DNA, genes and protein synthesis Flashcards
(42 cards)
What is a gene?
A section of DNA that contains coded information for making polypeptides and functional RNA.
The coded information is in the form of specific sequences of bases along the DNA molecule.
Genes, along with environmental factors, determine the nature and development of all organisms.
A gene is a section of DNA located at a particular position, called a locus, on a DNA molecule.
What is the function of a gene?
Polypeptides make up proteins and so genes determine the proteins of an organism.
Enzymes are proteins.
As enzymes control chemical reactions they are responsible for an organism’s development and activities.
What does a gene do?
It is a base sequence of DNA that codes for:
The amino acid sequence of a polypeptide.
Or a functional RNA, including ribosomal RNA and transfer RNAs.
Why did scientists suggest the genetic code?
Only 20 different amino acids regularly occur in proteins.
Each amino acid must have its own code of bases on the DNA.
Only four bases are present in DNA.
If each base coded for a different amino acid, only four different amino acids could be coded for.
Using a pair of bases, 16 (4^2) different codes are possible, which is still inadequate.
Three bases produce 64 different codes, more than enough to satisfy the requirements of 20 amino acids.
What is the genetic code?
In trying to discover how DNA bases coded for amino acids, scientists suggested that there must be a minimum of three bases that coded for each amino acid.
What is a triplet?
As the code has three bases for each amino acid, each one is called a triplet.
As there are 64 possible triplets and only 20 amino acids, it follows that some amino acids are coded for by one triplet.
What is a degenerate code - features?
The genetic code is a degenerate code because most amino acids are coded for by more than one triplet.
What is methionine?
The start of a DNA sequence that codes for a polypeptide is always the same triplet.
This codes for the amino acid methionine.
If this first methionine molecule does not form part of the final polypeptide, it is later removed.
What are stop codes?
Three triplets do not code for any amino acid.
These are stop codes and mark the end of a polypeptide chain.
They act like a full stop in a sentence.
What is the non-overlapping code?
The genetic code is non-overlapping.
Each base in the sequence is read only once.
Thus six bases numbered 123456 are read as triplets 123 and 456, rather than as triplets 123, 234, 345, 456.
How is the code universal?
The genetic code is universal, with a few minor exceptions each triplet codes for the same amino acid in all organisms.
This is indirect evidence for evolution.
What are the other features of the genetic code?
A few amino acids are coded by a single triplet.
The remaining amino acids are coded for by between two and six triplets each.
A triplet is always read in one particular direction along the DNA strand.
What are exons?
Much of the DNA in eukaryotes does not code for polypeptides.
For example, between genes there are non-coding sequences made up of multiple repeats of base sequences.
Even within genes, only certain sequences code for amino acids.
These coding sequences are called exons.
What are introns?
Within the gene these exons are separated by further non-coding sequences called introns.
What is DNA like in prokaryotes?
E.g. bacteria, DNA molecules are shorter, form a circle and are not associated with protein molecules.
They therefore do not have chromosomes.
What is DNA like in eukaryotes?
The DNA molecules are longer, linear, and occur in association with proteins called histones to form chromosomes.
The mitochondria and chloroplasts also contain DNA, which like prokaryotic DNA, is short, circular and not associated with proteins.
How are chromosomes seen?
They are only visible as distinct structures when a cell is dividing.
The rest of the time they are widely dispersed throughout the nucleus.
When they first become visible at the start of cell division, they appear as two threads, joined at a single point.
What are chromatids?
Each thread is called a chromatid because DNA has already replicated to give two identical DNA molecules.
The DNA in chromosomes is held by histones.
The considerable length of DNA found in each cell (around 2m) is highly coiled and folded.
What is the structure of chromosomes?
The double helix is wound around histones to fix it into position.
This DNA-histone complex is then coiled, then looped and further coiled before being packed into the chromosome.
A lot of DNA can be condensed into a single chromosome.
It contains a single, very long molecule of DNA, with many genes along its length.
Each gene occupies a specific position along the DNA molecule.
How many chromosomes are there?
The number is always the same for normal individuals of a species, but it varies from one species to another.
Humans have 46 chromosomes, potato plants have 48, and dogs have 78.
In most species there is an even number of chromosomes in the cells of adults.
What are homologous pairs?
Sexually produced organisms are the result of the fusion of a sperm and an egg, each of which contributes one complete set of chromosomes to the offspring.
Therefore one of each pair is derived form the chromosomes provided by the mother and the other from the father.
The total number is referred to as the diploid number - 46.
It is always two chromosomes that carry the same genes, but not necessarily the same alleles of the genes.
What is an example of a homologous pair?
A pair may each possess genes for tongue rolling and blood group, but one chromosome may carry the allele for non-roller and blood group A, while the other carries the allele for roller and blood group B.
What happens during meiosis with chromosomes?
The halving of the number of chromosomes is done in a way which ensures that each daughter cell receives one chromosome from each homologous pair.
In this way, each cell receives one gene for each characteristic of the organism.
When these haploid cells combine, the diploid state, with paired homologous chromosomes, is restored.
What is an allele?
One of a number of alternative forms of a gene.
Each gene exists in two different forms, called an allele.
Each individual inherits one allele from each of its parents.
The two alleles may be the same or different.
When they are different, each allele has a different base sequence, therefore a different amino acid sequence, so produces a different polypeptide.
