IB Biology Topic D1.2.14: Non-coding Sequences in DNA Do Not Code for Polypeptides Notes

Most of Your DNA Does Not Code for Polypeptides, Yet It Is Far From Junk

1. Most of the genome is non-coding DNA, meaning it is not translated into polypeptides.
2. These sequences still carry out essential functions in gene expression, chromosome stability, and protein synthesis.
   1. Calling them "junk DNA" is misleading, because losing them would disrupt how cells build and control proteins.

Regulatory Sequences Control When and How Strongly Genes Are Expressed

1. Regulatory sequences are non-coding DNA that switch genes on or off and tune their output.
2. Promoters
   1. These are the sites where RNA polymerase binds to begin transcription.
3. Enhancers
   1. These distant sequences increase transcription of a gene.
4. Silencers
   1. These sequences decrease or shut down transcription of a gene.

Introns Are Removed From mRNA but Still Shape Gene Function

1. Introns are non-coding sequences within a gene that are removed during RNA processing.
2. The remaining coding sequences, the exons, are joined together to form the mature mRNA.
3. Alternative splicing
   1. Because introns separate the exons, exons can be joined in different combinations so one gene can produce several polypeptides.
4. Regulation
   1. Some introns contain enhancers or silencers that influence gene expression.
5. Evolutionary flexibility
   1. Mutations can accumulate in introns without changing the coding sequence, giving genes room to evolve.

Telomeres Cap Chromosome Ends and Limit How Often a Cell Can Divide

1. Telomeres are repetitive non-coding sequences at the ends of chromosomes.
2. They stop chromosomes from fraying or sticking to one another.
3. Telomeres shorten a little with each round of DNA replication.
   1. Once they become too short, the cell can enter senescence or undergo programmed cell death, which limits further division.
4. Cancer cells often reactivate the enzyme telomerase to rebuild their telomeres and keep dividing indefinitely.

Genes for rRNA and tRNA Build the Machinery of Translation

1. Some non-coding genes are transcribed into functional RNA rather than being translated into a polypeptide.
   1. The genes for rRNA and tRNA are examples, because their end product is the RNA molecule itself.
2. Ribosomal RNA (rRNA)
   1. rRNA forms the core of ribosomes, the structures that assemble polypeptides.
   2. It catalyses the formation of peptide bonds, making it a ribozyme, an RNA with enzyme activity.
3. Transfer RNA (tRNA)
   1. Each tRNA carries a specific amino acid and has an anticodon that pairs with a matching mRNA codon.
   2. This pairing ensures the correct amino acid is added to the growing polypeptide.