Biology - Genetics, Variation & Independence

A Level AQA Biology Topic 4

DNA in Prokaryotes

Possess a single, circular chromosomal DNA molecule - a nucleotide

Contain one or more Plasmids - Small circular DNA that contains a few genes. More accessible for proteins required for gene expression and therefore contain genes that are required often, quickly, and/or in emergencies. Genes for antibiotic resistance are often found in plasmids

Prokaryotic DNA is not associated with any proteins, e.g, histones

DNA in Eukaryotes

Found in the nucleus, very long and linear, associated with proteins and form chromosomes - the main proteins are large positively charged histones which organise and condense the DNA

A tightly coiled combination of DNA and proteins is called chromatin, which makes up chromatids and chromosomes

Chromatids are organised into sections, which are a series of genes

Ends of chromatids in chromosomes are ‘sealed’ with protective structures called telomeres

Mitochondrial & Chloroplasts DNA

Mitochondria and chloroplasts of eukaryotic cells contain DNA

Similar to the DNA in prokaryotes - short, circular, and not associated with proteins

Mitochondrial DNA is located in the matrix of the mitochondria

Chloroplasts’ DNA is located in the stroma

Genes

The base sequence of DNA that codes for the amino acid sequence of a polypeptide or functional RNA molecule

mRNA - the base sequence on messenger RNA molecules is used by ribosomes to form polypeptide chains

tRNA - amino acids are carried to the ribosome by transfer RNA molecules

rRNA - ribosomal RNA molecules form part of the structure of ribosomes

The genes in DNA control protein structure and therefor protein function

Genetic Code

• Non-Overlapping - A base is only used once

• Universal - Almost every organism uses the same genetic code, the same triplet code codes for the same amino acid in all organisms

• Degenerate - Multiple triplet codes code for the same amino acid

The genetic code contains introns (coding DNA) and exons (non-coding DNA). These exons are included in pre-mRNA and then taken out by splicing

Genome & Proteome

Genome - The complete set of genes present in a cell, but not every gene is expressed in every cell

Proteome - The full range of proteins that a cell can produce, usually larger than the genome

RNA

• mRNA - Transcript copy of a gene that encodes for a specific polypeptide, carries genetic code from DNA in the nucleus to the ribosomes, where it is used to synthesise proteins during translation, single-stranded

• tRNA - Has a folded shape and single stranded, with hydrogen bonds between complementary bases holding the single strand together in certain regions. The specific anticodon found on the tRNA molecule is complementary to the specific triplet of bases on an mRNA molecule, allowing amino acids to bind to it

Transcription 

1. A section of DNA unwinds; this section contains the gene from which a specific polypeptide will be produced. Unwinding is caused by the breaking of hydrogen bonds between complementary base pairs by DNA helicase

2. The gene to be transcribed is now exposed, a complementary copy of the code is made by creating a molecule of mRNA - free activated nucleotides pair up with their complementary DNA bases on the unzipped DNA molecule, this is the template strand

3. The RNA nucleotides are bonded by DNA polymerase, which catalyzes the condensation reaction between nucleotides to form phosphodiester bonds

4. When the gene has been transcribed, the mRNA molecule is complete, the hydrogen bonds between the mRNA and DNA break, and the DNA reforms

5. The mRNA leaves the nucleus via a pore in the nuclear envelope

Splicing

Splicing occurs before the pre-mRNA leaves the nucleus

The non-coding sections are removed, and the coding sections are rejoined

After splicing, mRNA only contains introns before leaving the nucleus and joining the ribosome; this is called mature mRNA

Alternative splicing - the exons can be spliced in different ways to produce different mature mRNA molecules, affecting the overall protein shape and function

Translation

1. After transcription, the mRNA molecule leaves the nucleus and is attached to a ribosome

2. Within the cytoplasm, there are free molecules of tRNA which has an anticodon at one end and an amino acid binding site at the other. The tRNA molecules bind with their specific amino acids and bring them to the mRNA molecule on the ribosome

3. The anticodon on each tRNA molecule binds with a complementary triplet code on the mRNA molecules; two tRNA molecules fit onto the ribosome at any one time

4. A peptide bond is then formed between the two amino acids, which requires ATP, which is provided by the mitochondria within the cell

5. This process continues until a ‘stop codon’ on the mRNA is reached, and translation stops

6. The amino acid chain then forms the final peptide

Genetic Mutations

• Deletion - A nucleotide is deleted from the DNA sequence, which changes the amino acid. A knock-on effect occurs as all subsequent triplet codes are altered. This is called a frame shift mutation. The protein structure and function are changed

• Substitution - A base in the DNA sequence is swapped with a different base, changing only one triplet code, and thus only one amino acid is altered. could be silent if the amino acid sequence does not change, could create a stop codon, which will change the protein structure and function

• Addition - A base is added to the DNA sequence, changing all triplet codes from that point, altering the DNA sequence creates a knock-on effect (frame shift). The protein structure and function will change

Chromosomal Mutation

Non-Disjunction - when chromosomes fail to separate during meiosis, ends up with one extra copy of a particular chromosome or no copies of a particular chromosome

Genetic Diversity in Meiosis

During meiosis I, homologous chromosomes line up in the centre of the cell; they are then separated and pulled apart into different cells. Independent segregation is when each homologous pair lines up randomly, so the way one pair segregates does not affect how another pair segregates

Crossing over, where homologous chromosomes exchange genetic material during meiosis I, increases genetic variation. Homologous chromosomes pair up and form bivalents, chromatid then breaks and rejoin to the chromatid of its homologous chromosome, and alleles are exchanged

Meiosis I

1. Prophase - DNA replication has occurred, DNA condenses and becomes visible as chromosomes, arranged side by side in homologous pairs, crossing over may occur here. The spindle is formed by the centrioles, and the nuclear envelope breaks down, and the nucleolus disintegrates 

2. Metaphase - Homologous pairs of chromosomes line up randomly along the equator of the spindle; independent segregation occurs here

3. Anaphase - Microtubules pull whole chromosomes to opposite ends of the spindle; the centromeres do not divide

4. Telophase - The chromosomes arrive at opposite poles, spindle fibres start to break down. Nuclear envelope forms around the groups of chromosomes, and nucleoli reform

The cell then divides by cytokinesis, forming two haploid cells (daughter cells), each with half of the original number of chromosomes. The cells are haploid as they contain half the number of centromeres

Meiosis II

1. Prophase II - Nuclear envelope breaks down and chromosomes condense, a spindle forms at a right angle to the old one

2. Metaphase II - Chromosomes line up in a single file along the equator of the spindle

3. Anaphase II - Centromeres divide, and all individual chromatids are pulled to opposite poles - each chromatid is now an individual chromosome. Creates four groups of chromosomes that have half the number of chromosomes compared to the original parent

4. Telophase II - Nuclear membranes form around each group of chromosomes. Cytokinesis occurs, cytoplasm divides as new cell surface membranes are formed, creating four haploid cells

Environmental Adaptions

• Anatomical - Structural features of an organism’s body that help it survive

• Physiological - Internal biological processes that increase survival or reproduction

• Behavioural - Actions or behaviours that help an organism survive

Phylogenetic Classification

The study of how organisms are evolutionarily related is often shown using a phylogenetic tree

Uses DNA, RNA, and protein similarities to identify evolutionary links

More similar DNA sequences = more recent common ancestor

Hierachy of Taxa

• Domain

• KIngdom

• Phylum

• Class

• Order

• Famil

• Genus

• Species

Immunology

1. Pure albumin is extracted from various species and injected into separate rabbits

2. Each rabbit produces antibodies specific to albumin

3. Antibodies are then mixed with different albumin samples, and the resulting precipitate is weighed (antigen-antibody complex)

The heavier the precipitate, the more antigen-antibody complexes made, the greater the similarity between the albumin and antibody, so they are more closely related by evolution, vice versa

Ecosystem Definitions

Ecosystem - All the living and non-living components of a particular area

Species richness - Number of species within a community

Community - A Group of populations of different species living in the same place at the same time that interact with each other

Population - All the individuals of the same species living in a particular area at a specific time, which can potentially interbreed

Niches - All the conditions and resources required for an organism to survive and reproduce (its “role”)