Topic 4A- DNA, RNA and Protein Synthesis

What is a gene?

A sequence of DNA bases that codes for a polypeptide

What is a chromosome?

• A large structure of DNA wrapped around histones

• This forms chromatin which is coiled further to form chromosomes

What is the structure of DNA in eukaryotes?

• DNA is found within the nucleus surrounded by a nuclear envelope

• It is a double helix structure wrapped around histones to form nucleosomes

• Nucleosomes are coiled to form chromatid which is organised into chromosomes

• It contains non-coding DNA regions called introns

How is the structure of DNA adapted for its size?

DNA is condensed and coiled compactly so that a large amount can be stored and organised in a small space

What is the position of a gene on a chromosome called?

A locus

What are homologous pairs of chromosomes?

• These chromosomes are found in pairs

• On of the pair is inherited from the mother, the other from the father

• The chromosomes are the same size and carry the same genes on the same loci

• They carry different alleles

What is the proteome?

All the proteins that an organism can produce

What is the structure of prokaryotic DNA?

• Prokaryotic organisms like bacteria and archaea have no true nucleus

• DNA is shorter and circular

• Not wrapped around histones

• Condensed by supercoiling

• The DNA is found is circular DNA and plasmids

What feature do chloroplasts and mitochondria share with nuclei?

They both contain DNA

What is a nucleoid?

Circular DNA

What is the structure of mitochondria and chloroplast DNA?

• Short

• Circular

• Not associated with histones

• No introns present

• One nucleoid

What is endosymbiosis?

• One organism living within another

• It is though that eukaryotic organelles are derived from prokaryotic cells incorporated in larger prokaryotic cells

What is the function of the chromosomes?

Caries the genetic information to build a large number of polypeptides and functional RNA molecules

What are the features of the genetic code?

• Degenerate

• Non-overlapping

• Universal

What does DNA being degenerate mean?

Some amnio acids are coded for by multiple triplets

What does DNA being non-overlapping mean?

The bases a triplet are only read once- triplets don’t share bases

What does DNA being universal mean?

The same triplet will code for the same amino acid in any organism

How does DNA regulate where the start and end of a gene is and what’re these regions on DNA called?

• Some regulatory DNA triplets code for the start and end of a gene

• These are called promoter and terminator regions

What are non-coding regions called?

• Intron

• Introns do not code for a polypeptide chain

What are coding regions called?

Exons

What are multiple base repeats?

Non-coding DNA between genes in eukaryotic DNA which is repeated many times

What are the three types of RNA?

• rRNA (ribosomal RNA)

• tRNA

• mRNA

What is the structure of mRNA?

• Single stranded

• Helical

• Made in the nucleus and carries genetic information into the cytoplasm through pores in the nuclear envelope due to it’s small size

• A codon is a group of 3 bases

• Chemically unstable

• Length varies based on the length of the gene

• Contains bases AUGC

• Has no hydrogen bonds

What is the structure of tRNA?

• Smallest polynucleotide strand

• Clover leaf shape- hydrogen bonds hold it’s shape

• Made in the nucleus and found throughout the cell

• Contains anticodons and an amino acid binding site

• More stable than mRNA, less stable than DNA

• Contains bases AUGC

How is the structure of DNA related to it’s function?

• Coiled and wrapped around histones to make DNA more compact

• Long to store more genetic information

• Many weak hydrogen bonds can be easily broken for DNA replication

• DNA strand acts as a template during transcription

• DNA bases are no exposed to it is more stable

How is the structure of mRNA related to it’s function?

• Small so can fit through nuclear pores

• Has groups of 3 bases called codons

How is the structure of tRNA related to it’s function?

• Complimentary binding site for amino acids that are carried to ribosomes

• Has anticodons complimentary to codons on mRNA so amino acids are lined up in a specific order

What happens in transcription?

• RNA polymerase binds at promoter region and exposes a DNA strand by breaking hydrogen bonds

• One strand acts as a template and forms complimentary base pairs with RNA nucleotides present in the nucleus- A to U (instead of T) and G to C

• RNA polymerase joins adjacent RNA nucleotides together by condensation reactions to form phosphodiester bonds and release water molecules

• When RNA polymerase reaches the terminator region at the end of the gene it detaches and the production of pre-mRNA is complete

• Introns are the spliced out to form mature mRNA

Why don’t prokaryotes need to splice mRNA strands?

Prokaryotes do not contain introns

What happens in translation?

• In eukaryotes, the mRNA passes through the pores of the nuclear membrane, from this point, translation in prokaryotes and eukaryotes is the same

• mRNA attaches to the ribosome at the start of a codon

• A tRNA molecule carrying an amino acid binds with the mRNA codon with it’s anticodon at the ribosomes

• A second tRNA binds to the next codon, lining up the next amino acid

• The two amino acids join via a condensation reaction forming a peptide bond and a molecule of water with energy provided by the hydrolysis of ATP

• The first tRNA then detaches and goes back into the cytoplasm to pick up another amino acid

• The ribosome moves along the mRNA molecule as the process repeats

• A chain of amino acids linked by peptide bonds folds as it is produced

• At a stop codon on mRNA, the polypeptide chain is complete and moves away from the ribosome

How and why is transcription different in prokaryotes?

• mRNA is made in the cytoplasm so mRNA can be longer

• mRNA does not need to be spliced as prokaryotes don’t have introns

• mRNA is less modified as there is no need to protect it from degradation