Nucleotides & Nucleic Acids

What elements do nucleic acids contain?

1. Carbon

2. Hydrogen

3. Oxygen

4. Nitrogen

5. Phosphorus

Nucleic Acids

Large polymers formed from many nucleotides (monomers) linked together in a chain

Structure of RNA nucleotide

• Ribose pentose sugar, phosphate group, & 1 of the 4 nitrogenous bases

• Uracil replaces thymine

• RNA molecule made up of a single polynucleotide chain

Structure of DNA nucleotide?

• Deoxyribose pentose sugar

• Phosphate group

• Nitrogenous group (A-T, C-G)

Amount of H bonds formed between complementary base paris

• Thymine & Adenine form 2

• Cytosine & Guanine form 3

Pyrimidine bases

• Thymine & Cytosine & Uracil

• Smaller bases containing single carbon-nitrogen ring structures

Purine bases

• Adenine & Guanine

• Larger bases, containing double carbon-nitrogen ring structures

What is ADP & its structure as a phosphorylated nucleotide?

Adenosine Diphosphate

• Contains the nitrogenous base adenine, the pentose sugar ribose & 2 phosphate groups

What is ATP & its structure as a phosphorylated nucleotide?

Adenosine triphosphate

• Contains nitrogenous base adenine, pentose sugar ribose & 3 phosphate groups

How are polynucleotides synthesized?

• Nucleotides linked together by condensation reactions

• Phosphate group at 5th carbon of pentose sugar of 1 nucleotide forms a covalent bond w. OH group at 3rd carbon of pentose sugar of adjacent nucleotide

  • These bonds = phosphodiester bonds

• Forms a long, strong sugar-phosphate backbone w. a base attached to each sugar

Breakdown of polynucleotides into nucleotides

Breaking phosphodiester bonds using hydrolysis reactions

DNA structure

• Double helix, consisting of 2 anti-parallel polynucleotide strands joined together by hydrogen bonding between complementary bases

• 2 anti-parallel strands twist to form a double helix shape

How is DNA purified? (6)

1. Grind sample in mortar & pestle

   • Breaks down cell walls

2. Mix sample w. detergent

   • Breaks down cell membrane, releasing cell contents into solution

3. Add salt

   • Breaks H bonds between DNA & water molecules

4. Add protease enzyme

   • Breaks down proteins associated w. DNA in nuclei

5. Add layer of ethanol on top

   • Causes DNA to precipitate out of solution

   • Will be seen as white strands between layer of sample & alcohol

6. DNA can be picked up by spooling it onto glass rod

DNA replication (5)

1. DNA helicase breaks H bonds between 2 polynucleotide strands, forming 2 single strands

2. Free floating DNA nucleotides join to exposed bases on each orig. template strand by complementary base pairing

3. Nucleotides on new strand join together by DNA polymerase, forming the sugar-phosphate backbone & complete polynucleotide chain

4. Strands twist to form double helix

5. Each new DNA molecule contains 1 strand from orig. DNA molecule & 1 new strand

Why is it important to replicate DNA? (2)

1. So each new cell has full amount of DNA

2. Important for passing genetic info from generation to generation

Whys it important DNA replication is accurate?

To conserve genetic info

What is the occurrence of random, spontaneous mutations?

Often, although they don’t always have an affect but alter the sequence of amino acids

• May cause abnormal protein to be produced

Genetic Code

Sequence of base triplets (codons) in DNA or mRNA which codes for specific amino acids

Nature of genetic code? (3)

1. Non overlapping

   • Triplets don’t share bases

2. Degenerate

   • Are more possible combinations of triplets than there are amino acids

3. Universal

   • Same base triplets code for same amino acids in all living things

How many possible combinations of triplets are there?

64

How many amino acids are there?

20

How does a gene determine the sequence of amino acids in a polypeptide (the primary structure of a protein)?

• A gene is a sequence of DNA nucleotides that codes for a polypeptide

• The sequence of amino acids in a polypeptide which is the primary structure that forms a protein

Order of protein synthesis

1. Transcription

2. Translation

Transcription (5)

1. DNA helicase moves along DNA & unzips strands by breaking H bonds

   • Exposes DNA bases on template DNA strand

2. DNA polymerase binds & free RNA nucleotides enter 1 by 1 forming temporary H bonds w. complementary base pairs

3. mRNA forms as RNA polymerase makes phosphodiester bonds

4. H bonds break & mRNA moves to ribosome via a nuclear pore

5. DNA double helix reforms

Antisense Strand

• Complementary copy of the sense strand & doesn’t code for a protein

• Acts as a template strand during transcription

  • Ensures complementary RNA strand formed carries same base sequence as sense strand

  • From 3’ to 5’

Sense strand

DNA strand that has to code for protein to be synthesized

• Runs from 5’ to 3’

Translation (7)

1. Mature mRNA binds to small ribosome unit ; large ribosome unit sits on top

2. Codons presented 1 at a time

3. tRNA enters & anti codon binds temporarily to complementary codon, bringing the corresponding amino acid

4. Peptidyl transferase binds adjacent amino acids using peptide bonds, forming a polypeptide

5. H bonds break

6. Peptidyl transferase catalyzes release from tRNA

7. Process continues until a stop codon on mRNA is reached eventually forming a polypeptide chain

What occurs post translation?

• As the amino acids are joined together forming the primary structure of the protein, they fold into secondary & tertiary structures

• This folding & bonds that are formed are determined by the sequence of amino acids in the primary stucture

• Protein may be further modified in Golgi Apparatus before it is fully functional & able to carry out specific role

What is mRNA & its role?

• A single polynucleotide strand made in nucleus during transcription

• Carries genetic code from RNA in nucleus to cytoplasm

• Groups of 3 adjacent bases = codons

What is tRNA & what is its role?

Transfer RNA

Carries amino acids to the ribosome during protein synthesis

What is rRNA & what is its role?

Ribosomal RNA ; is in the ribosome

Combines w. proteins to form ribosomes which are responsible for protein synthesis in cells

• Forms 2 subunits in a ribosome

• Helps to catalyze formation of peptide bonds between amino acids