DNA in Eukaryotes

• Nucleus - surrounded by nuclear envelope

• DNA double helix - wrapped around histone proteins = nucleosomes - coil to form chromatin organised into chromosomes

• DNA is compact to large amounts can be stored in small spaces

• DNA is linear

• DNA froms several chromosomes with no plasmids

DNA in Prokaryotes

• No nucleus - free-floating in cytoplasm

• DNA is shorter, circular and not wrapped around histone proteins

• Condensed by supercoiling

• DNA is found in main chromosome and smaller structures - plasmids

Genes

Gene = section of DNA that codes for one polypeptide - polypeptides determine nature and development of organisms

Gene also codes for a functional RNA - fixed position (locus) on a particular chromosome

Individuals contain 2 copies of the each gene - each gene at the same position on a pair of homologous chromosomes - same length and contain same genes

Alleles = two or more alternative forms of a gene that arise by mutation

Genome = the complete set of genes in a cell - including mitochondria or/and chloroplasts

Proteome = the full range of proteins produced by the genome

The Genetic Code

DNA carries the genetic code to allow the cell to synthesis proteins/enzymes

Difference between DNA strands is their length and sequence of bases

DNA is triplet coded - 3 bases code for one amino acid

• Universal = a triplet specifies for one amino acid in all organisms

• Non-overlapping = bases in DNA code are only read once and triplets don’t share bases

• Degenerate = more than one triplet can code for one amino acid

Specific sequence of bases controls the sequence of amino acids in proteins/primary structure

The specific tertiary structure and function of a protein is formed as bonds form between R groups of different amino acids

Differences in the base sequences of alleles of a single gene - due to mutation may result in non-functional proteins - like non-functional enzymes

Non-coding DNA

Eukaryote DNA - most doesn’t code for synthesis of proteins due to non-coding DNA between genes - contains multiple repeats of the base sequences(VNTR) every individual number and length of core sequences are unique

Eukaryotic DNA contains non-coding sections = introns within genes

Exons are sections of DNA that are expressed to produce proteins whilst introns intervene

Prokaryotes have no introns in their DNA

The Role of RNA

1. Messenger RNA

• Messenger RNA consists of thousands of nucleotides in a linear strand

• mRNA is formed by transcription of a gene in DNA in the nucleus - complementary to the DNA in its base sequence

• mRNA is variable in length - copy of one gene so it is shorter than DNA

• Amino acid is coded for by a triplet of bases on mRNA = codon

• mRNA is single-stranded and therefore has unpaired bases and is easily broken down in the cytoplasm - exists temporarily until protein synthesis

2. Ribosomal RNA

• rRNA with ribosomes = ribosomes - site of mRNA translation and protein synthesis

• rRNA is coded by numerous genes in different chromosomes

3. Transfer RNA

• tRNA is relatively small molecule = around 80 nucleotides

• Single-stranded which folds

• tRNA molecule forms hydrogen bonds within complementary sections of the molecule to stabilise the molecule

• One end attaches to an amino acid

• Several Types tRNA to carry a single specific amino acid - base of the tRNA is sequence of three bases = anticodon

• Each amino acid carried have a different sequence of bases on the anticodon of tRNA

Protein Synthesis

Proteins = polypeptide chains - DNA in nucleus contains instructions to make all the organisms proteins

Transcription : in the nucleus of the cell = formation of pre-mRNA which is complementary to the sequence of bases on the DNA - carries genetic code out of nucleus to the ribosome - DNA never has to leave nucleus so less chance of getting damaged

RNA processing : in the nucleus of the cell = non-functioning sequences of bases are spliced from pre-mRNA into mRNA - mRNA then leaves the nucleus and attaches to a ribosome

Translation : on ribosomes = involves translation of mRNA message into a specific sequence of amino acids to form a polypeptide

Transcription

Transcription in eukaryotes = making pre-mRNA using DNA as a template - portion of gene is transcribed

• Two DNA strands separated by DNA helicase - breaking hydrogen bonds

• One strand of the DNA acts as a template for pre-mRNA is made

• Free DNA nucleotides line up against DNA template by complementary base pairing

• The adjacent RNA nucleotides bond together to form pre-mRNA molecule carrying a sequence of bases that is complementary to the DNA template

• RNA polymerase - catalyses formation of phosphodiester bonds between RNA nucleotides = forming sugar-phosphate backbone of the mRNA molecule

Splicing

In pre-mRNA in eukaryotic cells - introns are removed by enzymes and other molecules before mRNA moves into the cytoplasm

Remaining Exons join together in a number of different combinations - splicing

After Splicing = mRNA molecules leaves nucleus through the nuclear pores

Transcription is different in prokaryotes

Prokaryotic Genes do not contain non-coding sections so they don’t produce pre-mRNA that requires splicing - does not occur in the nucleus

Translation

• A ribosome binds to mRNA

• A specific tRNA molecule with a complementary anticodon to the first codon on mRNA arrives at the ribosome - tRNA carries the specific amino acid

• A second tRNA molecule joins in the same way with a another amino acid and both amino acids join in a condensation reaction forming a peptide bond using ATP

• A ribosome can only hold 2 tRNA molecules at one time - the first tRNA leaves, leaving the ribosome behind and goes and gets another ribosome and joins next to the second one and forms another peptide bond until a stop codon is reached

• A polypeptide chain is formed and detaches from the ribsome

This polypeptide chain then:

• Folds into further secondary and tertiary structures in the rough endoplasmic reticulum

• Modified/packaged into vesicles by the golgi apparatus