Chap 6: Nucleic acids & protein synthesis

polynucleotide

a chain of nucleotides joined by phosphodiester bond

nucleotide structure

nucleic acids

monomers are nucleotides

ATP structure

adenine + ribose = adenosine

3 phosphate = trisphosphate

ATP = adenosine triphosphate

purines

pyrimidines

complementary base pairing

A - T (2 H bonds)

C - G (3 H bonds)

→ In RNA, there are no T, therefore, A bonds with U

DNA nucleotide structure

DNA structure (5)

• double helix

• sugar-phosphate backbone

• anti-parallel chains (one 5’ to 3’, one 3’ to 5’)

• H bonds between complementary base pairs

• nucleotides are linked by phosphodiester

phosphodiester bond in DNA

• phosphodiester bond alternating deoxyribose sugars and phosphate group

• phosphodiester bonds link 5’ of one sugar to the 3’ of another sugar via a phosphate group

→ give DNA strand a direction of 5’ end or 3’ end

semi-convervative replication def

the method by which a DNA molecule is copied to form two identical molecules, each containing one strand from the original strand

describe DNA replication (7)

1. two strands of DNA are unwinded by breaking the H bonds

2. DNA polymerase attaches nucleotides to leading strands in the direction of 5’ to 3’

3. DNA polymerase can only runs in 5’ to 3’ direction

4. the nucleotides are attaches by complementary base pairing

5. the lagging strand is synthesised in Okazaki fragments

6. the Okazaki fragments are hold to the original strand by H bonds between bases

7. the DNA ligase connect the Okazaki bonds by phosphodiester bond

RNA nucleotide

RNA structure (4)

• single strand

• ACGU bases

• ribose sugar

• no H bonds as there are no complementary base pairing

gene

sequences of DNA nucleotides that codes for a polypeptide

polypeptide

is coded for by a gene

triplet

3 bases of DNA (e.g: ACT)

functions of triplets (3)

1) start codon: where transcription begins

2) stop codon: where transcription ends

3) code for a specific amino acid

genetic code is non-ambiguous

1 codon will code for 1 amino acid, but 1 amino acid can be coded by many codons

codons that code for amino acids are…

the same across all organisms

transcription

from DNA to RNA

translation

from RNA to polypeptide

codon

a sequence of 3 adjacent nucleotides in mRNA that codes for one amino acid

• DNA triplets → RNA codons

describe transcription process (5)

1) RNA polymerase unwinds DNA by breaking H bonds → expose the gene to be transcribed

2) free (activated) nucleotides complementary pair up with the template strand by H bonds

3) RNA polymerase bonds the nucleotide until stop codon

4) when mRNA is complete → the H bonds are broken

5) mRNA leaves nucleus through nuclear pores

location of transcription

nucleus

location of translation

cytoplasm

describe translation process

1) mRNA attches to a ribosome, 2 codons exposed to ribosome at a time

2) tRNA attaches to specific amino acid → brings to mRNA on ribosome

3) the anticodon complementary pair with base of mRNA

4) 2nd tRNA brings another amino acid next to first amino acid → peptide bond between 2 amino acids

5) the ribosome moves along the mRNA to read the next codon

6) 1st tRNA (w/o amino acid) leaves ribosome → another one enter

7) repeat until stop codon

non-transcribed strand

the other strand that is not used in transcription

transcribed/template strand

the strand that is used in transcription

primary transcript

the RNA nucleotides synthesised after transcription that contains introns and exons

introns

non coding sequence so they do not code for an amino acid

exons

coding sequence so they code for an amino acid

DNA splicing

occurs after transcription, modification of primary transcript, remove introns and join exons tgt

→ functional mRNA will be formed

gene mutation

a change in the sequence of base pairs in the DNA molecule that may result in an altered polypeptide

deletion/insertion mutation

=> cause frameshift mutation

may introduce stop codon → alter primary structure → shorter polypeptide → tertiary structure altered → non-functional polypeptide

substitution mutation

=> may/may not change the polypeptide formed

• may lead to stop codon

• may cause silent mutation: base is changed, amino acid produced is the same so no effects on polypeptide