DNA part 2 Flashcards
What are histones?
Eukaryotic DNA associated with proteins
Describe the function of histones
Package the DNA into globular structures called nucleosomes.
What does the association of histones with DNA contribute to?
Supercoiling. During condensation of chromosomes in the early stages of mitosis and meiosis the tails of histones in adjacent nucleosomes link up and pull the nucleosomes togther
What is transcription?
The process by which a gene is copied into a complementary RNA using DNA as template.
Where does transcription take place?
In eukaryotes it takes place in the nucleus during interphase
Describe the process of transcription.
- RNA polymerase binds to a sequence at the start of the gene called the promoter. In eukaryotes the binding of the RNA polymerase to the promoter is mediated and controlled by transcription factors
- RNA polymerase moves along the gene, separating the DNA strands and assembling an RNA molecule along one of the DNA strands.
- The template DNA strand which is used is the antisense strand
- RNA nucleotides are assembled based on complementary base pairing as in DNA replication, except that uracil and not thymine pairs with adenine.
- RNA polymerase links together the RNA nucleotides by making phosphodiester bonds between the 5’ end of the free RNA nucleotide and the 3’ end of the growing mRNA. So transcription occurs in a 5’ to 3’ end.
- Free nucleotides exist in cells as NTPs. RNA polymerase covalently binds the NTP in a reaction that involves the release of 2 phosphates.
- The RNA strand separates from the DNA strand as it is produced
- Transcription is terminated at the end of the gene and the DNA, RNA and RNA polymerase separate. The DNA rewinds.
Which strand is transcribed and which is not?
Transcribed- antisense strand
not transcribed- sense strand
What happens in post- transcriptional modifications in eukaryotes?
In eukaryotes the mRNA is modified after transcription and this happens in the nucleus before the mRNA exists through a nuclear pore.
What is the immediate product of transcription?
pre-mRNA
What are the coding sequences interrupted by?
Non-coding sequences called introns. The remaining coding parts of the mRNA are exons
Describe the post-transcriptional modifications
- Introns are removed from pre-mRNA in the nucleus
- Exons are spliced together to form the mature mRNA
- This process is called mRNA splicing
- Some genes have more exons and different combinations of them can be spliced together to produce different proteins. This is alternative splicing
What does alternative splicing cause?
A single gene codes for multiple proteins. This increases the total number of proteins an organism can produce from its gene.
What happens when post- transcriptional modification at the end?
addition of a 5’ cap usually before transcriptional has been completed and a poly- A tail is added at the 3’ end after the transcript has been made
What is the genetic code?
A triplet code: 3 bases in the mRNA code for one amino acid
What is a codon?
3 bases on the mRNA that code for one amino acid
What are most amino acids coded by?
Two or more codons- so the genetic code is called a degenerate
What is the start codon?
AUG for initiation of translation- this codes for the amino acid methionine
How many stop codons are there?
Three
Describe the genetic code
It is universal meaning that almost all organisms use the same genetic code- allowing for gene transfer between species to create genetically modified organisms which produce a polypeptide from a different species.
There are however minor variations in some organisms
What is translation?
The synthesis of polypeptides on ribosomes
In which stage does translation take place?
Interphase
What are the three components of translation?
mRNA
tRNA
ribosome
What is the role of complementary base pairing in translation?
The amino acid sequence of the synthesised polypeptide is determined by the base sequence of the mRNA according to the genetic code- 3 bases on mRNA, called a codon- code for one amino acid.
Each tRNA molecule has a special triplet of bases called an anticodon and carries a specific amino acid that corresponds to this anticodon.
Translation depends on complementary base pairing between codons on mRNA and anticodons on tRNA and hydrogen bonds are formed.
Describe the structure of tRNA.
- Double- stranded sections formed by base pairing and single- stranded loops
- A triplet of bases called the anticodon which is part of a loop of 7 unpaired bases
- There are two other loops
- The base sequence CCA at the 3’ end forms a site for attaching the amino acid.