A3.1 Diversity of organisms

Define organism

Any biological system that functions as a life form, all organisms are composed of cells.

Define population

A group of organisms of the same species in the same area

Define community

Populations of two or more species occupying the same area at the same time

Define variation

Differences between members of a group

List sources of genetic variation within a species

• Mutations in the sequences of genes in the DNA

• Movement of genes between different populations

• Meiosis processes

• Random fertilisation

Compare discrete and continuous variation

• Discrete variation: Traits that can be put into discrete qualitative categories influenced by one or a few genes or sometimes by the environment

• Continuous variation: Traits that vary along a quantitative column influenced by the environment and complex interactions between multiple genes

Compare intraspecific and interspecific variation

• Intraspecific variation: Genetic variation within a species is inheritable

• Interspecific variation: Differences in traits between species, and the amount of variation depends on how closely related they are.

Define species according to the morphological species concept

Classification of organisms based on their morphology and shared characteristics.

Define binomial nomenclature

A universal language for naming species by their genus and species

State four rules of binomial nomenclature formatting

• Genus begins with a capital letter

• Species in lowercase

• Underline if handwritten

• After one use in text, genus can be abbreviated to the first letter

Outline why the binomial naming system is used in science rather than local names

• Reflects evolutionary relationships because all members of the same genus will share a common ancestor

• Consistency in communication around the world

Define species according to the biological species concept

A species is a group of organisms that can interbreed to produce a fertile offspring

Describe limitations of the biological species concept, with mention of hybrids and geographical separation

• Natural and artificial hybrids

• Does not reflect the variation within a species across geographical ranges

• Inapplicable to asexually reproducing species

• Ethical constraints of interfering with animal behaviour for scientific studies

• Cannot test for extinct populations

• Chronospecies are different stages in the same evolving lineage and its unclear where one species begins and ends.

• The BSC ignores divergence during speciation as a long, gradual process where genes exchange

Define speciation

If two populations of the same species do not interbreed, traits may start to differ and diverge to the point where they become different species

Explain the difficulties in distinguishing between populations and species during speciation

• Genetic divergence is gradual, and it can be difficult to assess if two populations are considered different species.

• When two populations that have been diverging come in contact again, hybrids could be produced

• Even after speciation, related species can be hard to distinguish from each other.

State the relationship between chromosome number and a species

Chromosome number is a distinguishing characteristic of a species and can be used to deduce evolutionary relationships

State the fundamental genetic requirement for members of a species to successfully interbreed

They must have compatible genetic profiles

Outline the structural similarities required between the chromosomes of two parents for successful reproduction.

The genetic information must be organized into the same number of chromosomes, with comparable sizes and identical gene loci positions

Explain why organisms with different diploid numbers are usually unable to interbreed

Because their chromosomes have different numbers, they cannot form homologous pairs during meiosis

Explain why the typical number of chromosomes in a diploid cell is an even number

Diploid organism inherit two complete sets of chromosomes, one from each parent.

State the number of chromosomes in humans and in chimpanzees

Humans have 46 chromosomes while chimpanzees have 48

Evaluate the evidence for the hypothesis that chromosome 2 in humans arose from the fusion of chromosomes 12 and 13 with a shared primate ancestor

• The combined length of chromosomes 12 and 13 in chimpanzees matches the length of human chromosome 2

• The centromere location of human chromosome 2 matches that of chimpanzee chromosome 12

• The banding of the arms of human chromosome 2 matches that of the chimpanzee chromosomes 12 and 13

• Human chromosome 2 contains telomeric DNA in a position where fusion of chromosomes 12 and 13 would have occurred

• Human chromosome 2 also contains non-coding DNA in the position that would correspond to the centromere location of a fused chromosome 13

Define karyotyping

The process of pairing and ordering the complete set of chromosomes within a cell to provide a snapshot of an organism’s genetic profile

State the term used for the visual representation of an organism's photographed chromosomes

Karyogram

Outline how cells are prepared for karyotyping to ensure chromosomes are visible

Chemical inhibitors are used to isolate the cell during division while the chromosomes are condensed

Explain how the timing of cell arrest affects the appearance of chromosomes in a karyogram

It will determine whether chromosomes appear with sister chromatids or not.

List the characteristics by which chromosomes are paired and arranged on the karyogram

• They are arranged in homologous pairs

• They are ordered from largest to smallest

• They are organised by their centromere positions

• Chromosomes are stained to generate banding patterns for further identification

List the three key pieces of genetic information that can be determined using a karyogram

• Type of species by chromosome number

• Females are XX and Males are XY

• Any chromosomal abnormalities

Define autosome and sex chromosome

An autosome is any non-sex chromosome, and a sex chromosome determines the sex.

Define genome

The complete set of genetic material in an organism

Define gene

A segment of DNA that holds the instructions to create a functional product

Define Allele

A different version of the same gene, one from each parent.

Define Single-Nucleotide Polymorphism (SNP)

A mutation to a single nucleotide base that is present in a large proportion of the population

State the main cause of SNPs

DNA replication errors during cell division can lead to that mutation becoming permanent in that lineage of cells

State the effect of SNPs

SNPs create a unique genetic profile for all individuals, causing overall diversity.

Describe reasons why a larger genome does not necessarily indicate the presence of more genes

The genome is the total genetic material present, including non-coding DNA and non-nuclear sequences.

Compare variation in genome sizes and gene sequences within and between species

• Genome size and organisation vary between species in terms of total amount of DNA and chromosomes

• Gene sequences vary between and within species, but variation between species is greater

State the units for measuring genome size

Base Pairs (bp) / Kilobases (kb) / Megabases (Mb) / Gigabases (Gb)

Describe the variation in genome size between different groups of organisms

Viruses and bacteria tend to have very small genomes, while most eukaryotic organisms have larger genomes

Explain why plant genome sizes can vary dramatically

• Plants can self-fertilise, resulting in the exact genome being copied into the offspring.

• Plants can become polyploid, meaning they possess more than two complete sets of chromosomes

Explain why genome size is not a valid indicator of genetic complexity

Genome size varies greatly between organisms and does not directly reflect their complexity or size

What is genome sequencing

The process of determining the DNA sequence of an organism's entire genome from a cell

State two developments that have increased the use of whole-genome sequencing.

• Increasing speed of sequencing

• Decreasing costs of sequencing

Describe one current use of whole genome sequencing in research

Investigating evolutionary relationships by comparing DNA sequences between organisms to determine how closely related they are.

Explain one potential future use of whole genome sequencing in medicine

Personalised medicine uses an individual's genetic information to help select treatments suited to their genetic profile and helps predict proneness to diseases

Outline how whole genome sequencing is important for public health

t can be used to track diseases and protect populations

Define asexual reproduction

When a parent passes on its full set of genes to the offspring, resulting in a genetically identical offspring.

Discuss difficulties in applying the biological species concept to asexually reproducing species like bacteria

Bacteria reproduce asexually, so the biological species concept cannot be applied to them because they do not interbreed.

Define horizontal gene transfer

Movement of DNA between individuals through specialised mechanisms

Discuss difficulties in applying the biological species concept to bacteria that have horizontal gene transfer.

Bacteria can exchange plasmids, so genetic information can be shared without interbreeding, so the biological species concept does not apply to them

Explain how species of bacteria can be classified

• Morphology

• Metabolic requirements

• Structures

State a chromosome requirement of the biological species concept

Members of a species generally have the same chromosome number, allowing compatible chromosomes to pair during meiosis to produce a fertile offspring

Explain how differences in chromosome number can prevent successful reproduction between organisms.

Organisms with different chromosome numbers would not be able to produce compatible gametes and hence would be unable to generate a viable or fertile offspring.

Describe hybridisation

Interbreeding of closely related species, such as a horse and a donkey, to produce an infertile offspring like a mule due to the differences between parental chromosomes which interfere with meiosis.

Explain the use of a dichotomous key in the identification of a specimen

A dichotomous key is a method of identification where groups of organisms are divided into two categories repeatedly

Define DNA barcode

DNA barcodes are short, specific DNA sequences used to identify species by comparing them with reference sequences in databases

Define environmental DNA (eDNA)

Environmental DNA is genetic material released by organisms into their surroundings

Explain how DNA barcoding of eDNA can be used to investigate biodiversity in a habitat

Environmental DNA is extracted, amplified, and sequenced which is then analysed and compared with reference DNA barcodes in databases to identify species present

Explain why using DNA barcodes and eDNA allows biodiversity to be investigated rapidly

• DNA from multiple species can be detected in a sample

• Less disturbance to organisms

• It targets short, standardised DNA fragments

List the features of a DNA barcode

• 300-600 bp in length

• Unique identification sequence for each species

• A conserved sequence across multiple species