Chapter 8 - DNA, Genes and Protein Synthesis Flashcards
What is a gene?
A section of DNA that codes for a polypeptide
What is the genetic code?
What is a codon?
The genetic code refers to the sequence of bases that code for amino acids. - order of baes of DNA.
consists of codons
Codons are the triplets of bases on mRNA that code for a particular amino acid
What is a locus?
The fixed position of a DNA molecule occupied by a gene
What is an allele?
Different versions of the same gene, found at the same locus on a chromosome
What are exons?
Sections of DNA that code for amino acids
What are introns?
Where are they found?
Sections of DNA that do not code for amino acids
Between exons within genes
What is a genome?
The complete set of genes within a cell
What are the features of the genetic code?
- Universal - Each DNA triplet codes for the same amino acid in all organisms (with a few minor exceptions).
- Degenerate - more than one triplet codes for the same aminon acid
- Non-overlapping - Each tripletin the DNA sequence is only read once (e.g. CGTATC is read as CGT and ATC)
How was it proved that genetic code is degenerate?
There are 20 different amino acids
Scientists were able to conclude that each amnio acid is coded for by one or more combination of triplets. There are therefore 64 possible triplets (4^3) - sequence of 3 bases, 4 possible bases - meaning that each amino acid is represented by more than one triplet.
DNA and chromosomes
How does DNA vary between prokaryotes and eukaryotes
DNA molecules are wound around proteins known as histones to form a DNA-histone complex.
These complexes coil further to form chromatin, which helps pack the DNA into chromosomes. Each chromosome contains just a single molecule of DNA.
Eukaryotic and prokaryotic cells store DNA in slightly different ways.
Eukaryotic cells:
Store DNA in the nucleus.
Contain long, linear DNA molecules.
Associate the DNA with histones.
DNA contains introns.
Prokaryotic cells:
Store DNA in the cytoplasm.
Contain shorter, circular DNA molecules.
Do not associate the DNA with histones.
DNA does not contain introns.
Eukaryotic cells also contain DNA in their mitochondria and chloroplasts. This DNA, like in prokaryotes, is short, circular, and not associated with histones.
Homologous chromosomes
What are homologous pairs of chromosomes?
A homologous pair is two chromosomes that carry the same genes but not necessarily the same alleles of the genes
What is a mutation?
Any changes in the base sequence of a gene
What is a proteome?
The complete set of proteins that cam be produced by a cell.
What are the two types of RNA important in protein synthesis?
Messenger RNA (mRNA)
Transfer RNA (tRNA)
Describe the tructure of mRNA
It is a long single strand synthesised during transcription
Contains a base sequence complementary to a DNA sequence.
Contains codons, which are sets of three bases that code for an amino acid.
Small enough to leave the nucleus.
How does mRNA leave the nucleus?
Via pores in the nuclear envelope and then enters the cytoplasm where it associates with ribosomes - here it acts as a template for protein synthesis
Role of mRNA?
Its role is to carry genetic information from the DNA to the ribosomes (where proteins are made).
Describe structure of tRNA?
Relatively small molecule - single stranded chain made up of around 80 nucleotides that is folded into a clover leaf shape, with one end end of the chain extending beyond the other.
Used in translation
Uses hydrogen bonds between complementary base pairs to hold it in shape.
Contains a specific sequence of three bases at one end, known as the anticodon.
Contains an amino acid binding site at the opposite end.
Each tRNA is specific to one amino acid and has an anticodon that is specific to that aminoa acid
Role of tRNA?
Its role is to transport amino acids to ribosomes to build up a polypeptide chain.
Transcription
process of making pre-mRNA using part of DNA as a template
Describe the process step by step of making pre-mRNA
-RNA polymerase breaks hydrogen bonds between DNA bases, breaking the two strands apart and exposing the bases on each strand.
-RNA polymerase adds RNA nucleotides to complementary bases on one strand- known as the template strand.
-The RNA polymerase moves along the template strand and joins the nucleotides together until the pre-mRNA strand is complete
- As the RNA polymerase adds the nucleotides one at a time to build a strand of pre-mRNA, the DNA strands rejoin behind it
(After the nucleotides are added by RNA polymerase, the DNA helix behind is reattached.
RNA polymerase continues to move along the DNA strand until the pre-mRNA strand is complete.)
- When the RNA polymerase reaches a particular sequence of bases on the DNA that it recognises as a stop triplet code, it detatches and the prodcution of pre-mRNA is complete.
-At the end of transcription, we are left with the original DNA helix (two strands) plus one complete strand of pre-mRNA.
Do prokaryotes require splicing and why?
No because they do not have exons.
Transcription results directly in the prodcution of mRNA from DNA
Why is it required in eukaryotes?
pre-MRNA contains both introns and exons from the DNA sequence
Splicing is used to remove introns to make mRNA only with exons
Describe process of splicing
Splicing involves the removal of introns from pre-mRNA, while the remaining exons are attached together.
Exons are removed by a cluster of proteins. These proteins then attach the remaining introns together.
What is an anticodon?
Set of 3 bases that are complementary to a codon on mRNA.
The anticodon is attached to a molecule that carries an amino acid
