3.4 Genetic Information, Variation And Relationships Between Orgainisms Flashcards
What is the size order of the components of the genetic code?
Gene, DNA, chromosome, nucleus
What is DNA?
Deoxyribonucleic acid, polymer made up of nucleotide monomers
What is prokaryotic DNA like?
DNA molecules are short, circular and not associated with proteins
What is eukaryotic DNA like?
Found in the nucleus, DNA molecules are very long, linear and associated with proteins called histones. together a DNA molecule and it’s associated proteins form a chromosome
What is a loci?
A gene occupies a fixed position, called a locus, on a particular DNA molecule
What is a chromosome?
DNA that is coiled up very tightly, 23 pairs, consist of maternal and paternal chromosomes, cells with 24 pairs are called diploids
What is a gene?
A short section of DNA that codes for one polypeptide
What is an allele?
A different version of a gene
What is meant by homologous?
Same structural features and pattern of genes but not identical (contain different alleles)
What can the genetic code be described as?
Non overlapping, a triplet code, universal
What is meant by the genetic code being a triplet code?
One triplet codes for one amino acid, three nucleotide bases= one amino acid
What is meant by the genetic code being universal?
Each triplet code always coded for the same amino acid, it is the same in all living organisms
What is meant by the genetic code being non overlapping?
Each base is discrete (123,456 not 123,234,345)
What is meant by degeneracy?
64 possible triplet codes and only 20 different amino acids in proteins, some amino acids coded for by more than one triplet (e.g tyrosine=TAT or TAC), act as punctuation marks, indicate start and stop (start codon=ATG)
What is protein synthesis?
Transcription (formation of pre mRNA in the nucleus) and translation (formation of polypeptides at the ribosomes)
What is the difference between mRNA and DNA?
mRNA has the bases GCAU whereas DNA has GCAT, mRNA has ribose DNA has deoxyribose, mRNA is single stranded, DNA is double helix, DNA has the base pairing G+C and A+T the but mRNA has no base pairing (can predict the percentage of bases in DNA but not in mRNA), mRNA is shorter than DNA
Describe the process of transcription:
Firstly DNA helicase breaks the hydrogen bonds between the bases in the DNA double helix, free RNA nucleotides complementary base pair with the template strand forming weak hydrogen bonds, the RNA polymerase will resynthesise the sugar phosphate back bone, it must be RNA polymerase because the DNA polymerase is not specific to the ribose sugar, the weak hydrogen bonds between RNA bases and the template strand break resulting in pre mRNA, the template strand re forms hydrogen bond with the other DNA strand, pre mRNA is then splices to remove non coding DNA (introns) and joins together coding DNA (exons) so mRNA only contains coding DNA and it is small enough to leave the nucleus via nuclear pores
What is translation?
The process by which mRNA is used to make a specific protein
Describe the process of translation:
mRNA leaves the nucleus through nuclear pores, mRNA associated with the ribosome fitting between the two sub units of the ribosome, one codon of the mRNA is read by a tRNA molecule, the tRNA brings an amino acid to the codon and attaches to it by a complimentary anticodon, the ribosome moves along the mRNA strand and another complementary tRNA molecule attaches to the next codon, two amino acids at the ribosome join by a peptide bond during a condensation reaction, the first tRNA molecule can move back into the cytoplasm to pick up a new amino acid, this process continues until a stop codon is reached (no amino acid or complementary anticodon available), energy released from ATP hydrolysis is used to form the polypeptide chain and fold it into its specific 3D shape
Why is splicing not necessary in prokaryotes?
mRNA is produces directly from the DNA, there are no introns in prokaryotic DNA
What is a mutation?
Any change in one or more nucleotide base or a change in the sequence of the bases in DNA, they are random, spontaneous, natural and either positive or negative
What are the three types of mutation?
Insertion, deletion and substitution
What is meant by insertion?
An extra base is added, affects the amount whole strand, every triplet code changes as there is a frameshift
What is meant by deletion?
Remove a base, affects the whole strand, a nucleotide is lost from the DNA sequence, causes a frame shift
What is substitution?
Only affects one triplet code as one base is replaced, has the least effect as only only one amino acid is affected or none at all if it is degenerate
What is meant by frameshift?
Every amino acid after the insertion or deletion will move one place
What is a mutagen?
A physical or chemical agent that changes the genetic material of an organism
What are examples of mutagenic agents?
Caffeine, X-rays, mustard gas, UV radiation
What is chromosomal mutation?
Changes in the structure or number of whole chromosomes e.g. polyploidy and non disjunction
What is polyploidy?
Changes in the structure or number of the whole chromosome, cells have multiple sets of chromosomes, 3n=triploid which is common in plants
What is non disjunction?
Homologous pairs fail to separate, down syndrome = extra chromosome 21
What is meiosis?
The process by which a diploid nucleus (2n) divides to produce four haploid daughter nuclei (n)
Describe what happens in meiosis:
After interphase is he genetic material is replicated so there is two chromatids per chromosome , in prophase 1 genetic material condenses and the nuclear membrane breaks down, in metaphase 1 chromosomes associated in homologous pairs line up on the equator attached to spindle fibres at the centromere, in anaphase 1 spindle fibres contract and homologous chromosomes are pulled to opposite poles, in telophase 1 and cytokinesis the nuclear envelope reforms, cytoplasm splits and the cell membrane forms. The same process occurs in meiosis two (the second division) the same process occurs except this time the cells are only 2n and spindle fibres attach at the centromere to pull apart sister chromatids and haploid cells are formed (n).
What are the two forms of genetic variation?
Crossing over (the exchange of alleles) and independent segregation (random arrangement of chromosomes)
What is meant by genotype?
The genetic make up of an organism, the genes and alleles they have
What is meant by phenotype?
The observable characteristics, genotype and the environment affect this
What are the three causes of genetic variation?
Mutations, sexual reproduction, meiosis
What happens in crossing over (meiosis)?
Homologous pairs of chromosomes line up, chromatids of each pair become twisted, section of chromatid breaks off and rejoins chromatid of the other homologous chromosome, sections of chromatids exchanged, sections have different alleles, new combinations of linked alleles cause variation
What is independent segregation?
Gregor Mendel in the 19th century said genes are inherited independently of one another BUT genes close together have a high likelihood of being inherited together, this occurs during anaphase
What happens in independent segregation?
Homologous pairs of chromosomes line up along the equator completely randomly, therefore the combination of chromosomes pulled to each pole is random, meaning daughter cells produced are genetically different as the genes are the same but alleles are different. Chromatids are then separated in meiosis 2
How do you predict how likely genetic variation is? (formula)
(2^n)^2
Why can’t you predict the genetic variation from crossing over?
Because it is random
Why can you predict the variation from independent segregation and sexual reproduction?
Either chromatid making up a homologous pair could be pulled into one or two piles (independent segregation 2^n) and gamete come together during sexual reproduction ((2^n)^2)
What is meant by genetic variation?
The total number of different alleles in a population, occurs due to mutations, there must first be a change in the DNA (a random mutation) resulting in a difference
Explain how a mutation results in a change in characteristic (4 marks)
First a mutation occurs which is a random change in the DNA, this caused a change in the triplet code and therefore the codons in mRNA, this means that different anticodon on tRNA will make a different sequence of amino acids and therefore a different polypeptide. Changes the secondary, tertiary and quaternary structure, changes the protein formed/its function. This results in increased genetic diversity
Why is genetic diversity important?
Within a species all genes are the same but they still have genetic diversity, total number of genetic characteristics differs because there are different alleles present, high genetic diversity = large number of different alleles, low genetic diversity = small number of different alleles
What are the positives of high genetic diversity?
Less spread of disease, more complex food web, more likely to survive in different conditions e.g. adapting for climate change prevents extinction and increases stability, can survive in different habitats prevents overcrowding and competition
What are negatives of high genetic diversity?
Less have the desirable characteristics i.e food for humans and selective breeding, fewer organisms adapted to an environment at a specific time or place
What is meant by phenotype?
Observable characteristics that an organism has I.e. blue eyes or brown eyes affected by genetics and environment
What is meant by genotype?
Genetic make up of an organism (eye colour, beak size)
What is meant by natural selection
Not all alleles are equally likely to be passed to the next generation, the better adapted to the environment tend to survive and breed at the expense of those less well adapted, changing the allele frequency
What is meant by allele frequency?
The number of times an allele occurs within a gene pool
What is meant by gene pool?
The total number of alleles in a particular population at a specific time
Explain how natural selection occurs
Random mutation of alleles within the gene pool may result in a new allele of a gene which could code for a characteristic which is advantageous over other phenotypes in the environment. These individuals will be better adapted and there’s more likely to survive in their competition with others. These individuals therefore survive and breed at the expense of individuals without this mutation. The offspring are more likely to inherit the new allele which is advantageous and they in turn are more likely to survive and so reproduce successfully. Over generations the number of individuals with the new advantageous allele will increase at the expense of individuals without this, over time the frequency of the new advantageous allele in the population will increase while that of the non advantageous ones decreases
How does a change in the environment affect selection?
The change in the environment does not cause the mutation it just makes it more advantageous
What is a normal distribution curve?
No bias, data surrounds central point (mean,median and mode are all the same/central), curve is symmetrical, curve is bell shaped
Why is there still variation (50% above mean and 50% below mean) in a normal distribution curve?
Most characteristics are a result of multiple genes (polygenes), each gene is affected by the environment, changes in the environment = changes in the mean
What happens on directional selection?
Mutation occurs, change in environment occurs, selection favours one direction from the mean, the mean characteristic of the population changes, one extreme is selected for, allele frequency changes
How is antibiotic resistance in bacteria a result of directional selection?
A random mutation occurs in the gene pool resulting in a different or new allele coding for antibiotic resistance as a different protein/enzyme is produced to break down the antibiotic. The introduction of antibiotics into the environment means that this mutated characteristic has become advantageous meaning that these bacteria have a higher chance of survival and reproduction at the expense of bacteria without this mutation. This means that offspring are more likely to inherit this characteristic, increasing the allele frequency, moving the mean, median and mode toward the extreme of having high antibiotic resistance
What is stabilising selection?
The mean median and mode have not changed, but there is a higher frequency of the mean, median and mode and a lower frequency at the extremes
Why does stabilising selection occur?
The environment is stable so it is more advantageous for the population to mostly have the characteristics suited to that environment, selection acts against both extremes in a range of phenotypes, mean is favoured, phenotypic variation is reduced, environment has remained stable