Ch. 10-1DNA

DNA made of repeating subunits called nucleotides

Each DNA molecule consists of two long chains of nucleotides

DNA nucleotide has three main parts:◦ Sugar molecule

(Deoxyribose)◦ Phosphate group◦ Nitrogen-containing base

Deoxyribose & phosphate group are identical in ALL DNA nucleotides

Nitrogen-containing base may be one of four different kinds:◦ Adenine (A)◦ Guanine (G)◦ Cytosine (C)◦ Thymine (T)

Nitrogen-Containing Base

Deoxyribose

Adenine & Guanine = PURINES◦ Have two rings of carbon

Thymine & Cytosine = PYRIMIDINES◦ Have one ring of carbon

1953: James Watson & Francis Crick suggested model for DNA

Rosalind Franklin & Maurice Wilkins X-ray photographed DNA

1962 Watson, Crick & Wilkins received Nobel Prize

Covalent bonds btwn deoxyribose sugar & phosphate molecules◦ Sugar & phosphate form a “backbone”

where nitrogen-containing bases attach Hydrogen bonds btwn 2

nucleotides

-Bases on one chain of DNA face bases on other chain of DNA forming hydrogen bonds btwn them

-Hydrogen bonds are relatively weak

-Adenine pairs with Thymine

-Cytosine pairs with Guanine

Adenine --------------- Thymine

(purine) (pyrimidine)

Cytosine --------------- Guanine

(pyrimidine) (purine)

Pairing of one base to specific other base = complementary base pairing

Two base-pairing rules:◦ Adenine ---- Thymine◦ Cytosine ---- Guanine

Copying of DNA: Replication Two nucleotide chains unwind and each chain serves

as template for new nucleotide chain point at which two chains separate: Replication fork◦ Two chains are separated by enzyme Helicase as

helicase moves along DNA, they break hydrogen bonds btwn bases

DNA polymerase: bind to separated chains of DNA, move along chains & assemble nucleotides complementary to existing DNA chains◦ Complementary bases are bonded by hydrogen

bonds DNA polymerases begin replication simultaneously

at many points along the separated nucleotide chains (~6000 sites at same time in fruit fly DNA)

When replication complete: two new exact copies of original DNA molecule are produced and cell is ready to undergo cell division

Example: DNA Molecule of ATTCCG would have complementary strand of TAAGGC

DNA rep occurs with high degree of accuracy—1 error in every 10,000 paired nucleotides

Change in nucleotide sequence at even one location: mutation: can have serious effects in new cells

DNA proofreading & repair processes help keep error rate to one error/1 billion nucleotides!

What are the main functions of DNA? Identify the types and locations of covalent

bonds and hydrogen bonds in a DNA molecule.

List the base-pairing rules. What roles do enzymes play in DNA

replication? How would the deoxyribose sugar-phosphate

backbone of nucleotide chains look if purines paired with only purines and pyrimidines paired with only pyrimidines?

What are the main functions of DNA? Stores and transmits genetic information

Identify the types and locations of covalent bonds and hydrogen bonds in a DNA molecule: Covalent bonds exist between the deoxyribose sugar and phosphate molecules. Hydrogen bonds join the nitrogen bases

List the base-pairing rules: Guanine and cytosine pair; thymine and adenine pair

What roles do enzymes play in DNA replication? Helicase attaches to DNA and breaks hydrogen bonds between the complementary bases and the chains separate. DNA polymerases bind to the separated chains of DNA. DNA polymerase identifies an unpaired base on a template chain, locates a nucleotide that has the complementary base in the surrounding medium, and attaches the free nucleotide to the new nucleotide chain that is being formed. Other enzymes are responsible for identifying and repairing errors in replicated DNA.

How would the deoxyribose sugar-phosphate backbone of nucleotide chains look if purines paired with only purines and pyrimidines paired with only pyrimidines? The backbone would appear uneven, similar to the appearance of a roller coaster.

Sugar = Ribose (not deoxyribose) Single strand of repeating nucleotides Adenine pairs with Uracil (not thymine)

◦ Adenine----Uracil; Cytosine----Guanine

mRNA (Messenger RNA): carries genetic information from DNA in nucleus cytosol

tRNA (Transfer RNA): binds to specific amino acids; 45 varieties

rRNA (Ribosomal RNA): most abundant form; make up ribosomes where proteins are made

Process by which genetic information is copied from DNA to RNA: Transcription

RNA polymerase: primary transcription enzyme, make RNA copies from DNA sequences◦ Initiates transcription by binding t specific regions

of DNA called promoters: mark beginning of the DNA chain that will be transcribed (beginning of gene)

◦ When RNA polymerase binds to promoter, DNA molecule separates—only one of separated DNA chains—template—is used for transcription

RNA polymerase attaches to first DNA nucleotide of template chain

It begins adding complementary RNA nucleotides to newly forming RNA molecule

Same base-pairing rules, except one exception: C—G, A—U

Transcription continues until RNA polymerase reaches DNA region called termination signal: specific sequence of nucleotides that marks end of gene ( in eukaryotes)

At termination signal, RNA polymerase releases both DNA molecule and newly formed RNA molecule

Following transcription, mRNA moves through pores of nuclear membrane into cytosol of cell where it will direct synthesis of proteins

Define transcription. List the main steps involved in this process.

In what ways does the structure of RNA differ from that of DNA?

Describe the structure and function of each of the three types of RNA.

List three roles of RNA polymerase in transcription.

What basic principle ensures that the transcribed RNA molecule is carrying the right genetic message?

Does it matter which of the separated DNA chains is used for transcription? Why?

Define transcription. List the main steps involved in this process: The process of producing RNA from DNA

In what ways does the structure of RNA differ from that of DNA? RNA usually consists of a single chain of nucleotides, has ribose instead of deoxyribose as its sugar, and contains uracil rather than thymine.

Describe the structure and function of each of the three types of RNA: mRNA consists of RNA nucleotides in the form of a single uncoiled chain. mRNA carriers genetic information from the DNA in the nucleus to the cytosol of a eukaryotic cell. rRNA is a single chain of about 89 nucleotides folded into a hairpin shape that binds to a specific amino acid. rRNA consists of RNA nucleotides in a globular form and makes up the ribosomes where proteins are made.

List three roles of RNA polymerase in transcription: RNA polymerase initiates transcription by binding to promoters. When RNA polymerase binds to a promoter, the DNA molecule in that region separates. RNA polymerase attaches to the first DNA nucleotide of the template chain. Then it begins adding complementary RNA nucleotides to the newly forming RNA molecule. At the termination signal, RNA polymerase releases DNA and RNA.

What basic principle ensures that the transcribed RNA molecule is carrying the right genetic message? Accuracy of the genetic message is assured by complementary base pairing.

Does it matter which of the separated DNA chains is used for transcription? Why? Yes, because templates are complementary, they do not contain identical sequences of nucleotides A sequence complementary to the template will code for different information.

Ch. 10-3PROTEIN SYNTHESIS

Proteins are polymers which are made of one or more polypeptides which are made of amino acids linked to each other by peptide bonds

20 different amino acids Polypeptides consist of hundreds

or thousands of the 20 AA arrangedin particular sequence sequencewill determine the function of protein

amino acids

polypeptides

polymers

During protein synthesis (translation), sequence of nucleotides in mRNA is translated into sequence of amino acids

Genetic code—used by most organisms to translate mRNA into proteins; correlation btwn nucleotide sequence & AA sequence

Combination of 3 mRNA nucleotides = CODON◦ Each codon codes for specific amino acid◦ There are 64 different codons that code for the 20

different amino acids◦ The near-universality of the genetic code supports

idea that all organisms are evolutionarily related

CODON TABLE

ANSWER THE FOLLOWING:

1) CCA=

2) AUU=

3) GUC=

4) AUG=

5) CGA=

Start codon: (AUG); engages ribosomes to start translating an mRNA molecule; codes for amino acid methionine

Stop codon: (UAA, UAG, UGA); cause ribosome to stop translating an mRNA

Translation: process of assembling polypeptides from information encoded in mRNA

Begins when mRNA leave nucleus and migrates to ribosomes where proteins are made

tRNA carry AA from cytosol to ribosomes

tRNA has region that bonds to specific AA

Opposite side of tRNA has sequence of 3 nucleotides called anticodon: complementary to and pairs with its corresponding mRNA codon◦ tRNA with anticodon of AAA

would bind to mRNA codon sequence of UUU and would be specific for AA phenylalanine ensures that AA are added to growing polypeptide in order prescribed by mRNA transcript

Made of both rRNA and proteins Can be free floating or attached to RER Membrane proteins & proteins that will be

exported for outside use are produced by ribosomes on RER

Ribosomes have 3 binding sites:◦ One site holds an mRNA so that its codons are

accessible to rRNA◦ Other two sites hold tRNAs whose anticodons pair

with the mRNA codons

Ribosomes attaches to start codon (AUG) on mRNA start codon pairs with anticodon UAC on a tRNA (because tRNA that has UAC anticodon carries AA methionine, the first AA in every polypeptide is methionine) it may be removed later

Ribosome moves along mRNA and each codon is paired with its tRNA anticodon this causes specified AA to attach to previously translated AA with a covalent bond called a peptide bond

Compare transcription with translation. Distinguish a codon from an anticodon,

and explain the significance of each. How does the structure of tRNA relate to

its function in translation. Explain the significance of the start codon

and stop codons. Do all polypeptides begin with the amino acid coded for by the start codon?

What would translation of the mRNA transcript UAACAAGGAGCAUCC produce?

Compare transcription with translation: Transcription is the process of producing RNA from DNA. Translation is the process by which polypeptides are assembled from the information encoded in mRNA.

Distinguish a codon from an anticodon, and explain the significance of each: A codon is a series of three nucleotides in mRNA that code for a specific amino acid. An anticodon on tRNA is a set of three bases complementary to the codon.

How does the structure of tRNA relate to its function in translation: The structure of tRNA allows an amino acid at one end to be positioned in a polypeptide when the anticodon at the other end of the tRNA pairs with a codon.

Explain the significance of the start codon and stop codons. Do all polypeptides begin with the amino acid coded for by the start codon? The start codon (AUG) engages a ribosome to start translating an mRNA molecule. Stop codons (UAA, UAG, UGA) cause the ribosome to stop translating mRNA. All polypeptides do not begin with the amino acid coded for the by start codon.

What would translation of the mRNA transcript UAACAAGGAGCAUCC produce? No protein would be produced because the mRNA begins with a stop codon.