Section 4: Genetic information, variation and relationships between organisms Flashcards
Describe what a gene is
a base sequence of Dna that codes for the amino acid sequence of a polypeptide or functional RNA
How many naturally occurring amino acids are there
20
What is the reasoning that three bases code for an amino acid
only 20 different amino acids
each amino acid must have it own code of bases on DNA
only four bases present in DNA
if each base coded from a different amino acid only four amino acids coded for
using a pair of b ashes only codes for 16 (4 squared)
three bases produce 64 (4 cubed) different codes
What are the features of the genetic code
Degenerate code because most amino acids are coded for by more than one triplet
A triplet is always read in one particular direction along the DNA code
The start of the DNA sequence that codes for a polypeptide is always the same triplet
‘Stop codes’ three triplets dont code for any amino acid
The code is non overlapping
The code is universal
Within a gene there are non-coding sequences. What are they called?
Introns
What do DNA molecules associated with to form chromosomes
the protein histones
what holds to chromatid threads together
centromere
What is the locus
the specific position each gene occupies along the DNA molecule
What are homologous pairs of chromosomes
two chromosomes that always carry the same genes but not necessarily the same alleles of the genes
What is an allele
one of a number of alternative forms of a gene
Contrast the DNA of a prokaryotic cell with that of a eukaryotic cell
In prokaryotic cells the DNA is smaller, circular and is not associated with proteins
In eukaryotic cells it is larger, linear and associated with proteins / histones to form chromosomes
State the function of the protein found in chromosomes
It fixes the DNA into position
histones
What is a genome and proteome
genome = complet set of genes in a cell
proteome = full range of proteins produced by the genome
Describe the structure of DNA (3marks)
• DNA is a double helix made of two antiparallel strands.
• It is composed of nucleotides containing a deoxyribose sugar, phosphate group, and nitrogenous base.
• Bases pair by hydrogen bonds: A-T (2 bonds) and C-G (3 bonds).
Explain how a mutation in a gene can result in a non-functional protein. (4 marks)
• A mutation changes the sequence of bases in DNA.
• This alters the codons in mRNA, changing the amino acid sequence.
• It can affect the tertiary structure of the protein.
• This may change the active site or prevent proper folding, making the protein non-functional.
Describe the process of transcription. (4 marks)
• RNA polymerase binds to the DNA at the promoter region.
• Hydrogen bonds between DNA strands break, exposing the template strand.
• Free RNA nucleotides complementary base pair with the template strand.
• RNA polymerase joins nucleotides together to form pre-mRNA.
What happens during translation? (5 marks)
• mRNA binds to a ribosome.
• tRNA with a complementary anticodon binds to the codon on mRNA.
• Each tRNA carries a specific amino acid.
• Peptide bonds form between amino acids, creating a polypeptide.
• Process continues until a stop codon is reached.
What is the difference between pre-mRNA and mRNA? (2 marks)
• Pre-mRNA contains both introns and exons.
• mRNA has introns removed during splicing.
What are the roles of tRNA and mRNA in protein synthesis? (4 marks)
• mRNA carries the genetic code as codons from DNA to the ribosome.
• tRNA has an anticodon that is complementary to the codon on mRNA.
• tRNA carries specific amino acids to the ribosome.
• tRNA helps assemble the amino acids into a polypeptide chain.
Explain the semi-conservative replication of DNA. (4 marks)
• DNA helicase breaks hydrogen bonds, separating the strands.
• Each strand acts as a template.
• Free nucleotides complementary base pair with exposed bases.
• DNA polymerase joins nucleotides, forming two identical DNA molecules.
Explain what is meant by the genetic code being universal and non-overlapping. (2 marks)
• Universal: The same codons code for the same amino acids in almost all organisms. (1 mark)
• Non-overlapping: Each base in the sequence is read only once, in triplets. (1 mark)
Explain what is meant by the term degenerate code. (2 marks)
• Most amino acids are coded by more than one codon. (1 mark)
• For example, both UUU and UUC code for phenylalanine. (1 mark)
What is the role of RNA polymerase during transcription? (2 marks)
RNA polymerase joins RNA nucleotides together to form pre-mRNA. (1 mark)
It also separates the DNA strands by breaking hydrogen bonds. (1 mark)
How do promoters and terminators regulate transcription? (3 marks)
• Promoters are specific DNA sequences where RNA polymerase binds to initiate transcription. (1 mark)
• Terminators signal the end of transcription, causing RNA polymerase to detach. (1 mark)
• This ensures the correct gene is transcribed. (1 mark)
Describe how post-translational modifications can produce a functional protein. (3 marks)
• The polypeptide chain folds into a specific 3D structure (e.g., secondary and tertiary structures). (1 mark)
• Disulfide bridges, hydrogen bonds, and ionic bonds stabilize the structure. (1 mark)
• Functional groups or molecules, such as carbohydrates, may be added (e.g., glycoproteins). (1 mark)
A mutation changes one base in a promoter region. Explain how this could affect protein synthesis. (3 marks)
• A mutation in the promoter may prevent RNA polymerase from binding. (1 mark)
• Transcription of the gene will not occur. (1 mark)
• This means no mRNA is produced, so the protein cannot be synthesized. (1 mark)
The genetic code is described as “degenerate.” Explain why this reduces the impact of point mutations. (2 marks)
• Some mutations do not change the amino acid because multiple codons code for the same amino acid. (1 mark)
• This reduces the likelihood of altering the protein structure. (1 mark)
Name the organelle involves in translation
ribosome
What are the 3 steps of protein synthesis
Transcription
Splicing
Translation
What is the structure of a tRNA molecule
anticodon at bottom
amino acid binding site at top
single-stranded clover leafed shape
Where does translation take place
on ribosomes in the cell cytoplasm
What are the 2 types of gene mutations that occur spontaneously during DNA replication
base substitution and base deletion
Chromosome mutations can arise spontaneously and take two forms:
Polyploidy = changes in whole sets of chromosomes occur when organism have three of more sets of chromosomes rather than 2
Non-disjunction = Changes in the number of individual chromosomes when individuals fail to separate during meiosis . one more or one fewer chromosome
Explain why a mutation that is transcribed onto mRNA may not result in any change to the polypeptide that it codes for
Genetic code is degenerate substitution codon may still code for same amino acid so polypeptide is unchanged
Explain why errors in DNA replication can be far more damaging that errors in transcription
These errors may be inherited and may therefore have a permeanent affect on the whole organism. Errors in transcription usually affect only specific cells, are temporary and are not inherited. They are therefore less damaging
State two differences between mitosis and meiosis
mitosis produces two daughter cells with the same number of chromosomes as the the parent cell and as each other
Meiosis usually produces four daughter cells, each with half the number of chromosomes as the parent cell
What is haploid and diploid
diploid cell has two complete sets of chromosomes 46
haploid cell has one full set of chromosomes 23
In what ways can meiosis bring out genetic variation
independent segregation of homologous chromosomes
new combination of maternal and paternal alleles by crossing over
Describe independent segregation of homologous chromosomes
During meiosis 1 each chromosome lines up alongside its homologous partner
They arrange themselves randomly - random assortment
Its a matter of chance what paternal and maternal chromosomal origin go into what daughter cell
Describe how meiosis produces genetic variation (4marks)
• Crossing over between homologous chromosomes in prophase I exchanges alleles. (1 mark)
• Independent segregation of homologous chromosomes during metaphase I creates different combinations of maternal and paternal chromosomes. (1 mark)
• Random fertilization of gametes further increases variation. (1 mark)
• Mutations during DNA replication can introduce new alleles. (1 mark)
Explain the importance of meiosis in the production of gametes. (3 marks)
• Meiosis halves the chromosome number to produce haploid cells. (1 mark)
• This ensures the chromosome number is restored during fertilization. (1 mark)
• It introduces genetic variation through crossing over and independent segregation. (1 mark)
What is meant by independent segregation, and how does it increase genetic diversity? (3 marks)
• Independent segregation is the random arrangement and separation of homologous chromosomes during meiosis I. (1 mark)
• Different combinations of maternal and paternal chromosomes are distributed into gametes. (1 mark)
• This increases genetic diversity by creating gametes with unique allele combinations. (1 mark)
A diploid cell in an organism has 12 chromosomes. How many chromosomes are present in:
(a) A gamete produced by meiosis?
(a) 6 chromosomes (haploid).
Describe the process of crossing over and explain its significance. (3 marks)
• Homologous chromosomes pair up during prophase I. (1 mark)
• Non-sister chromatids exchange sections of DNA at points called chiasmata. (1 mark)
• This creates new combinations of alleles on chromatids, increasing genetic variation. (1 mark)
Explain the role of the spindle fibers during meiosis. (3 marks)
• Spindle fibers attach to centromeres of homologous chromosomes in meiosis I. (1 mark)
• They pull homologous chromosomes to opposite poles of the cell. (1 mark)
• In meiosis II, they separate sister chromatids, ensuring haploid gametes are formed. (1 mark)
Compare the products of meiosis and mitosis. (4 marks)
• Meiosis produces four genetically varied haploid cells, while mitosis produces two genetically identical diploid cells. (1 mark)
• Meiosis involves two divisions, while mitosis involves one. (1 mark)
• Meiosis introduces genetic variation, while mitosis is for growth and repair. (1 mark)
• Meiosis occurs in gametes, while mitosis occurs in somatic cells. (1 mark)
Define genetic diversity. (1 mark)
The variety of alleles in a population.
Explain how a mutation can lead to the production of a non-functional protein. (3 marks)
• A mutation changes the base sequence of DNA.
• This alters the codons in mRNA, leading to a different amino acid sequence.
• The resulting protein may fold incorrectly, changing its shape or active site.
Explain how natural selection leads to adaptation. (4 marks)
• Within a population, variation exists due to mutations.
• Some individuals have advantageous alleles that increase survival and reproduction.
• These alleles are passed on to the next generation.
• Over time, the frequency of these alleles increases, leading to adaptation.
Explain the difference between directional and stabilizing selection. (4 marks)
• Directional selection favors individuals with alleles for one extreme phenotype.
• This shifts the mean phenotype over time (e.g., antibiotic resistance).
• Stabilizing selection favors individuals with alleles for the average phenotype.
• This reduces variation and maintains the mean phenotype (e.g., birth weight in humans).
How does migration increase genetic diversity in a population? (2 marks)
• Migration introduces new alleles into a population.
• This increases the variety of alleles available.
A population of insects becomes resistant to a pesticide. Explain how this resistance could evolve. (4 marks)
• Random mutations produce alleles for pesticide resistance.
• Insects with this allele survive and reproduce, while others die.
• The resistance allele is passed to the next generation.
• Over time, the frequency of the resistance allele increases.
Explain how genetic diversity is maintained in a population despite selection pressures. (3marks)
- Some alleles may be advantageous under different conditions
- Heterozygote advantage ( sickle cell carriers resistant to malaria)
- Gene flow from other populations reintroduces alleles
What are the three types of adaptions affecting natural selection
Anatomical
Physiological
Behavioural
Define species
They are capable of breeding to produce living, fertile offspring
What is the binomial system
The first name is the generic name and is the genus
The second name is the specific name and is the species
written in italic generic name capitalised and if species not known you write sp
Why is specific courtship behaviour so important
So they can recognise members of their own species
identify a mate capable of breeding
form a pair bond
synchronise mating
become able to breed
What are the two main forms of biolgical classification and what is the difference
Artificial classification - based on characteristics that don’t necessarily have the same evolutionary origins
phylogenetic classification - based on evolutionary relationships
What is each group within a phylogenetic biological classification called
Tax on
What are the four kingdoms in the taxonomic rank
Protocista, Fungi, Plantae and Animalia
List the taxonomic rank
Kingdom
phylum
class
order
family
genus
species
Suggest a way inn which courtship behaviour of one species might be used to determine which of two other species is most closely related to it
The courtship behaviour that most closely resembles that of the first one is most likely to be the closest relative
The eveloutionary are line of descent is known as________
phylogeny
Define species richness
The number of different species in a particular area at a given time (community)
What is meant by species diversity
The number of different species and the number of individuals of each species within any one community
What practices have removed habitats and reduced species diversity
Removal of hedgerows
creating monocultures
filling in ponds / draining marsh and other wetland
over-grazing of land
use of pesticide
leaching
absence of crop rotation
What are conservation techniques farmers can implement
Maintain existing hedgerows
Plant hedges over fences
Leave wet corners of fields rather than draining them
plant native trees on land with low species diversity rather than high
Reduce use of pesticides
use organisms over inorganic fertilisers
Crop rotation that includes a nitrogen-fixing crop
intercropping
natural meadows
let seeds disperse naturally
How can you compare genetic diversity within, and between, species
Observable characteristics
Base sequence of DNA
base sequence of mRNA
amino acid sequence of proteins
How can you prevent sampling bias when carrying out random sampling
Divide study area into grid numbered lines
Using random numbers obtain a series of coordinates
Take samples of each pair of coordinates
What is the mode
The most frequent value
What is the median
The central or middle value
List two reasons why a sample may not be representative of the population as a whole
chance variation
sampling bias
Give three structural differences between mRNA and tRNA
mRNA is straight chain and tRNA is clover-leaf shape
mRNA is codon and tRNA is anti-codon
mRNA has no amino acid binding site tRNA has an amino acid binding site
mRNA has no hydrogen bonds tRNA has hydrogen bonds
