DNA, genes and chromosomes Flashcards
what is a gene
diagram of gene in notes
-section of DNA that contains a code for making a polypeptide and functional RNA
-the code is a specific sequence of bases
-polypeptides make proteins and so genes determine the proteins of an organism.
what is the locus
the location of a particular gene on a chromosome.
what is an allele
diagram in notes
-one of a number of alternatives forms of a gene
-most alleles occur in two(alleic pair)
-occasionally in more different forms (multiple alleles)
chromosomes
diagram in notes
- in a eukaryotic cell nucleus DNA is stored as chromosomes
-Humans have 23 pairs of chromosomes and 46 in total.
-contains a centromere and chromatids.
homologous chromosomes
- pairs of matching chromosomes
for example the two copies of chromosome one are homologous pairs. - exactly the same size
-exactly the same genes ( at the same loci - position)
-they might have different alleles so not identical.
how many pairs of Homologous chromosomes do humans have
23
DNA storage in eukaryotic cells
- stored as chromosomes inside the nucleus,
-chromosomes are linear in shape
-the DNA is tightly coiled and wrapped around proteins called histones to create the chromosome.
Start codon
start codon- at the start of every gene there are three bases which help to initiate translation
stop codon
stop codon- at the end of every gene there are three bases that do not code for an amino acid and this is a stop codon. A stop codon marks the end of a polypeptide chain and causes ribosomes to detach and therefore stop translation.
The genetic code
triplet code
an amino acid is coded for by three DNA bases which are described at the ‘triplet code’
on mRNA a sequence of three bases, that code for one amino acid are codons.
Three key features of the genetic code
- degenerate
2.universal
3.non- overlapping
degenerate
-multiple triplet codes can code for the same amino acid
- as there are four bases and a sequence of 3 bases codes for one amino acid this works out as a total of 64 possible triplet combinations of triplet codes to code for the 20 amino acids.
-this is far more than the 20 different amino acids that exist and as a result each amino acid is coded for by more than one triplet of bases
- this is what is meant by the genetic code being degenerate . e.g tyrosine is coded for by ATA and ATG.
why is the code being degenerate an advantage?
-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.
universal
the same triplet of bases codes for the same amino acid in all organisms
why is the genetic code being described as universal an advantage
means that genetic engineering is possible
e.g inserting the human gene for insulin into bacteria
overlapping
-each base in a gene is only part of one triplet of bases that codes for one amino acid
-therefore each codon (triplet of bases) is read as a discrete unit
why is overlapping an advantage
if a point of mutation occurs it will only effect one codon and therefore one amino acid.
what are introns
-sections of DNA that do not code for amino acids and therefore polypeptide chains.
Found in eukaryotic DNA but not in prokaryotic DNA
-these get removed (spliced) out of mRNA molecules.
what are exons.
sections of DNA that do code for amino acids.
genome
is the complete set of genes in a cell
the whole genetic makeup.
-widely varies
e.g bacteria contain on average 600000 DNA base pairs whereas humans contain 3 billion DNA base pairs - lots more of non-coding DNA.
proteome
the full range of proteins that a cell is able to produce.
mRNA (messenger RNA)
-copy of a gene from DNA,
-created in the nucleus
-leave the nucleus to carry the copy of the genetic code of one gene to a ribosome in the cytoplasm.
-DNA is too large to leave the nucleus and would be at risk of being damaged by enzymes
therefore permanently destroying the genetic code
-mRNA is much shorted because it is only the length of one gene
-therefore it can then leave the nucleus as it is small enough to fit through the nuclear pores.
mRNA is single stranded
every three bases in the sequence code for a specific amino acid
these three bases are called codons.
tRNA (transfer)
-found only in the cytoplasm
-single stranded but folded to create a shape that looks like a cloverleaf
-this shape is held in place by hydrogen bonds
the function of tRNA is to attach to one of the 20 amino acids and transfer this amino acid to the ribosome to create a polypeptide chain
specific amino acids attach to specific tRNA molecules
-this is determined by the three bases found on the tRNA molecule which are complentary to the three bases on mRNA
These are called anticodons because they are complmentary to the codon on mRNA.
protein synthesis.
-proteins created on ribosomes
-the production of proteins from the DNA code occurs in two main stages :
translation
(splicing)
transcription
Transcription
where one gene on the DNA is copied into mRNA.
translation
where the mRNA joins with a ribosome, and corresponding tRNA molecules bring the specific amino acid that the codon codes for.
transcription process
-first stage of protein synthesis
-a complementary mRNA copy of one gene on the DNA is created in the nucleus.
-mRNA is much shorter than DNA so it is able to fit through the nuclear pores in the nuclear membrane and carry the genetic code to the ribosome in the cytoplasm to enable the protein to be made
1.the DNA helix unwinds to expose the bases to act as a template
2.only one chain of the DNA acts as a template
3.Like with DNA replication this unwinding and unzipping is catalysed by DNA helicase.DNA helicase breaks the hydrogen bonds between bases.
4.Free mRNA nucleotides in the nucleus align opposite to the exposed complementary DNA bases
5. the enzyme RNA polymerase bonds together, a phosphodiester bond in a condensation reaction, the RNA nucleotides to create a new RNA polymer chain.
6.one entire gene is copied
once copied the mRNA is modified and then leaves the nucleus through the nuclear envelope pores.
translation process
the second stage in which the polypeptide chain is created using both the mRNA base sequence and the tRNA.
1.once spliced mRNA has left the nucleus, it attaches to a ribosome in the cytoplasm
2. the ribosome attaches at the start codon- 3 bases
3. the tRNA with the complementary anticodon to the start codon aligns opposite the mRNA, which is held in place by the ribosome.
4.The ribosome will move along the mRNA molecule to enable another complementary tRNA molecule to attach to the next codon on the mRNA.
5. The two amino acids that have been delivered by the tRNA molecule are joined by a peptide bond. This is catalysed by an enzyme and requires ATP.
6.This continues until the ribosome reaches the stop codon at the end of the mRNA molecule.The stop codon does not code for for an amino acid and therefore, the ribosome detaches and translation ends.
the polypeptide chain is now created and will never enter the golgi body for folding and modification.
in what ways can genetic variation be introduced
-meiosis
-mutations
-random fertilisation of gametes.
meiosis what is it
the type of cell division that creates genetically different gametes.
-unlike mitosis there are two nuclear divisions in this process which results in four haploid daughter cells
(haploid (n) = one copy of each chromosome)
(diploid(2n) = two copies of each chromosome.
what genetic differences are introduced by the two key process in meiosis
-independent segregation of homologous chromosomes
-crossing over between homologous chromosomes.
Crossing over
- occurs in meiosis 1
-when homologous pairs line up oposite each other at the equator in meiosis 1, and the parts of the chromatids can become twisted around each other
-this puts tension on the chromatids causing pairs of the chromatid to break.
-the broken parts of the chromatids recombine ( genetic recombination) with another chromatid. This results in a new combinations of alleles in the gametes.
Independent segregation
-in meiosis 1, homologous pairs of chromosomes line up opposite each other at the equator of the cell
-it is random on which side of the equator the paternal and maternal chromosomes from each homologous pair lie
-these pairs are separated so one of each homologous pair ends up in the daughter cell
-this creates large numbers of possible combinations of chromosomes in the daughter cells produced
This can be calculated at 2(n.o of hom chromosomes) squared.
there are approximately 8,388,608 combinations of chromosomes in humans
gene mutation
- a mutation in a gene is a change in the base sequence of the DNA. Gene mutations randomly occur during DNA replication
-These random mutations are more likely to occur if you are exposed to mutagenic agents( interfere with DNA replication - this increases the likelihood but is not the cause.)
These include high energy radiation ionising radiation (gamma rays and X rays) and chemicals ( e.g cigarette smoke)
what could a gene mutation lead to?
a base being deleted or substituted for a different one.
deletion - deleted a base
insertion - added a base
substitution - changed a base.
what is a frameshift
when the whole DNA shifts
a ‘silent’ base substitution
the new codon still codes for the same amino acid
this is because the genetic code is degenerate leading to lower effect.
what does degenerate mean
multiple codons can code for the same aino acid
What does base deletions lead to
a frameshift meaning that the removal of one base changes all of the subsequent codons which is more harmful as multiple amino acids may be incorrectly coded for
and a complete different primary structure is formed.
chromosome mutations
mutations in the number of chromosomes can arise spontaneously by chromosome non-disjunction during meiosis
this can occur in two forms:
1.polyploidy -changes in the whole sets of chromosomes
2. aneuploidy - changes in the number of individual chromosomes.
what is non-disjunction
when the chromosomes or chromatids do not split equally during anaphase.
polyploidy
-changes in whole sets of chromosomes occur when organisms have three or more sets of chromosomes rather than the usual two
-mainly occurs in plants
Aneuploidy
-this is a change in the number of individual chromosomes
-sometimes individual homologous pairs of chromosomes fail to separate during meiosis.
-this is non disjunction and usually results in a gamete having one more or one fewer chromosome.
- on fertilisation with a gamete that has the normal number of chromosomes , the resultant zygote will have fewer chromosomes than normal in their body cells, e.g downsyndrome is caused by 3 copies of chromosomes 21.
Genetic diversity
-The number of different alleles in a population
-the genetic diversity population is what enables natural selection to occur.
what is a gene pool
-all the genes and alleles in a population at a particular time
what is allele frequency
The proportion of of organisms within the population carrying a particular allele
what is natural selection
the process that leads to evolution in populations.
-evolution is the change in the allele frequency over many generations in a population
what does natural selection result in
species becoming better adapted to their environment
For example antibiotic resistant to bacteria.
Whats the three groups that adaptions can be
-anatomical- spikes on a hedgehog
-physiological - hedgehogs hibernating and slowing their metabolic reactions in winter
-behavioral-hedgehogs curl up when in danger.
Describe the process of natural selection
A 5 mark question.
- Random mutations occur within the population
2.This introduces genetic variation to the population - provide an organism with an advantageous trait to survive in its environment
- the new allele provides a reproductive selective advantage. Therefore the individual with the allele is more likely to reproduce and pass on the advantageous allele to their offspring.
- Over many generations there will be an increase in the frequency of this allele within the population
This change in the allele frequency in a population is evolution which results in species that are better adapted to their environment.
Two key types of selection
- directional selection
2.stabilising selection
an example of selection pressure
the type of food available determines which alleles are successful
what is stabalising selection
-when the middle (median) trait has a selective advantage and therefore continues to be the most frequent in the population
-This impact can be represented by a normal distribution graph
-The range decreases as the the extreme traits are lost over time
whats a common example of stabalising selection
-human birth weight
-Middling birth weight is a selective advantage as extremely low birth weights may be disadvantageous to survival due to the underdevelopment of the baby.
-Therefore babies born with a middling rate are more likely to survive and pass on the advantageous allele to their offspring later in life.
Modal Trait within stabalising selection
-has a selective advantage
-occurs when there is no change in the environment
-The modal trait remains the same
-standard deviation decreases as the number of individuals with the extreme trait decreases.
Directional selection
-when one extreme trait has a selective advantage
-occurs when there is a change in the environment
-the modal trait changes
An example of directional selection
-antibiotic resistance to bacteria
-a random mutation creates an allele that provides resistance to antibiotics in bacteria.
-if this population of bacteria are exposed to this antibiotic, only those with the resistance allele survive and all others die.
-This allele is then passed on over many generations and results in most of the bacteria carrying this allele
mitosis 1
interphase 1
DNA replicated
Prophase 1
chromosomes condense and become visible
chromosomes appear as chromatids joined at the centromere
Metaphase 1
chromosomes line up along the equator
attach to spindle fibers by their centromere
Anaphase 1
the centromere splits
chromatids / homologous chromosomes are pulled to opposite poles
telophase 1 and cytokinesis
chromatids and chromosomes uncoil
meiosis 2
prophase 2
two diploid daughter cells
metaphase 2
chromosomes line up along the equator and attach to spindle fibers
anaphase 2
sister chromatid are pulled apart to opposite poles
Telophase 2 and cytokinesis
nuclear envelope reforms production of 4 haploid cells.
crossing over from diagram in booklet
-two chromatid per chromosome joined at the centromere
-crossing over
-chromatid wrap around each other
-break off
-different combinations of alleles
(two chromatids from each chromosome cross over)
Resulting in variation.
What does a stabilizing selection graph look like
- normal curve
- an extreme trait has decreased in frequency and has been selected against
the graph will be the normal cure and then a tall and more pointed curve on top of the old curve
what does a directional graph look like
-shift toward one of the extreme values
-increased frequency of extreme trait
other extreme has been selected against- frequency of trait has decreased.
what enzyme is required to break apart the double helix
DNA helicase
what enzyme is required to re-synthesize the backbone in mRNA
RNA polymerase
what term is used to describe the structure of a protein made of two or more polypeptides
quaternary structure
identify one event that occurs during division 2 of mitosis but not
during division 1
separation of chromatid
what two structures make up the sugar phosphate backbone on pre-mRNA
-ribose sugar
-phosphate
