Biology - Genetics, Variation and relationships between organisms.

Genome:

Complete set of genes in a cell.

Proteome

Complete set of proteins .

What is a gene?

A gene is a short section of DNA that contains a code for making a polypeptide and functional RNA. The code is a specific sequence of bases. So genes determine the proteins of organisms.

What is a locus?

The location of a particular gene on a chromosome is called the locus.

What is an allele?

An Allele is one of a number of alternative forms of a gene most genes occur in two occasionally more different form.

Describe Chromosomes.

In a eukaryotic cell nucleus, DNA is stored as chromosomes. Humans have 23 pairs of chromosomes, 46 in total.

What is a homologous pair of chromosomes?

When chromosomes are exactly the same size, have exactly the same genes but might have different alleles. Pairs of matching chromosomes are called homologous pairs.

How is DNA stored in a eukaryote?

• DNA is stored as chromosome inside of the nucleus.

• Chromosomes in eukaryotic cells are linear in shape.

• To tightly coil the DNA to fit in the nucleus as chromosomes the DNA is tightly wound around proteins called histones.

• This complex on DNA wrapped around a histone is called a nucleosome.

How is DNA stored in a prokaryotic cell?

Prokaryotes also carry DNA in chromosomes, but the DNA molecules are short and circular. The DNA is *not* wound around histones. Instead it supercoils to fit in the cell.

Describe how DNA is found in chloroplasts and mitochondria?

The DNA is similar to prokaryotic DNA in that it is short, circular and is not histone bound.

What is RNA?

RNA is a polymer of a nucleotide formed of a ribose, a nitrogenous base and a phosphate group.

What are the nitrogenous bases in RNA?

Adenine, Guanine, Cytosine and Uracil. RNA has the base uracil instead of thymine,

Describe the structure of RNA?

RNA is a relatively short polynucleotide chain and it is single stranded.

What is the function of RNA?

The function of RNA is to copy and transfer the genetic code from DNA in the nucleus to the ribosomes. Some RNA is also combined with proteins to create ribosomes.

What are three types of RNA?

mRNA - messenger RNA

tRNA - transfer RNA

rRNA - ribosmal RNA

What is the structure and function of mRNA?

mRNA is a copy of a gene from DNA.

it is created in the nucleus and it then leaves the nucleus to carry the copy of the genetic code of one gene to a ribosome in the cytoplasm.

What is the structure of tRNA?

• a single polynucleotide strand with a clover shape

• has an amino acid attached to it and an anticodon that’s complementary to an mRNA codon.

What are the properties of the genetic code?

1. It’s universal - the code is the same for all organisms.

2. It’s degenerate - more codons than amino acid so some codons encode the same amino acid e.g CAA and CAG both encode glutamine.

3. Non- overlapping - each base is only part of one codon so each codon is read as a discrete unit.

What are the coding and non coding parts of DNA called?

Coding - exons

Non-coding introns

What are the 3 overall steps in protein synthesis.

1. Transcription = DNA to pre-mRNA

2. Splicing = pre-mRNA to mRNA

3. Translation = mRNA to protein (using tRNA)

Stage 1: Transcription

1. DNA helicase ‘unwinds’ the two DNA strands by breaking hydrogen bonds between complementary base pairs. Only 1 DNA strand acts as the template.

2. Free mRNA nucleotides align and pair with their complementary base.

3. RNA polymerase joins the mRNA nucleotides to form pre-mRNA.

Stage 2: Splicing

1. Introns (non-coding) are removed

2. Exons are attached back together via a condensation reaction.

3. mRNA is formed which then exits the nucleus through the nuclear pores in the nucleus.

   * this does not occur in prokaryotes as they do not have introns, mRNA production is direct

Stage 3: Translation

1. Ribosome attached to the start codon of the mRNA (AUG)

2. tRNA with complementary anticodon aligns with mRNA and brings an amino acid.

3. Ribosomes moves along mRNA, allowing another tRNA to attach to the next codon.

4. The 2 amino acids attached to the tRNA molecules are joined by peptide bonds.

5. This continues until the ribosome reaches a stop codon, where the ribosome detaches and translation ceases.

6. Polypeptide chain has now been created.

Gene mutation:

• A change in the base sequence of DNA

• Can occur during replication

Substitution mutations:

• single substitution of one base for another

• meaning a change in the amino sequence

• change in the primary thus tertiary structure, possibly affecting protein structure and function

Why might some substitution mutations not affect the amino acid sequence at all?

The degenerate nature of the genetic code means that different codons can code for the same amino acid and so will not affect anything. \*single amino acid change is more likely to have less of an effect on protein structure and therefore function.

Deletion mutations:

• deletion of a base in a DNA sequence

• causing a frameshift

• has a larger impact on amino acid sequence

• example is the muscle-wasring disorder Duchenne Muscular Dystrophy

What are the two types of mutations that can occur in chromosomes?

1. Polyploidy - changes can occur in the whole set of chromosomes so that an individual has three of more sets of chromosomes instead of two. This is common in plants, with many modern wheats arising to be polyploidy.
2. Non-disjunction - non-disjunction occurs when chromosomes fail to separate correctly in meiosis. As a result the gametes and any zygotes formed will have one more or one less chromosome than they should.

meiosis:

• formation of cell division that produces 4 daughter cells that are genetically different from each other.

• involves two separate divisions

• haploid (n) cells are formed

• produces gametes

Why does meiosis produce genetically different daughter cells?

* This is due to the processes of independent assortment and crossing over.

What happens in independent assortment?

there are various combinations of chromosome arrangement. During meiosis 1 homologous chromosomes line up in pairs, the arrangement of these pairs is random, meaning that the division into the daughter cells is also random. •

What happens in crossining over?

* T he process whereby a chromatid breaks during meiosis and rejoins to the chromatid of its homologous chromosome so that its alleles are exchange.

Meiosis:

Meiosis I = homologous chromosomes separate

Meiosis II = sister chromatids separate

Genetic Diversity:

Is the total number of different alleles in the population.

Population:

A group of individuals of the same species that live in the same place and are able to breed with one another. The greater the number of alleles in a population the greater the genetic diversity.

Natural selection:

The process in which fitter individuals who are better adapted to the environment survive and pass on the advantageous genes to future generations.

Evolution:

The process by which the frequency of alleles in a gene pool changes over time as a result of natural selection.

Evolution via natural selection:

• There’s a variety of different phenotypes within a population

• An environmental change occurs and as a result of that the selection pressure changes.

• Some individuals posses advantageous alleles which give them a selective advantage allowing them to survive and reproduce.

• The advantageous alleles are passed onto their offspring

• Over time, the frequency of alleles in a population changes and this leads to evolution.

Selection pressures:

Environmental factors that affect the chance of survival of an organism.

What are the types of selection:

1. Directional selection

2. Stabilising selection

Directional selection:

directional selection occurs when the environmental conditions change and the phenotypes best suited to the new conditions are more likely to survive. As a result these individuals will breed and produce offspring. Overtime the mean of the population will move in the direction of these individuals. An example of this is bacteria being resistant to antibiotics. A single bacteria will have had a mutation that meant it was not killed by penicillin as it could produce the enzyme penicillinase. As a result it was able to grow and populate, and the frequency of the allele that enabled penicillinase production increased in the population. Therefore the population moved to have greater penicillin resistance.

Stabilising selection:

In Stabilising selection the phenotypes with successful characteristics are preserved and those of greater diversity are reduced. This selection doesn’t occur due to changes in the environment. If the environment stays the same then the individuals closest to the mean are favored because they have the alleles that have given them the survival advantage. The furthest from the mean are selected against. A example is new born babies weights. Those that have a birth weight of around 3kg are more likely to survive than those at the extremes.

What is the niche of a species?

It’s role within the environment. Species which share the same niche compete with each other and a better adapted species survive. The idea that better adapted species survive is the basis of natural selection.

What are the three types of adaptations?

1. Anatomical - Physical adaptations either internal or external e.g length of the loop of henle - in dessert mammals this is very long to produce concentrated urine and minimize water loss.

2. Behavioral adaptations are changes in behavior which improve the organism’s chance of survival e.g. mating calls.

3. Physiological adaptations are processes inside an organism’s body that increase its chance of survival e.g. regulation of blood flow through the skin

Species:

A group of similar organisms that are able to reproduce to produce fertile offspring.

How do we name species?

Using the binomial naming system. Every species is given a name with two parts, based on Latin and Greek.

• The generic name is the genus to which the organism belongs. If a species is closely related it will share the same name

• The specific name is the species to which the organism belongs. When hand writing these the names must be underlined and the first letter of the generic name must be capitalised.

Hierachial system:

A system in which larger groups contain smaller groups with no overlap between groups.

What is the order of the hierarchal system.

Domain

Kingdom

Phylum

Class

Order

Family

Genus

Species

Biodiversity:

The total amount of variation in species that exist in a specific region.

* can relate to a range of habitats.

Species richness:

A measure of the number of different species in a community.

Index of diversity:

Ecological parameter that measures the number and variation of species in a habit at any time.

* Describes the relationship between the number of species in a community and the number of individuals in each species.

Index of diversity formula:

d = N(N-1) divided by the sum of n(n-1)

Species abundance:

Number of each individual in a population.

Species evenness:

How close in numbers each species in an environment is

Courtship:

In order for members of the same species to recognise one another during mating courtship behaviour is often used. Members of the same species share similar physical, biochemical and behavioural characteristics. Courtship behaviour takes place before mating and enables individuals to:

• recognise members of their own species

• Synchronise mating

• forming a pair bond

• become able to breed