Biology - Nucleic Acids - A-Level OCR A

What is a Double Helix

The shape of a DNA molecule due to coiling of the two sugar-phosphate backbone strands into a right-handed spiral configuration

A double helix shape is caused by...

the coiling of the two sugar phosphate backbone strands into a right-handed spiral configuration

What is a Nucleotide

molecule consisting of a five-carbon sugar, a phosphate group and a nitrogenous base.

What is the nucleotide pentose sugar in RNA

Ribose

What is the nucleotide pentose sugar in DNA

Deoxyribose

What are Phosphorylated nucleotides

Nucleotides that contain more than one phosphate group

Examples of phosphorylated nucleotides

ADP (Adenosine Diphosphate)

ATP (Adenosine Triphosphate)

What are the four bases of DNA

Adenine, Thymine, Guanine, Cytosine (GC-AT)

What are the four bases of RNA

Adenine, Uracil, Guanine, Cytosine

What is the structure of DNA

- Made of repeating monomeric units called nucleotides

- Two polynucleotide strands that are antiparallel

- Each nucleotide consists of a phosphate group, a deoxyribose sugar and a nitrogenous base

- Phosphodiester bonds between sugar and phosphate group

- Long, can carry a lot of genetic information

What are the two purines?

Adenine and Guanine (2 rings)

What is a purine?

a nitrogenous base that has a double-ring structure

What are the two pyrimidines?

Cytosine and Thymine (one ring)

What is a pyrimidine?

a nitrogenous base that has a single-ring structure

What is the importance of hydrogen bonds between bases?

- Adenine always pairs with Thymine by means of two hydrogen bonds

- Guanine always pairs with Cytosine by means of three hydrogen bonds

- A purine always pairs with a pyrimidine, giving equal sized rungs on the DNA ladder. These can then twist into the double helix coil, giving the molecule stability

- Hydrogen bonds allow the molecule to unzip for transcription

How is DNA is organised in eukaryotic cells

- Majority of DNA content (genome) is organised in the nucleus

- Each DNA molecule is wound into chromosomes around special histone proteins

- There is also a DNA loop without the histone proteins in the mitochondria and chloroplasts

How is DNA is organised in prokaryotic cells

- DNA is in a loop and is within the cytoplasm

- It is described as 'naked'

How does DNA replicate?

semi-conservative replication

What is Semi-conservative replication?

in each new DNA double helix, one strand is from the original molecule, and one strand is new.

How does Semi-conservative replication works

- The DNA molecule unwinds

- The DNA molecule unzips - hydrogen bonds between bases are broken

- Free phosphorylated nucleotides are bonded to the exposed bases

- DNA polymerase catalyses the addition of the new nucleotide bases. The unzipped DNA strands are used as templates

- Leading stand is synthesised continuously

- Hydrolysis of the activated nucleotides supplies energy to make the phosphodiester bonds between the sugar residue of one molecule and the phosphate group of the next

Which enzyme catalyses the unwinding of DNA

Gyrase Enzyme

Which enzyme catalyses the unzipping of DNA

DNA Helicase

What is a gene?

A segment of DNA that codes for a specific trait

What is a polypeptide?

A polymer (chain) of many amino acids linked together by peptide bonds.

What is transcription?

synthesis of an RNA molecule from a DNA template

What is translation?

Formation of a protein by assembling amino acids into a particular sequence according to the coded instructions carried from DNA to the ribosome by mRNA

What is a protein?

Large polypeptide of 100 or more amino acids

Ways in which RNA is structurally different to DNA

- Ribose sugar instead of deoxyribose sugar

- Nitrogenous base Uracil replaces Thymine

- Polynucleotide chain is usually single-stranded

- Polynucleotide chain is shorter

- Three forms of RNA

What are the three forms of RNA?

Messenger RNA (mRNA), Transfer RNA (tRNA), Ribosomal RNA (rRNA)

How much of an organisms dry mass does protein account for?

75%

As long as the primary structure of polypeptide chains is correct, it will fold correctly and be able to be held in its tertiary structure, enabling it to carry out its function. Examples:

- The shape of an enzyme molecules active site, antibody molecule having a shape complimentary to the antigens, receptor on cell membrane must have shape complimentary to shape of cell-signalling molecule, ion-channel protein must have hydrophilic amino acids lining the inside of the channel, and lipophillic amino acids on the outside portion

Nature of the genetic code

- Genetic code is universal

- Degenerate

- Non-overlapping

What does it mean for the genetic code to be universal?

Almost all living organisms use the same triplet of DNA bases codes for the same amino acid

What does it mean for the genetic code to be degenerate?

There is more than one base triplet, which reduces the effect of point mutations

What does it mean for the genetic code to be non-overlapping?

It is read starting from a fixed point in groups of three bases, if a base is added or delete, it causes a frame shift and the amino acid it codes for is changed

Transcription process

- Gene unwinds and unzips

- Hydrogen bonds between complementary nucleotide bases break

- The enzyme RNA polymerase catalysed the formation of temporary hydrogen bonds between RNA nucleotides and their complementary unpaired DNA bases. This DNA strand is called the template strand

- A length of RNA that is complementary to the template strand of the gene is produced. It is therefore a copy of the DNA strand - the coding strand

- The mRNA now passes out of the nucleus, through the nuclear envelope, and attaches to the ribosome

Translation process

- tRNA molecules bring the amino acids and find their place when the anticodon binds by temporary hydrogen bonds to the complementary codon on the mRNA molecule

- As the ribosome moves along the length of mRNA, it reads the code and when two amino acids are adjacent to each other a peptide bond forms between them

- Energy, in the form of ATP, is needed for polypeptide synthesis

- The amino acids sequence for the polypeptide is therefore ultimately determined by the sequence of triplets of nucleotide bases on the length of DNA

- After the polypeptide has been assembled, the mRNA breaks down. It's component molecules can be recycled into new lengths of mRNA, with different codon sequences

- The newly synthesised polypeptide is helped, by chaperone proteins to the cell, to fold correctly into its 3D shape or tertiary structure in order to carry out its function