DNA, Genes and Chromosomes

4.1 DNA, Genes and Chromosomes

• Nucleotide structure is the same in eukaryotes and prokaryotes, with deoxyribose attached to a phosphate and base.
• Adjacent nucleotides are linked by phosphodiester bonds, and complementary bases are linked by hydrogen bonds.
• DNA in mitochondria/chloroplasts is similar in structure to DNA in prokaryotes.
• Prokaryotic DNA is short, circular, and not associated with proteins, unlike eukaryotic DNA.
• Eukaryotic DNA is longer and linear.
• Histone proteins are associated with eukaryotic DNA, but not prokaryotic DNA.
• Eukaryotic DNA contains introns, while prokaryotic DNA does not.
• A chromosome consists of long, linear DNA and its associated histone proteins, found in the nucleus of eukaryotic cells.
• A gene is a sequence of DNA (nucleotide) bases that codes for the amino acid sequence of a polypeptide or for functional RNA like ribosomal RNA or tRNA.
• A locus is the fixed position a gene occupies on a DNA molecule.
• The genetic code involves a sequence of 3 DNA bases (a triplet) that codes for a specific amino acid.
• The same base triplets represent the same amino acids in all organisms
• Each base is part of only one triplet, so each triplet is read as a discrete unit.
• An amino acid can be coded for by more than one base triplet.
• Non-coding base sequences of DNA do not code for amino acid sequences/polypeptides.
• They exist between genes as non-coding multiple repeats or within genes as introns.
• In eukaryotes, much of the nuclear DNA does not code for polypeptides.
• Exons are base sequences that code for amino acid sequences (in a polypeptide).
• Introns do not code for amino acids, in eukaryotic cells.

4.2 DNA and Protein Synthesis

• The genome is the complete set of genes in a cell, including those in mitochondria and/or chloroplasts.

• The proteome is the full range of proteins that a cell can produce, coded for by the cell's DNA/genome.

• Transcription produces messenger RNA (mRNA) from DNA in the nucleus

• Translation produces polypeptides from the sequence of codons carried by mRNA at ribosomes.

• Both tRNA and mRNA are single polynucleotide strands.

• tRNA folds into a 'clover leaf' shape, whereas mRNA is linear/straight.

• tRNA has hydrogen bonds between paired bases, whereas mRNA does not.

• tRNA is a shorter, fixed length, whereas mRNA is a longer, variable length (more nucleotides).

• tRNA has an anticodon, mRNA has codons.

• tRNA has an amino acid binding site, mRNA does not.

• Hydrogen bonds between DNA bases break during transcription in eukaryotic cells.

• Only one DNA strand acts as a template.

• Free RNA nucleotides align next to their complementary bases on the template strand.

• In RNA, uracil is used in place of thymine, pairing with adenine in DNA.

• RNA polymerase joins adjacent RNA nucleotides, forming phosphodiester bonds via condensation reactions.

• Pre-mRNA is formed and then spliced to remove introns, creating mature mRNA.

• Pre-mRNA is produced in eukaryotic cells, while mRNA is produced directly in prokaryotic cells

• Genes in prokaryotic cells do not contain introns, so no splicing occurs.

• In translation, mRNA attaches to a ribosome, and the ribosome moves to a start codon.

• tRNA brings a specific amino acid, and its anticodon binds to a complementary mRNA codon.

• The ribosome moves to the next codon, and another tRNA binds, joining 2 amino acids by a condensation reaction, forming a peptide bond, and using energy from ATP hydrolysis.

• tRNA is released after its amino acid is joined to the polypeptide chain, forming the polypeptide until a stop codon is reached

• ATP: Its hydrolysis to ADP + Pi releases energy, so amino acids join to tRNAs, and peptide bonds between amino acids form.

• tRNA: It attaches/transports a specific amino acid in relation to its anticodon where the tRNA anticodon complementary base pairs to mRNA codon, forming hydrogen bonds and 2 tRNAs bring amino acids together so a peptide bond can form

• Ribosomes: mRNA binds to that and it. allows space for 2 codons. It allows tRNA with anticodons to bind and catalyses formation of peptide bond between amino acids (held by tRNA molecules) before moving along (mRNA to the next codon)/translocation

• tRNA anticodons are complementary to mRNA codons

• Sequence of codons on mRNA are complementary to the sequence of triplets on the DNA template strand.

• In RNA, uracil replaces thymine.

4.3 Genetic Diversity Can Arise as a Result of Mutation or During Meiosis

• A gene mutation is a change in the base sequence of DNA on chromosomes, and can arise spontaneously during DNA replication in interphase.

• Examples of gene mutation include base deletion or substitution.

• A mutagenic agent increases the rate of gene mutation, for example, ultraviolet (UV) light or alpha particles.

• A mutation changes the sequence of base triplets in DNA, altering the sequence of codons on mRNA, changing the sequence of amino acids in the polypeptide.

• This changes the position of hydrogen/ionic/disulphide bonds and impacts the protein’s tertiary structure (shape).

• Mutations affect Enzymes , as active site changes shape so substrate can't bind, enzyme-substrate complex can't form.

• DNA base/nucleotide (pair) is replaced by a different base/nucleotide (pair).

• Substitution changes one triplet and therefore one mRNA codon

• One amino acid in the polypeptide changes or does not change due to the degenerate nature of the genetic code, or if the mutation is in an intron.

• One nucleotide/base (pair) is removed from the DNA sequence, changing the sequence of DNA triplets from the point of mutation (frameshift) onwards

• This changes the sequence of mRNA codons and therefore the amino acids in the sequence and positions of hydrogen/ionic/disulphide bonds resulting in change of the tertiary protein structure.

• Homologous chromosomes are the same length, have the same genes at the same loci but different alleles.

• Diploid cells have 2 complete sets of chromosomes, represented as 2n.

• Haploid cells have a single set of unpaired chromosomes, represented as n.

• During interphase, DNA replicates, creating 2 copies of each chromosome (sister chromatids) joined by a centromere.

• In meiosis I (first nuclear division) homologous chromosomes are separated.

• Chromosomes arrange into homologous pairs.

• Crossing over occurs between homologous chromosomes.

• Independent segregation of homologous chromosomes takes place.

• The outcome is 4 genetically varied daughter cells that are normally haploid (if the parent cell was diploid).

• Meiosis II (second nuclear division) separates chromatids.

• Homologous chromosomes are separated during meiosis I (first division).

• Homologous pairs of chromosomes associate/form a bivalent.

• Chiasmata form at the point of contact between (non-sister) chromatids.

• Alleles/equal lengths of (non-sister) chromatids are exchanged between chromosomes which creates new combinations of (maternal & paternal) alleles on chromosomes.

• Homologous pairs randomly align at the equator.

• Creates different combinations of maternal and paternal chromosomes/alleles in daughter cells.

• Mutation and meiosis increase genetic variation in a species

• Increased by random fertilisation/fusion of gametes or creating new allele combinations/new maternal and paternal chromosome combinations.

• Mitosis produces 2 daughter cells

• Meiosis produces 4 daughter cells, has 2 divisions while Mitosis only has 1

• Mitosis maintains the chromosomes while meiosis halves it during homologous separation produces genetically identical daughter cells while meiosios produces genetivcalyl varied ones, duce to crossing over and indipent segregation

• Mutations in chromosome numbers arise spontaneously by chromosome non-disjunction during meiosis, when homologous chromosomes or sister chromatids fail to separate during meiosis.

• Some gametes have an extra copy (n+1) of a particular chromosome, and others have none (n-1).

• The number of possible chromosome combinations in daughter cells following meiosis is calculated by 2ⁿ, where n = number of pairs of homologous chromosomes (half the diploid number).

• The number of possible chromosome combinations after random fertilisation of two gametes = (2")²

• Crossing over creates new combinations of (maternal & paternal) alleles on chromosomes.

4.4 Genetic Diversity and Adaption

• Genetic diversity is the number of different alleles of genes in a population.

• Alleles are variations of a particular gene (same locus) with different DNA base sequences and arise through mutation.

• A population is a group of organisms of the same species in a particular space at a particular time that can interbreed to produce fertile offspring.

• Genetic diversity enables natural selection and in certain environments, a new allele might benefit if possessor.

• Evolution- changing allele frequency

• Natural selection produces better adaptions resulting in more succes during reproduction

• Advantageous allele is inherited by members of the next generation (offspring) increases frequency over many generations

• Natural slection can happen via Mutation, Advantage ( in certain named environments)

• Reproduction as the possessors are likely to survive and have increased reproductive success & Inheritance is transferred in the advantageous allele member.

• Natural selection leads to Anatomical adaptations (structural/physical features), Physiological adaptations (processes/chemical reactions)

• Adaptation has 2 selections Directional and Stabilizing, with directional being antibiotic resistance in bacteria and stabilizing is human birth weight.

• Directional is when an extreme variation of a trait eg. bacteria with high level of resistance to an antibiotic is resistant, a change will cause the population to towards extreme trait, on the graph distribution curve shifts towards extreme trait.

• Stabilizing is when organisms with an average/modal variation of trait i.e babies that are average weight is stablizing, normally stays stable where those with an increased frequency of organisms will/alleles for average trait leading to curve being similar but with less variation.

4.5 Species and Taxonomy

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

• Different species cannot mate because chromosome numbers are different preventing reproduction (unless odd chromosome number).

• Courtship behaviour Allows recognition to mate in same speices and of opposite gender, as well as stimualtes and synchronises mating through release/production of sperm and indicates maturity, or sexual pheromones

• Species (attempted to be) arranged into groups, called taxa, based on their evolutionary origins (common ancestors) and relationships using a hierarchy

• Examples Domain (largest/broadest) Kingdom, Phylum, class, order, family, genus and species which are smallest.

• Every speciouse is universially identified using a consisting of the name of its genus and species

• This prevents confusion as some organisms have more than one common name.

• Branch point = common ancestor and Branch = evolutionary path

• If 2 specious have common ancestory that allows them to be relative by more closely related examples include C&D

• Genomics has advanved so more DNA base sequences = more distantly related/earlier common ancestor

• Can change with more build ups as well as mutations (change in DNA base sequences)

• Compared to Immounolofy with Protein tertiary structure

4.6 Biodiversity Within a Community

• Biodiversity: variety of living organisms genetic, ecosystems in habitats ranging from small habitats to earth.
• Community: all populations different species that live in an area.
• Species Richness: it is a measure of number of different species in a community.
• A index of Diversity helps describe relationship between; numer individuals in each species (population size).
• To understand takes into account different populations that present in small or high levels.
• Formula for diversity = total number of organisms of all species/ organisms of each with sum
• If one number is high more speciouse is highly diverse
• Removal forest decrease reduction food and niche etc

4.7 Investigating Diversity

• Comparing: frequency of measurable or observable characteristics. -Comparing base sequence of DNA,mRNA, acid sequence of a specific protein encoded by DNA and mRNA

• More differences in sequences that distant is common of the early ancestor during building or change

• As mutations in genome technologies allowed it tp be replated by DNA investigation

• Collect data from random samples remove bias Use same method of measurement each time Largel use sample sizes, calculate the run and sample till becomes fairly constant/ ethicaly to not harm others.

• Caluctae the mean and spread , high spread show higher varitaion leading to two differing results.

Explore the structure of DNA in eukaryotes and prokaryotes, highlighting their differences. Learn about phosphodiester bonds, hydrogen bonds, and the unique characteristics of mitochondrial/chloroplast DNA. Understand the composition of chromosomes and the coding function of genes and their loci.