Genetic Information, Variation and Relationships Between Organisms

Describe DNA in eukaryotic cells

Nucleus contains long, linear DNA molecules with associated proteins as chromosomes, DNA is wound around histones (proteins) for strength and so it can fit, coiled very tightly to make compact chromosomes

Describe DNA in mitochondria and chloroplasts

Have their own DNA, circular and shorter and not associated with proteins

Describe DNA in prokaryotic cells

Carry DNA as chromosomes, shorter, circular, not associated with proteins, condenses to fit in cell by supercoiling

What is a gene?

A base sequence of DNA that codes for:
The amino acid sequence of a polypeptide
A functional RNA

How many bases make up a triplet?

3

How many different amino acids there are?

20

How many combinations of amino acids are there?

64

Name and describe 3 features of the genetic code

Universal - same bases for all species
Non-overlapping - read in triplets
Degenerate - different triplet codes for the same amino acid

What is functional RNA and what does it do?

RNA molecules other than mRNA (e.g. tRNA and rRNA) which perform special tasks during protein synthesis

What is non-coding DNA?

Introns - removed by splicing
Exons - do code for amino acids

Define alleles

Different versions of the same gene

What are homologous chromosomes?

Pairs of matching chromosomes, same size and same genes, but could have different alleles

What is the locus?

The fixed position of a gene on each chromosome in a homologous pair

What is the genome?

Complete set of genes in a cell

What is the proteome?

Full range of proteins a cell is able to produce

Describe RNA

Single polynucleotide strand, contains uracil instead of thymine

What is the role of messenger RNA (mRNA)?

Made during transcription, carries genetic code from DNA to ribosomes, where it is used to make a protein during translation, single polynucleotide strand, groups of 3 adjacent bases = codons

What is the role of transfer RNA (tRNA)?

Involved in translation, carries amino acids used to make proteins to ribosomes, single polynucleotide folded, H-bonds hold it in shape, have specific anticodon and amino acid binding site

Where does transcription occur in eukaryotes?

Nucleus

Where does transcription occur in prokaryotes?

Ribosomes in cytoplasm, directly produces mRNA from DNA (no splicing occurs)

Describe the process of transcription

1. RNA polymerase attaches to DNA at beginning of gene, breaks H-bonds, separating strands

2. RNA polymerase lines up free nucleotides, complementary mRNA formed, T is replaced with U, RNA polymerase joins together free bases, forming an mRNA strand

3. RNA polymerase moves down DNA, mRNA assembles, H-bonds reform

4. RNA polymerase reaches stop signal and detaches

5. In eukaryotes, splicing occurs, introns are removed

Where does translation take place in prokaryotes and eukaryotes?

Ribosomes in cytoplasm

Describe the process of translation

1. mRNA attaches to ribosome and tRNA molecules carry amino acids to it, ATP provides energy for bond between amino acid and tRNA to form

2. tRNA (carrying amino acid) with complementary anticodon attaches to mRNA, a second tRNA does the same

3. The 2 amino acids join by a peptide bond

4. Process continues as ribosome moves along mRNA, produces polypeptide chain, until it reaches a stop signal

5. Polypeptide chain is complete

Normal body cells have a ________ number

Diploid (2n)

Gametes have a _________ number

Haploid (n)

What is the human haploid and diploid number?

Diploid = 46
Haploid = 23

What happens during sexual reproduction?

2 gametes joining together at fertilisation to form a zygote, which divides and develops into a new organism

What happens during fertilisation?

Haploid egg fuses with haploid sperm, to make a cell with normal diploid number of chromosomes (1/2 from father, 1/2 from mother)

Describe the process of meiosis

1. Before meiosis starts, DNA unravels and replicates, so there are 2 copies chromosome = sister chromatids

2. DNA condenses to form double-armed chromosomes, each made from 2 sister chromatids joined by centromeres

3. First division - chromosomes arrange themselves in homologous pairs then homologous pairs separated, halving the chromosome number

4. Second division - Pair of sister chromatids are separated (centromere divides)

5. 4 haploid cells that are genetically different from each other are produced

Where does meiosis occur?

In reproductive organs of multicellular, eukaryotic organisms

What is a mutation?

Change in DNA base sequence of chromosomes

What is base addition?

One or more bases added, changes whole code

What is base deletion?

One or more bases deleted, changes whole code

What is base substitution?

One base is substituted for another, only 1 triplet will be changed, sometimes changes degenerate code

What do mutagenic agents do?

Increase probability of a mutation occurring

Name 4 examples of mutagenic agents

UV radiation, ionising radiation, some chemicals, some viruses

What do chromosome mutations lead to?

When meiosis goes wrong, cells produced contain variations in numbers of whole or parts of chromosomes, leads to inherited conditions

What happens during non-disjuction in meiosis 1?

Homologous pairs fail to separate

What happens during non-disjunction in meiosis 2?

Zygote haas 3 copies of a chromosome

Describe crossing over of chromatids

1. During meiosis 1, homologous chromosomes pair up

2. Chromatids twist and entangle (form chiasma) and swap over same length of genes

3. New combinations of alleles formed

Describe independent segregation of chromosomes

When homologous pairs are separated in meiosis 1, it is completely random which chromosome from each pair ends up in which daughter cell, so the 4 daughter cells have different combinations of alleles, leads to genetic variation

What is variation?

Difference between same species organisms

Define genetic diversity

Total number of different alleles of genes in a population

Define population

Total number of organisms in a habitat

Define allele frequency

Frequency of specific alleles in a population

Why is genetic diversity important?

If a population has low genetic diversity, it might not be able to adapt to a change in the environment and the whole population could be wiped out by a single event

How is genetic diversity within a population increased?

Mutations in DNA forming new alleles, gene flow (different alleles being introduced into a population when individuals from another population migrate into it and reproduce)

What are genetic bottlenecks?

An event that causes a big reduction in a population, reduces number of different alleles in a gene pool which reduces genetic diversity, the survivors reproduce and a larger population is created from a few individuals

What is the founder effect?

When a few organisms from a population start a new colony and there are only a small number of different alleles in the initial gene pool, genetic diversity is reduced

Why is natural selection good?

It increases proportions of advantageous alleles within a population

Describe the process of natural selection

1. Presence of advantageous alleles
2. Survival of the fittest
3. More likely to survive, reproduce and pass on genes to offspring
4. Frequency of allele increases
5. Leads to evolution

What is evolution?

The gradual change in species over time, leads to diversity of living organisms on earth

What is directional selection?

Where individuals with alleles for characteristics of an extreme type are more likely to survive and reproduce, this could be in response to an environmental change

What is stabilising selection?

Where individuals with alleles for characteristics towards the middle of the range are more likely to survive and reproduce, occurs when environment isn't changing and it reduces the range of possible characteristics

What is phylogeny?

The study of evolutionary history of groups of organisms

What do phylogenic trees show? Draw an example

Closely related species diverged away from each other more recently

What is taxonomy?

The science of classification, naming organisms and putting them into groups

What is speciation?

The production of a new species

What does phylogenetic classification show?

Organises species on how closely they are related

Name the groups of classification

Domain
Kingdom
Phylum
Class
Order
Family
Genus
Species

As you go down the groups of classification, what happens?

More groups, but fewer organisms in each group, organisms in each group become more closely related

Why are classification systems constantly being updated?

Discoveries of new species and new evidence about known organisms (e.g. DNA sequencing data)

What taxons does the binomial naming system use? And why is this system used?

Genus and species, universal

How do you write out a binomial name properly?

First - genus, second - species, in italics or underlined, 1st letter - uppercase, 1st letter of second word - lowercase

What is courtship behaviour?

Behaviour carried out by organisms to attract a mate of the same species

Name 2 simple courtship behaviours

Releasing a chemical using sound, visual displays

Name 2 complex courtship behaviours

Dancing, building

Who carries out courtship behaviours?

Male or female, may sometimes involve both sexes

How can courtship behaviour be used to classify species?

Courtship behaviour is species specific, the more closely related a species is, the more similar their courtship behaviour

Define biodiversity

Number of different ecosystems and communities of organisms, number of different species and degree of genetic diversity within those species

Define habitat

Place where an organism lives

Define community

All the different species that live together in a habitat

What is species richness?

A measure of the number of different species in a community

What is the index of diversity? And how is it calculated?

Relationship between number of species in a community and number of individuals in a species

How is agriculture threatening biodiversity?

Woodland clearance, hedgerow removal, pesticides, herbicides, monoculture

How can diversity be investigated?

1. Frequency of measurable or observable characteristics

2. Base sequence of DNA / immunology

3. Base sequence of mRNA

4. Amino acid sequence of proteins encoded by DNA and mRNA