DNA carries the code for a protein?– the nitrogen bases

• & their order– change sequence amino

acid (usually) changes & protein made (usually) changes

• So, the big question is… – How does the DNA code in a cell’s nucleus

get to the ribosomes where proteins are synthesized?• transcription & translation

The DNA Code & Protein Synthesis

Transcription• What is transcription?

– DNA code is “transcribed”/copied into mRNA codons

• When does transcription happen?– when a protein is

needed

• Where does transcription happen?– in the nucleus

Transcription• What does transcription make?

– A single-stranded messenger RNA (mRNA)• made from instructions/order of the bases in the DNA

http://www.stolaf.edu/people/giannini/flashanimat/molgenetics/transcription.swf

Transcription• What are the steps in

transcription?1. DNA unzips

2. free RNA nucleotides pair w/ exposed bases on DNA• until “stop codon” is

reached

3. mRNA separates from DNA• & moves out of nucleus

– to ribosome in cytoplasm

4. DNA strands rejoin

http://www.stolaf.edu/people/giannini/flashanimat/molgenetics/transcription.swf

mRNA Carries DNA’s Code Using “Codons”

• What is a codon?– 3 consecutive bases

on mRNA that codes for a particular amino acid• 64 possible combos

– Some amino acids have more than one codon

mRNA Carries DNA’s Code Using “Codons”

• How can we figure out the amino acid coded for by a codon?– a codon

chartWhat amino acid does the AUG codon code for?

It must be in

this row

It must be in this

square

It must be in this

amino acid

mRNA Codons & Amino Acids• What amino acid does the AUG codon code for?

mRNA Codons & Amino Acids

Translation• What is translation?

– process of reading mRNA codons to synthesize proteins

• with help of tRNA (transfer RNA) & ribosomes

• When does translation occur?– when mRNA from the

nucleus joins with ribosomes

• Where does translation happen?– at ribosomes in the

cytoplasm

1. mRNA comes from the nucleus & joins with ribosome in cytoplasm

2. Ribosome “reads” the codon until it finds the start codon (AUG)

3. tRNA (with complementary anticodon) joins to the mRNA codon

– tRNA carries the amino acid specified by the mRNA codon

http://www.stolaf.edu/people/giannini/flashanimat/molgenetics/translation.swf

Steps of Translation

4. Ribosome moves along mRNA reading each codon until “stop” codon is reached

– tRNA brings each amino acid

5. Amino acids line up & bond to form polypeptide

6. Polypeptide chain forms protein (3-D)– Shape of protein is related to its function

http://www.stolaf.edu/people/giannini/flashanimat/molgenetics/translation.swf

Steps of Translation

tRNA anticodon

DNA codon

mRNA codon

transcription translation

mRNA codon

Let’s Practice… What would the mRNA strand made during

transcription look like?

What amino acids would be coded for using this mRNA?

What would the anticodons on the tRNA be for each amino acid?

Let’s Practice…• Given the DNA sequence below, determine the:

1. mRNA codon sequence

2. amino acids that would be coded for by each codon

3. anticodons on each tRNA which allowed it to “bring” that amino acid (using the mRNA codons)

• DNA = TAC CCA TTG GAT CCG ACT

• mRNA codon = • amino acid = • tRNA anticodon =

AUG GGU AAC CUA GGC UGA

Met/start Gly Asn Leu Gly stop

UAC CCA UUG GAU CCG ACU

Mutations• What are mutations?

– mistakes/changes in a gene on a chromosome• can occur spontaneously

during replication• can be caused by mutagens

(such as radiation, high temperatures, or chemicals)

• often corrected, but not always– once occurs, copied as if correct– can cause different protein

Types of Mutations• What are the 3 types of mutations?

1. deletion• What happens in a deletion mutation?

– a nucleotide (base) is left out» causes “frame shift” ~moves nucleotides after the deletion to the left causing a

different protein to be formed

2. insertion• What happens in an insertion mutation?

–a nucleotide (base) is added in» causes “frame shift” ~moves nucleotides after the insertion to the right causing a

different protein to be formed

Types of Mutations

Types of Mutations

3. point (base substitution)• What happens in a point

mutation?–one nucleotide (base) is

replaced by another» affects only 1 amino acid &

does not cause “frame shift”

» sometimes amino acid & protein can be the same (“silent mutation”)

» protein may be able to function normally or nearly normally

Sickle Cell Mutation• What does the sickle cell mutation do?

– affects hemoglobin

• What type of mutation causes sickle cell?– A point mutation

• changes one amino acid– changes the protein

“Silent” mutation (no change in polypeptide)

(point mutation)