2.1.3 Nucleotides and nucleic acids

What monomers combine to make nucleic acids?

Nucleotides

What are nucleotides made up of?

Phosphate group, pentose sugar, nitrogenous base

What are the 2 main types of nucleic acids

DNA, RNA

Describe the pentose sugar in DNA

Deoxyribose - H not OH on C2

Which bases can be found in DNA

Adenosine, thymine, cytosine, guanine

Describe the structure of DNA strands

Double stranded, double helix

Pentose sugar in RNA

Ribose - C2 has OH on bottom

Which bases can be found in RNA

Adenosine, uracil, cytosine, guanine

Structure of RNA

Single stranded

What are the 2 structural forms of nitrogenous bases

Purines, pyramidines

Is adenine a purine or pyramidine base

Purine

Is guanine a purine or pyramidine base

Purine

Is thymine a purine or pyramidine base

Pyramidine

Is cytosine a purine or pyramidine base

Pyramidine

Is uracil a purine or pyramidine base

Pyramidine

Difference in structure between purine and pyramidine bases

Purines have double ring, pyramidines have single

Name all the purine bases

Adenine, guanine

Name all the pyramidine bases

Thymine, cytosine, uracil

Name 2 types of pentose sugars in nucleotides

Deoxyribose, ribose

How are nucleotides joined together

Condensation reaction forms a phosphodiester bond

Which type of bond occurs between adjacent nucleotides on the same strand

Phosphodiester bonds

Describe a phosphodiester bond between two nucleotides

2 ester bonds from the phosphate group; 1 to C5 of same pentose sugar; 1 to C3 of next pentose sugar

What kind of reaction occurs to break a phosphodiester bond

Hydrolysis reaction

Name 2 phosphorylated nucleotides

ADP, ATP

What makes up ADP

Adenosine + 2 phosphate groups

What makes up ATP

Adenosine + 3 phosphate groups

What are phosphorylated nucleotides made up out of

Pentose sugar, nitrogenous base, inorganic phosphates

What is adenosine

Adenine (nitrogenous base) + ribose (pentose sugar)

Name 4 aspects of the structure of DNA

Phosphate-sugar backbone; anti parallel strands; hydrogen bonding; double helix

Describe DNA’s phosphate sugar backbone

Phosphodiester bonds between alternating phosphate groups + C5/C3 of deoxyribose sugars

What does it mean that the DNA strands are ‘antiparallel’

The two DNA polynucleotide strands run in opposite directions; one is 5’ to 3’, other is 3’ to 5’

Describe hydrogen bonding in DNA molecule

Strands held together by H-bonds between nitrogenous bases; complementary base pairing rules A-T/C-G

How many hydrogen bonds are formed between adenine and thymine

2

How many hydrogen bonds are formed between cytosine and guanine

3

Describe DNA’s double helix shape

3D shape formed by twisting of 2 joined strands

When in the cell cycle does DNA replication occur

Interphase

What does helicase do

Catalyse the unzipping of the 2 strands

Which bonds break to cause the unzipping of strands in DNA replication

H bonds between bases break

What happens after helicase unzips the 2 strands in DNA replication

Free DNA nucleotides pair up via complementary base pairing; catalysed by DNA polymerase

Which direction does DNA polymerase work in

3’ to 5’ on original strand

Which direction is the new strand formed in by DNA polymerase

5’ to 3’

How does DNA polymerase help to form the new strand of DNA in DNA replication

DNA polymerase adds nucleotides as the helix unwinds

Which direction is the lagging strand

5’ to 3’

What is the original non-lagging strand called

Leading strand

What direction is the leading strand

3’ to 5’

How does DNA polymerase work on the lagging strand

Adds new nucleotides in fragments; can only work in 3’ to 5’ direction

What are the fragments created on the lagging strand called

Okazaki fragments

How are Okazaki fragments joined together

Ligaments catalyses formation of phosphodiester bonds between nucleotides

How are nucleotides on the cloned DNA strand joined together (DNA replication)

DNA polymerase catalyses formation of phosphodiester bond between nucleotides to form sugar-phosphate backbone

Formation of which bond involves condensation reactions between deoxyribose and phosphate groups

Phosphodiester bond

Why is DNA replication considered ‘semi-conservative’

2 full double helix DNA molecules produced; one original DNA strand retained in each

Name 2 advantages of semi conservative replication

Maximises accuracy; genetic continuity ensured between generations of cells

How does semi conservative replication maximise accuracy

Each new DNA strand created from existing template

How does semi conservative replication ensuring genetic continuity help cells

Cells replacing others can carry out same role as parent

What is a mutation

Random, spontaneous change to DNA base sequence

Examples of mutations

Base deleted/added/inserted in wrong order

What is a gene

Sequence of bases on a chromosome which codes for a specific polypeptide chain

Why is it called the genetic code

Within each gene, 3 bases (triplet) code for one amino acid. This is how the gene determines sequence for amino acids in a polypeptide (primary structure)

3 aspects for nature of genetic code

Universal; degenerate; non-overlapping

Describe universal nature of genetic code

Same triplet of bases code for same amino acids in almost all organisms; evidence for common ancestor

Describe degenerate nature of genetic code

Multiple base triplets can code for same amino acid (except methionine + tryptophan) so if mutation occurs, amino acids may not change

Describe non-overlapping nature of genetic code

DNA read from fixed point in codons; if a base is added/deleted, frame shift occurs (every subsequent base is changed)

2 stages of polypeptide synthesis

Transcription, translation

Why does transcription occur

DNA cannot leave the nucleus as it is large and valuable so an mRNA molecule is formed which is a copy of coding strand

Short stages of transcription

Unzips; free mRNA nucleotides bind; mRNA strand formed separates from template strand; leaves nucleus; double helix reforms

What happens when gene unzips for transcription

Hydrogen bonds between bases break

What happens after gene unzips in transcription

Free mRNA nucleotides in nucleus bind to template strand by complementary base pairing; catalysed by RNA polymerase

RNA polymerase role

Catalyses mRNA nucleotides binding to template strand in transcription (protein synthesis)

Which enzyme is involved in transcription

RNA polymerase

What happens after the mRNA strand separates from the template strand

Leaves nucleus via nuclear pore

Why should the mRNA strand be complementary to template strand

Complementary base pairing so it is a copy of the coding strand

What is the coding strand in protein synthesis

Contains the gene that codes for the protein being synthesised

What is the template strand in protein synthesis

Complementary to coding strand; mRNA nucleotides bind to template strand

When does double helix reform (protein synthesis) after being unzipped

Once mRNA strand has detached from template strand

Where does transcription occur

Nucleus

Where does translation occur

Ribosome

Where does mRNA strand go after leaving nucleus (protein synthesis)

Ribosome; fits in groove between 2 subunits

What do tRNA molecules do

Bring amino acids to ribosome; Anticodon complementary to codon on mRNA

Describe mRNA and tRNA at the ribosome

mRNA moves along ribosome, exposing next codon one by one; complementary tRNA brings its amino acid

How do amino acids brought by tRNA join together to make a polypeptide chain

Peptide bonds form

What happens to the tRNA molecules after the polypeptide chain has formed

Leaves mRNA and splits from amino acid

Short stages of translation

MRNA to ribosome; tRNA brings first amino; anticodon complementary to codon; mRNA moves along exposing next codon; amino acids form peptide bonds; tRNA leaves; polypeptide chain

What are the 3 forms of RNA involved in protein synthesis

mRNA; tRNA; rRNA

How is mRNA involved in protein synthesis

Forms strand complementary to template strand/copy of coding strand; takes code for protein out of nucleus

How is tRNA involved in protein synthesis

Carries amino acids to ribosomes; anticodons complementary to codons on mRNA

How is rRNA involved in protein synthesis

Ribosome is formed from rRNA and proteins