AQA a level biology topic 4

What is a gene?

A gene is a base sequence of DNA that codes for:

the amino acid sequence of a polypeptide

a functional RNA (including ribosomal RNA and tRNAs).

What is a locus?

Location of a gene on a chromosome

What is a allele?

Different forms of a gene

what is a homologous pair of chromosomes?

a pair of matching chromosomes

same size

same gene

different alleles

how is DNA stored in eukaryotic cells?

1) Eukaryotic cells contain linear DNA molecules that exist as chromosomes

Chromosomes are found in the nucleus.

2)The DNA molecule is really long so, it has to be wound up so it can fit into the nucleus.

how is DNA stored in eukaryotic cells?

3)The DNA molecule is wound around proteins called histones.

4)Histone proteins also help to support the DNA. The DNA (and protein) is then coiled up very tightly to make a com p act chromosome.

how is DNA stored in prokaryotic cells?

1) Prokaryotes also carry DNA as chromosomes — but the DNA molecules are shorter and circular.

2) The DNA isn't wound around histones — it condenses to fit in the cell by supercoiling.

how is mitochondria and chloroplasts DNA stored in cells?

The mitochondria and chloroplasts of eukaryotic cells also contain DNA which, like the DNA of prokaryotes, is short, circular and not associated with protein.

how is the Genetic Code Non-Overlapping, Degenerate and Universal

1) The genetic code is the sequence of base triplets (codons) in mRNA which code for specific amino acids.

2) In the genetic code, each base triplet is read in sequence, separate from the triplet before it and after it. Base triplets don't share their bases — the co de is non-overlapping.

how is the Genetic Code Non-Overlapping, Degenerate and Universal

3) The genetic code is also degenerate — there are more possible combinations of triplets than there are amino acids. This means that some amino acids are coded for by more than one base triplet

how is the Genetic Code Non-Overlapping, Degenerate and Universal

4) Some triplets are used to tell the cell when to start and stop production of the protein — these are called start and stop signals (or codons). They're found at the beginning and end of the mRNA.

how is the Genetic Code Non-Overlapping, Degenerate and Universal

5) The genetic code is also universal — the same specific base triplets code for the same amino acids in all living things.

What are introns?

sections of DNA that don't code for amino acids

found in eukaryotic DNA

get removed, spliced, out of mRNA molecules

What are exons?

Sections of DNA that code for amino acids

what is a genome?

The complete set of genes in a cell

What is a proteome?

The full range of proteins that a cell is able to produce

DNA contain genes

1) A gene is a sequence of DNA bases that codes for either a polypeptide or functional RNA.

2) The sequence of amino acids in a polypeptide forms the primary structure of a protein.

DNA contains genes

3)Different polypeptides have a different number and order of amino acids. It's the order of bases in a gene that determines the order of amino acids in a particular polypeptide.

4) Each amino acid is coded for by a sequence of three bases in a gene called a triplet.

DNA contains genes

5) To make a polypeptide, DNA is first copied into messenger RN A (mRN A). This is the first stage of protein synthesis.

DNA contains genes

6) Genes that don't code for a polypeptide code for functional RNA instead. Functional RNA is RNA molecules other than mRNA, which perform special tasks during protein synthesis, e.g. tRNA and ribosomal RNA (rRN A), which forms part of ribosomes.

Most DNA in Eukaryotic Cells Doesn't Code for Polypeptides

1) Some genes don't code for polypeptides at all — they code for functional RNA.

2)In eukaryotic DNA , genes that do code for polypeptides contain sections that don't code for amino acids.

3)These sections of DNA are called introns. There can be several introns within a gene.

Most DNA in Eukaryotic Cells Doesn't Code for Polypeptides

4)All the bits or a gene that do code for amino acids are called exons.

5) Introns are removed during protein synthesis — so they don't affect the amino acid order. (Prokaryotic DNA doesn't have introns.)

Most DNA in Eukaryotic Cells Doesn't Code for Polypeptides

6)Eukaryotic D N A also contains regions of multiple repeats outside of genes.

7) These are DNA sequences that repeat over and over

8)These areas don't code for amino acids either, so they're called non-coding repeats.

what is mRNA?

mRNA is made during transcription. It carries the genetic code from the DNA to the ribosomes, where it's used to make a protein during translation. mRNA is a single Base polynucleotide strand. In mRN A, groups of three adjacent bases are usually called codons

what is tRNA?

tRNA is involved in translation. It carries the amino acids that are used to make proteins to the ribosomes. tRNA is a single polynucleotide strand that's folded into a clover shape.

what is tRNA?

Hydrogen bonds between specific base pairs hold the molecule in this shape. Every tRNA molecule has a specific sequence of three bases at one end called an anticodon. They also have an amino acid binding site at the other end

What is transcription?

1)The DNA helix unwinds to expose the bases to act as templates

2) only one chain of DNA acts as a template

What is transcription?

3) unwinding is catalysed by DNA Helicase

4) DNA Helicase breaks the hydrogen bond between bases

What is transcription?

5) free mRNA nucleotides in the nucleus align opposite exposed complementary DNA bases

What is transcription?

6) RNA polymerase bonds together RNA nucleotides to create a new polymer chain

pre-mRNA is spliced to remove introns

mRNA moves out of the nucleus through a nuclear pore and attaches to a ribosome in the cytoplasm, where the next stage of protein synthesis takes place

What is translation?

1)mRNA attaches to a ribosome in the cytoplasm

2) the ribosome attaches at the start codon

What is translation?

3) the tRNA molecules with the complementary anticodon to start codon aligns opposite to the mRNA

4) the ribosome will move along the mRNA molecule to enable another complementary tRNA to attach to the next codon on the mRNA

What is translation?

5) the two amino acids that have been delivered by the tRNA molecules are joined by a peptide bond which is catalysed by an enzyme and requires ATP

What is translation?

6) this continues until the ribosome reaches a stop codon at the end of a mRNA molecule. The stop codon doesnt code for an amino acid so the ribosome detaches

the polypeptide chain is now created and will enter the golgi body for folding and modification

how is DNA passed from one generation the next?

By gametes

1) Gametes are the sperm cells in males and egg cells in females. They join together at fertilisation to form a zygote, which divides and develops into a new organism.

how is DNA passed from one generation the next?

2) Normal body cells have the diploid number (2n) of chromosomes — meaning each cell contains two of each chromosome, one from the mum and one from the dad.

3) Gametes have a haploid (n) number of chromosomes — there's only one copy of each chromosome.

how is DNA passed from one generation the next?

4) At fertilisation, a haploid sperm fuses with a haploid egg, making a cell with the normal diploid number of chromosomes. Half these chromosomes are from the father (the sperm) and half are from the mother (the egg).

how is DNA passed from one generation the next?

5) During sexual reproduction, any sperm can fertilise any egg-fertilisation is random. Random fertilisation produces zygotes with different combinations of chromosomes to both parents. This mixing of genetic material in sexual reproduction increases genetic diversity within a species

how are gametes formed?

1) Before meiosis starts, the DNA unravels and replicates so there are two copies of each chromosome, called chromatids.

how are gametes formed?

2) The DNA condenses to form double-armed chromosomes, each made from two sister chromatids. The sister chromatids are joined in the middle by a centromere.

3 ) Meiosis I (first division) — the chromosomes arrange themselves into homologous pairs

how are gametes formed?

4) These homologous pairs are then separated, halving the chromosome number.

5) Meiosis II (second division) — the pairs of sister chromatids that make up each chromosome are separated (the centromere is divided).

6) Four haploid cells (gametes) that are genetically different from each other are produced

what are the two ways to introduce variation?

independent segregation of homologous chromosomes

crossing over between homologous chromosomes

what is independent segregation?

1) Each homologous pair of chromosomes in your cells is made up of one chromosome from your mum (maternal) and one chromosome from your dad (paternal).

2) When the homologous pairs are separated in meiosis I, it's completely random which chromosome from each pair ends up in which daughter cell.

What is independent segregation

3) So the four daughter cells produced by meiosis have completely different combinations of those maternal and paternal chromosomes.

4) This is called independent segregation (separation) of the chromosomes.

5) This 'shuffling' of chromosomes leads to genetic variatio n in any potential offspring.

What is crossing over?

1)when the chromosomes of homologous pairs line opposite each other in the equator, parts of the chromatid can become twisted around each other

2)this puts tension on the chromatids causing pair of the chromatids to break

What is crossing over?

3)the broken part of the chromatid recombines with another chromatid

4)this results in new combinations of allels

how do you identify where meiosis is occurring in a life cycle?

Goes from diploid (2n) to haploid (n)

Meiosis compared to mitosis

Meiosis

-two nuclear divisions

-haploid cell (one set of chromosomes)

introduces genetic variation

Mitosis

-one nuclear division

diploid cell (two sets of chromosomes)

creates genetically identical cells

how is genetic variation increased further?

random fertilisation increases variation

it is random which egg and sperm will fuse in fertilisation

(this is before crossing over)

who can chromosome mutation occur?

mutations in the number of chromosomes can arise spontaneously by chromosome non-disjunction during meiosis.

non-disjunction is when chromosomes do not split equally during anaphase

What is polyploidy?

changes in whole sets of chromosomes occur when organisms have three or more sets rather than the usual two.

(occurs mainly in plants)

1)each homologous pair is doubled, due to DNA replication in interphase

What is polyploidy?

2)Non-disjunction in meiosis 1- ALL chromosomes fail to separate equally

3)normal division in meiosis 2- chromatids separate equally

What is aneuploidy?

change in the number of individual chromosomes

sometimes individual homologous pairs of chromosomes fail to separate during meiosis

this is non-disjunction and usually results in gametes having one more or one fewer chromosome

What is aneuploidy?

on fertilisation with a gamete that has the normal number of chromosomes, the resultant zygote will have more or fewer chromosomes than normal in all their body cells

What is aneuploidy?

1)each homologous pair is doubled, due to DNA replication in interphase

2)Non-disjunction in meiosis 1- ONE chromosomes fail to separate equally

3)normal division in meiosis 2- chromatids separate equally

What are gene mutations?

Gene mutations involve a change in the DNA base sequence of chromosomes.

Substitution — one base is substituted with another,

Deletion — one base is deleted,

What are gene mutations?

The order of DNA bases in a gene determines the order of amino acids in a particular protein. If a mutation occurs in a gene, the sequence of amino acids it codes for (and the protein formed) could be altered:

why not all mutations result in a change to the amino acid sequence?

The degenerate nature of the genetic code means that some amino acids are coded for by more than one DNA triplet. This means that not all substitution mutations will result in a change to the amino acid sequence of the protein — some substitutions will still code for the same amino acid.

why not all mutations result in a change to the amino acid sequence?

Substitution mutations won't always lead to changes in the amino acid sequence, but deletions will — the deletion of a base will change the number of bases present, which will cause a shift in all the base triplets after it.

What are mutagenic agents?

Mutations occur spontaneously, e.g. when D N A is misread during replication. But some things can cause an increase in the rate of mutations — these are called mutagenic agents. Ultraviolet radiation, ionising radiation, some chemicals and some viruses are examples of mutagenic agents.

what is genetic diversity?

is the number of different alleles of genes in a population.

enables natural selection

how can genetic diversity be increased?

• Mutations in the DNA — forming new alleles.

• Different alleles being introduced into a population when individuals from another population migrate into them and reproduce. This is known as gene flow.

how can genetic diversity be decreased?

1. Mark for general principle of - reduced variety/number of different alleles/DNA / reduced gene pool (in new population);

2. Founder effect;

3. A few individuals from a population become isolated/form colonies:

how can genetic diversity be decreased?

4. (Genetic) bottlenecks;

5. (Significant) fall in size of population

6. Selective breeding / artificial selection;

7. Using organisms with particular alleles/traits/phenotypes/characteristics;

What is natural selection?

the process that leads to evolution

evolution is the change in allele frequency over many generations in a population

natural selection results in species becoming better adapted to their environment

adaptations may be anatomical, physiological and behavioural

describe how natural selection works?

new alleles for a gene are created by random mutations.

1) Not all individuals are as likely to reproduce as each other. There's differential reproductive success in a population — individuals that have an allele that increases their chance of survival are more likely to survive, reproduce and pass on their genes (including the beneficial allele), than in dividuals with different alleles.

describe how natural selection works?

2) This means that a greater proportion of the next generation inherits the beneficial allele.

3) They, in turn, are more likely to survive, reproduce and pass on their genes.

describe how natural selection works?

4) So the frequency of the beneficial allele increases from generation to generation.

5) Over generations this leads to evolution as the advantageous alleles become more common in the population.

what is a genetic bottleneck?

1) A genetic bottleneck is an event that causes a big reduction in a population, e.g. when a large number of organisms within a population die before reproducing.

what is a genetic bottleneck?

2) This reduces the number of different alleles in the range of gene pool and so reduces genetic diversity.

3) The survivors reproduce and a larger population is created from a few individuals.

What is the founder effect?

1) The founder effect describes what happens when just 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.

What is the founder effect?

2) The frequency of each allele in the new colony might be very different to the frequency of those alleles in the original population — for example, an allele that was rare in the original population might be more common in the new colony. This may lead to a higher incidence of genetic disease.

What is the founder effect?

3) The founder effect can occur as a result of migration leading to geographical separation or if a new colony is separated from the original population for another reason, such as religion.

What is a Behavioural adaptation?

Ways an organism acts that increase its chance of survival and reproduction. For example, possums sometimes 'play dead' if they're being threatened by a predator to escape attack.

what is a Physiological adaptation?

Processes inside an organism's body that increase its chance of survival. For example, brown bears hibernate over winter. They lower their rate of metabolism (all the chemical reactions taking place in their body). This conserves energy, so they don't need to look for food in the months when it's scarce.

What is an anatomical adaptation?

Structural features of an organism's body that increases its chance of survival. For example, whales have a thick layer of blubber (fat) which helps them keep warm in the cold sea.

What is directional selection?

Directional selection is 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. Bacteria evolving antibiotic resistance is an example of directional selection.

describe directional selection in antibiotic resistance.

1) Some individuals in a population have alleles that give them resistance to an antibiotic.

2) The population is exposed to the antibiotic, killing bacteria without the resistant allele.

describe directional selection in antibiotic resistance.

3) The resistant bacteria survive and reproduce without competition, passing on the allele that gives antibiotic resistance to their offspring.

4) After some time, most organisms in the population will carry the antibiotic resistance allele.

what is stabilising selection?

Stabilising selection is where individuals with alleles for characteristics towards the middle of the range are more lik e ly to survive and reproduce. It occurs when the environment isn't changing, and it reduces the range of possible characteristics. An example of stabilising selection is human birth weight.

describe stabilising selection in human birth weight.

1) Humans have a range of birth weights.

2) Very small babies are less likely to survive — partly because they find it hard to maintain their body temperature.

describe stabilising selection in human birth weight.

3) Giving birth to large babies can be difficult, so large babies are less likely to survive too.

4) Conditions are most favourable for medium-sized babies — so weight of human babies tends to shift towards the middle of the range

What is a species?

A group of similar organisms that can breed and produce fertile offspring.

what is the importance of courtship behaviour?

enables them to recognise own species and own sex

synchronises mating behaviour-indicates sexually mature and in season

to ensure survival of offspring

-form a pair bond

-choose a strong a healthy mate

What is the binomial system?

1) The nomenclature (naming system) used for classification is called the binomial system — all organisms are given one internationally accepted scientific name in Latin that has two parts.

What is the binomial system?

2) The first part of the name is the genus name and has a capital letter. The second part is the species name and begins with a lower case letter.

3) The binomial system helps to avoid the confusion of using common names.

Why do different species look similar?

1) live in similar environment

2) have similar selection pressures

3) similar alleles will have the selective advantage

4) produces similar proteins and therefore will have similar characteristics

What is the classification system?

1) There are eight levels of groups used to classify organisms. These groups are called taxa. Each group is called a taxon.

What is the classification system?

2) The groups are arranged in a hierarchy, with the largest groups at the top and the smallest groups at the bottom. Organisms can only belong to one group at each level in the hierarchy — there's no overlap.

What is the classification system?

3) Organisms are first sorted into three large groups (or taxa) called domains.

What is the classification system?

4) Related organisms in a domain are then sorted into slightly smaller groups called kingdoms, e.g. all animals are in the animal kingdom. More closely related organisms from that kingdom are then grouped into a phylum, then grouped into a class, and so on down the eight levels of the hierarchy.

What is the classification system?

5) As you move down the hierarchy, there are more groups at each level but fewer organisms in each group. The organisms in each group also become more closely related.

What is the classification system?

6)The hierarchy ends with species — the groups that contain only one type of organism

The order is Kingdom, phylum, class, order ,family, genus, species

What is hierachy?

smaller groups within larger groups with no overlap

what are the ways to classify evolutionary relationships?

1) DNA sequence

2) mRNA sequence

3) comparing amino acid sequence

4) immunological- comparing similarity in self antibody shape

How can genome sequencing help to clarify evolutionary relationships?

Genome sequencing — Advances in genome sequencing have meant that the entire base sequence of an organism's DNA can be determined.

How can genome sequencing help to clarify evolutionary relationships?

The DNA base sequence of one organism can then be compared to the DNA base sequence of another organism, to see how closely related they are. Closely related species will have a higher percentage of similarity in their DNA base order.

how can comparing amino acid sequence help to clarify evolutionary relationships?

Comparing amino acid sequence — Proteins are made of amino acids. The sequence of amino acids in a protein is coded for by the base sequence in DNA. Related organisms have similar DNA sequences and so similar amino acid sequences in their proteins.

How can immunological comparisons help to clarify evolutionary relationships?

Immunological comparisons — Similar proteins will also bind the same antibodies. E.g. if antibodies to a human version of a protein are added to isolated samples from some other species, any protein that's like the human version will also be recognised (bound) by that antibody.

What is phylogeny?

phylogenetic classification arranges species into groups according to their evolutionary origins and relationships