3.4 Genetic information, variation and relationships between organisms

Question 1

What are the features of DNA molecules in prokaryotic cells

Answer 1

Short, circular and not associated with proteins

Question 2

What are the features of DNA molecules in eukaryotic cells?

Answer 2

Long, linear, found in the nucleus and associated with proteins called histones

DNA molecule and its histones form a chromosome

Question 3

What eukaryotic cell organelles apart from the nucleus contain DNA and how does this differ from the DNA in the nucleus?

Answer 3

Mitochondria/chloroplasts

It’s similar to the DNA of prokaryotes: short, circular and not associated with proteins

Question 4

A gene is a base sequence that codes for:

Answer 4

• the amino acid sequence of a polypeptide
• a functional RNA (including rRNA and tRNAs).

Question 5

Define locus

Define proteome

Answer 5

The fixed position that a gene occupies on a particular DNA molecule

Complete set of proteins that a cell can code for

Question 6

What is a triplet?

Answer 6

A sequence of 3 DNA bases that code for a specific amino acid

Question 7

Explain how the genetic code is universal, non-overlapping and degenerate

Answer 7

Universal because:
code is the same in almost all organisms
acts as evidence for evolution.

Non-overlapping because:
the first three bases are read as one codon, then the next three as the second etc
therefore each base is read only once and the bases do not overlap.

Degenerate because:
more than one triplet can code for a particular amino acid.

Question 8

Define genetic diversity
Benefit of it?
What can lower it?

Answer 8

number of different alleles of genes in a population
factor that allows natural selection to occur

Benefit:
-species able to adapt with changes in the environment
-e.g. if a new disease arises, some individuals will have characteristics to survive, and will reproduce passing on their alleles, so the species does not become extinct

Can be lowered by:
-small population size
-e.g. founder effect (the numbers start low)
-or genetic bottleneck (where the numbers decrease)

Question 9

Define species richness

Answer 9

Number of species in a community

Question 10

Define species diversity
What is the benefit of it?
How is it measured?
How does deforestation lower species diversity?
How does agriculture/farming lower species diversity?

Answer 10

-number of different species
-number of individuals for each species

-Stable ecosystem
-each species is less likely to become extinct (due to high genetic diversity)
-if a species does become extinct it will not affect the food chain as there are other species available

-Species Diversity Index
-takes into account the number of different species and how many individuals there are for each species
-the larger the species diversity index, the larger the species diversity

Deforestation:
-decreases plant species diversity
-less variety of habitats
-less variety of food sources
-decreases animal species diversity

Farming:
-deforestation to make space
-only a few plant and animal species kept
-selectively breed plants & animals
-use pesticides to kill other species

Question 11

Define ecosystem diversity

Answer 11

Variety of ecosystems

Question 12

Define taxonomy

Answer 12

the science of identifying, describing, and classifying organisms based on their characteristics and evolutionary relationships.

Question 13

Outline the principles of natural selection in the evolution of populations

Answer 13

-population of species has variation
-random mutation can result in new alleles of a gene
-many mutations are harmful but, in certain environments, the new allele of a gene might benefit its possessor
-leads to increased reproductive success
-people with the mutation are more likely to survive and reproduce
-advantageous allele is inherited by members of the next generation
-over many generations, the new allele increases in frequency in the population

Question 14

Outline the difference between directional and stabilising selection using an example

Answer 14

Directional selection:
-occurs when environmental conditions change, so phenotypes best suited to the new conditions are more likely to survive
-hence these individuals will breed and produce offspring
-overtime the mean population will move in the direction of these individuals
-e.g. bacteria being resistant to antibiotics
-a single bacteria will have had a mutation to prevent being killed by penicillin, e.g. producing the enzyme penicillinase
-allows it to grow and populate
-frequency of the allele that enabled penicillinase production increased alongside the population
-hence population moved to have greater penicillin resistance

Stabilising selection:
-phenotypes with successful characteristics are preserved
-phenotypes with greater diversity are reduced
-this selection doesn’t occur due to changes in environment
-if the environment stays the same then the individuals closest to the mean are favoured because they have the alleles that have given them the survival advantage
-the furthest from the mean are selected against
-e.g. new born babies weights
-those that have a birth weight of around 3kg are more likely to survive than those at the extremes

Question 15

Define biodiversity

Answer 15

Variety of habitats and species in an ecosystem

Question 16

What is classification and the two types?
What is the naming system used?

Answer 16

-places organisms into groups

Hierarchical/Linnean classification:
-large groups divided into smaller groups with no overlap
-domain, kingdom, phylum, class, order, family, genus, species

Phylogenetic classification:
-based on evolutionary relationships – how closely related different species are and how recent a common ancestor they have

Binomial naming system:
-using Genus name and Species name to name organism
-Genus name first in capital, Species name second in lower case
-e.g. tiger = Felix tigris

Question 17

Why are the offspring from 2 different species mating infertile?

Answer 17

-offspring will have a odd number of chromosomes
-therefore, cannot perform meiosis, cannot produce gametes
-example: horse + donkey = mule,

Question 18

Outline 3 ways of comparing relationship between different species
Why is one method better?

Answer 18

1) DNA Hybridisation: comparing DNA base sequence
-take DNA from 2 species to be compared
-radioactively label one of the DNA
-heat both sets so double strand separates
-cool so single strands join together
-look for Hybrid DNA (one strand from species A, one strand from species B)
-identify Hybrid DNA by 50% radioactivity
-heat Hybrid DNA to measure similarity

results = higher temperature required, as:
more hydrogen bonds present
more complementary base pairing
more similar the base sequence
more similar the species
more closely related
more recent a common ancestor

2) Amino Acid Sequence: comparing AA sequence for the same protein (e.g. haemoglobin in mammals)
results = more similar the AA sequence
more similar the DNA base sequence
more similar the species
more closely related
more recent a common ancestor

(comparing DNA sequence better then comparing AA sequence:
DNA sequence provides information on INTRONS and triplet code is DEGENERATE)

Protein Shape: comparing shape of the same protein (e.g. albumin) using immunological technique
- comparing species A and species B
- take albumin from species A
- place in a blood of rabbit
- rabbit will make antibodies against albumin of species A
- takes these antibodies and place in blood from species B
- if the albumin in species B has a similar shape to species A, the antibodies will bind to form antigen-antibody complexes, which will then form a precipitate

results = more precipitate
more complexes
more similar shape
more similar the species
more closely related
more common recent ancestor

Question 19

Define variation
Types of variation and causes

Answer 19

difference in characteristics between organisms

intraspecific = differences between organisms of the same species
Causes: genetic factors (same genes but different alleles) and environmental factors

interspecific = differences between organisms of different species
Causes: genetic factors (different genes and different alleles) and environmental factors

Question 20

What are the two types of characteristics and their properties

Answer 20

Discontinuous:
-characteristics fall into certain groups with no overlap (e.g. blood group) – determined by genetics only (a single gene)

Continuous:
-characteristics show a range (e.g. height) – determined by genetics (a few genes, polygenes) and environment

Question 21

What is a gene and how does it code for a protein?

Answer 21

-a section of DNA that codes for a protein
-made out of intron and exon
-intron = non-coding DNA (function e.g. turns gene on or off)
-exon = coding DNA (codes for protein)

-made out of a sequence of bases
-each 3 bases code for 1 amino acid (called triplet code)
-therefore sequence of bases
-determines sequence of triplet codes
-which determine the sequence of AAs
-hence the polypeptide chain/primary structure (folds to secondary, then to tertiary/quaternary)

Question 22

How does a mutation lead to a non-functional enzyme?

Answer 22

-change in base sequence
-change in sequence of triplet codes
-change in sequence of AAs
-change in primary structure
-change in hydrogen/ionic/disulfide bonds
-change in tertiary structure (3D shape)
-change in active site shape
-substrate no longer complementary
-can no longer form enzyme-substrate complex

Question 23

What is mRNA?

Answer 23

-messenger RNA
-single stranded complementary copy of a gene
-carries the code for assembling protein (on DNA called triplet code, on mRNA called codon)

Question 24

What is tRNA?

Answer 24

-transfer RNA
single stranded RNA folded over into a ‘clover leaf’ shape (held by hydrogen bonds between the bases)
-has an AA attachment site on the top
-has 3 specific bases on the bottom (anticodon)
-anticodon binds to complementary codons on mRNA

Question 25

Outline protein synthesis

Answer 25

Transcription: -occurs in nucleolus of nucleus -produces single stranded complementary copy of a gene (mRNA) from template strand of DNA Process: -DNA helicase hydrolyses hydrogen bonds between complementary bases in the gene -gene unwinds and leaves 2 separate strands (1 coding strand and 1 template strand) -only 1 strand acts as a template -complementary RNA nucleotides bind to exposed bases on the template strand -uracil is used instead of thymine -RNA polymerase joins the sugar-phosphate backbone of the RNA strand by forming phosphodiester bonds between nucleotides -forms pre-mRNA (contains introns and exons) -the copies of the introns are removed by splicing -mRNA leaves via nuclear pore Translation: -takes place on ribosomes -uses sequence of codons on mRNA to assemble the protein (tRNA brings in AAs) Process: -mRNA attaches to a ribosome -complementary tRNA carrying specific AAs bind to the codons on mRNA via their anticodon -the AAs on the tRNA are joined by peptide bonds -stop codon reached, so no more AAs -primary structure polypeptide chain formed, which can fold into secondary/tertiary

Question 26

What does meiosis produce? Benefits of meiosis? Stages? How is variation produced by meiosis?

Answer 26

-produces 4 genetically different cells, haploid (half the amount of chromosome/DNA) **Benefits:** -produces gametes which will be used in sexual reproduction in animals & plants -(2 gametes fuse to form a zygote, zygote develops into organisms) **Interphase:** -G1: protein synthesis -S: DNA replication (doubles set of DNA) -G2: organelle synthesis **Meiosis I:** Prophase I: DNA coils to form chromosomes, nucleus breaksdown, spindle fibres form, crossing over occurs Metaphase I: homologous pair of chromosomes line up in middle of cell and attach to spindle fibre via centromere Anaphase I: spindle fibres pull, homologous pair of chromosomes separate to opposite sides by independent assortment Telophase I: chromosomes uncoil, nucleus reforms (left with 2 nuclei) **Meiosis II:** Prophase II: DNA coils to form chromosomes, nucleus breaksdown, spindle fibres form Metaphase II: chromosomes line up in middle of cell and attach to spindle fibre via centromere Anaphase II: spindle fibres pull, centromere splits, sister chromatids move to opposite sides by independent assortment Telophase II: chromatids uncoil, nucleus reforms (left with 4 genetically different nuclei) **Cytokinesis:** separating cell into 4 (each receives a nucleus and organelles/cytoplasm) **Crossing over:** -occurs in Prophase I of Meiosis I -homologous pairs of chromosomes wrap around each other and swap equivalent sections of chromatids -produces new combination of alleles **Independent assortment:** -in Anaphase I of Meiosis I – the homologous pairs of chromosomes separate -in Anaphase II of Meiosis II – the chromatids separate -independent assortment produces a mix of alleles from paternal and maternal

Question 27

What happens to DNA mass in meiosis? What happens to chromosome number in meiosis?

Answer 27

Quarters Halves

Question 28

What is mutation and what causes it? Outline two types of mutations

Answer 28

-Change in DNA -causes: random or due to mutagens (e.g. radiation, chemicals) Chromosome mutation: -polyploidy in plants = inherit more than one diploid set of chromosomes -non-disjunction in animals = homologous pair of chromosomes don’t separate in meiosis, so inherit one extra or one less chromosome Gene mutation: -a change in the base sequence of DNA -2 types = substitution and insertion/deletion -substitution = replace one base for another, changes one triplet code, can be: -silent (new triplet code codes for same AA) -mis-sense (codes for a different AA, so protein shape changes slightly), -non-sense (codes for a stop codon, so polypeptide chain not produced) -insertion = adding a base, deletion = removing a base -both insertion/deletion causes frameshift, all the triplet codes after the mutation changes, so normal polypeptide chain/protein not produced

Question 29

If two organisms can produce fertile offspring, they….

Answer 29

belong to the same species

Question 30

Outline courtship behaviour and its importance

Answer 30

-behaviour that leads to successful mating -enables organisms to identify members of their own species -courtship behaviour leads to the formation of a pair bond -which results in better reproductive success, due to increased survival probability of offspring

Question 31

Order of Linnaean classification system

Answer 31

Domain Kingdom Phylum Class Order Family Genus Species

Question 32

difference between stabilising and directional selection

Answer 32

Stabilizing selection results in a decrease of a population 's genetic variance when natural selection favors an average phenotype and selects against extreme variations. In directional selection, a population's genetic variance shifts toward a new phenotype when exposed to environmental changes