A-level Biology Notes

Genes

What is a gene?

- a section of DNA that codes for a protein

- made out of intron and exon

- intron = non- coding DNA [function e.g. turns gene on or off]

- Exon = coding DNA [codes for protein]

How does a gene/ Exon code for protein

- made out of a sequence of bases

- each three bases code for one amino acid [called triplet code]

- therefore, sequence of bases determines sequence of triplet codes which determines the sequence of AAs = polypeptide chain/ primary structure [folds to secondary, then to tertiary/ quaternary]

properties of triplet code?

-degenerate= each AA has more than one triplet code

-non-overlapping = each base is read only once

- stop codes = occur at each end of sequence- do not code for an AA

how does a mutation lead to a nonfunctional enzyme?

- change in base sequence

- changes sequence of triplet codes

- change in sequence of AAs

- and in primary structure

- changing hydrogen/ionic/disulfide bonds

- change in tertiary structure [3D shape]

- change an active site shape

- substrate no longer complementary

- can no longer form enzyme- substrate complex

How is the protein assembled?

- by transcription and translocation

- transcription = production of a single stranded complimentary copy of a gene [called mRNA]

-translation = use sequence of codons on mRNA to assemble protein [tRNA brings in AAs]

DNA vs RNA?

- deoxyribose sugar versus ribose sugar

-thymine VS uracil

- single stranded VS single stranded

- one type VS 2 types [mRNA and tRNA]

what is mRNA?

- messenger RNA

- single stranded complimentary copy of the gene

- carries the code for assembling protein [on DNA called triplet code, on mRNA called codon]

what is tRNA?

- transfer RNA

- single stranded RNA folded over into a "cloverleaf shape" [held by hydrogen bonds between the bases]

- has an AA attachment sight on the top

- has three specific bases on the bottom [anticodon]

- anticodon binds to complementary codon on mRNA

what is transcription?

- occurs in nucleolus of nucleus

- producing the single stranded complimentary copy of a gene [called M RNA]

-DNA is double stranded, 1 strand called coding strand and one strand called template strand, the template strand will be used to build mRNA

- process, DNA helicase breaks the hydrogen bonds between complementary base in the gene

- the double strand of the gene unwinds

- leaves two separate strands [one coding strand and one template strand]

- RNA polymerase joins the sugar- phosphate backbone of the RNA strand

- leaves pre- mRNA [contains introns and exons]

- the copies of the introns are removed by splicing

- leaves mRNA

What is translation?

- takes place on ribosomes of rough endoplasmic reticulum

-uses The sequence off codons on the M RNA to assemble the protein [T RNA brings in AAs]

- process, mRNA leaves nucleus via nuclear pore

-mRNA attaches to a ribosome

- complementary T RNA carrying specific AAs binds to the codons on mRNA via their anticodon

- the AAs on the tRNA are joined by peptide bonds