B8 DNA, Genes And Protein Synthesis Flashcards
What is a gene
sections of DNA that contains code for making polypeptide and functional
RNA. code is specific sequence of bases.
What do polypeptides make
POLYPEPTIDES MAKE PROTEINS AND SO GENES DETERMINE THE PROTEINS OF AN ORGANISMS.
What is a locus
THE LOCATION OF A PARTICULAR GENE ON A CHROMOSOME IS CALLED THE LOCUS.
What is an allele
AN ALLELE IS ONE OF A NUMBER OF ALTERNATIVE FORMS OF A GENE. MOST GENES OCCUR IN TWO, OCCASIONALLY MORE, DIFFERENT FORMS.
IN A EUKARYOTIC CELL NUCLEUS, DNA IS STORED AS what
Chromosomes
How many pairs of chromosomes do humans have
23 pairs of chromosomes
46 in total
What is a homologous pair
PAIRS OF MATCHING CHROMOSOMES
ARE CALLED HOMOLOGOUS PAIRS (FOR
EXAMPLE THE TWO COPIES OF CHROMOSOME I ARE
HOMOLOGOUS PAIRS).
[Pairs of matching chromosomes are called homologous pairs. A homologous pair of chromosome are exactly the same size, have exactly the same genes BUT might have different alleles.]
What is DNA stored as?
DNA is stored as chromosomes inside of the nucleus.
What shape are chromosomes in eukaryotic cells
Chromosomes in eukaryotic cells are linear in shape.
What is a nucleosome
This complex on DNA wrapped around a histone is called a nucleosome.
How does the DNA fit into the nucleus
To tightly coil the DNA to fit in the nucleus as chromosomes, the DNA is tightly wound around proteins called histones.
DNA in prokaryotic cell
Prokaryotes also carry DNA in chromosomes, BUT the DNA molecules are shorter and circular. The DNA is NOT wound around histones. Instead it supercoils to fit in the cell
[In prokaryotes DNA is coiled and stored in chromosomes, but the chromosomes are not protein bound. Chromosomes in prokaryotic cells are short and circular in shape.]
DNA in chloroplasts and mitochondria
There is DNA found in chloroplasts and mitochondria. This DNA is similar to prokaryotic DNA, in that it is short and circular.
3 features of genetic code
Degenerate
Universal
Non-overlapping
What is a start condon
Start Codon
At the start of every gene there is a ‘start codon’ TAC in DNA or AUG in mRNA. This codes for the amino acid methionine. This methionine is later removed from the protein if it is not actually needed for the structure.
What is a stop codon
Stop Codon
At the end of every gene there are 3 bases that do no code for an amino acid and is known as a
‘stop codon’. These stop codons mark the end of a polypeptide chain and cause ribosomes to detach and therefore stop translation. These codons are ATT,ATC and ACT on DNA.
Genetic code is degenerate
There are 20 amino acids that the genetic code has to be able to code for. There are four DNA bases, (GCTA), and therefore three bases are needed to make enough combinations to code for at least 20 amino acids. This can be proven mathematically (4n).
If one base coded for one amino acid this would only allow for 4 amino acids to be coded for. This is insufficient to code for 20 amino acids.
If two bases coded for one amino acid this would allow for 16 amino acids to be coded for (4x4 combinations of code). This is insufficient to code for 20 amino acids.
If three bases coded for one amino acid this would allow for 64 amino acids to be coded for (4 x 4 x 4 combinations of code).
Degenerate - amino acids coded for by more than 1 base triplet
64 combinations is more than is needed to code for 20 amino acids, and as a result each amino acid is actually coded for by more than one triplet of bases. This is what is meant by the genetic code be degenerate. e.g. tyrosine is coded for by ATA and ATG.
This genetic code wheel enables you to work out all combinations of bases that code for each of the 20 amino acids.
Advantage of genetic code being degenerate
This is an advantage as if a point mutation occurs, even though the triplet of bases will be different, it may still code for the same amino acid and therefore have no effect
Genetic code is universal
The same triplet of bases codes for the same amino acid in all organisms, this is why the genetic code is described as being universal.
Advantage of genetic code being universal
This is an advantage as if it means genetic engineering is possible.
E.g. inserting the human gene for insulin into bacteria
Genetic code is non-overlapping
Each base in a gene is only part of one triplet of bases that codes for one amino acid. Therefore each codon, or triplet of bases, is read as a discrete unit.
ACG GCT TCA ACT
Advantage of genetic code being non-overlapping
This is an advantage as if a point mutation occurs, it will only affect one codon and therefore one amino acid.
What are introns
Introns are sections of DNA that do not code for amino acids and therefore polypeptide chains. Introns are found in eukaryotic DNA, but not in prokaryotic DNA.
These get removed, spliced, out of mRNA molecules.
What are exons
Exons are the sections of DNA that do code for amino acids.
Genome and proteome
The genome is an organisms complete set of DNA in one cell, where as the proteome is the full range of proteins in one cell.
The genome should never change, whereas the proteome of the cell is constantly changing depending on which proteins are currently needed.
The genome of organisms widely varies, for example bacteria contain on average 600,000 DNA base pairs where as humans contain 3 billion DNA base pairs.
Where are proteins created
On ribosomes
2 main stages in the production of proteins from the DNA code
I. Transcription - where one gene on the DNA is copied into mRNA.
- Translation - where the mRNA joins with a ribosome, and corresponding tRNA molecules brings the specific amino acid the codon codes for.
Where is A complementary mRNA copy of one gene on the DNA created
In the nucleus
mRNA is much shorter than DNA so it is able to what?
mRNA is much shorter than DNA so it is able to carry the genetic code to the ribosome in the cytoplasm to enable the protein to be made.
Transcription process
I. The DNA helix unwinds to expose the bases to act as a template.
- Only one chain of the DNA acts as a template.
- Like with DNA replication, this unwinding and unzipping is catalysesd by DNA helicase.
- DNA helicase breaks the hydrogen bonds between bases.
- Free mRNA nucleotides in the nucleus align opposite exposed complementary DNA bases.
- The enzyme RNA polymerase bonds together the RNA nucleotides to create a new RNA polymer chain. One entire gene is copied.
Once copied, the mRNA is modified and then leaves the nucleus through the nuclear envelope pores.
What happens following transcription
Following transcription, pre-mRNA has to be modified to become mRNA that is ready to leave the nucleus and take part in translation.
The introns are spliced out by a protein called a splicesome. This leaves behind just the exons, the coding regions.
What happens in translation
This is the stage in which the polypeptide chain is created using both the mRNA base sequence and the tRNA.
Translation process
This is an advantage as if a point mutation occurs, it will only affect one codon and therefore one amino acid.
