GENES

GENE: a section of DNA at a particular locus on DNA that codes for polypeptides and functional RNA in the form of a sequence of bases

HOW MANY BASES?

• 20 different amino acids occur in proteins
• each amino acid has own base on DNA
• there are only 4 different bases of DNA - adenine, thymine, cytosine and guanine
• if each base coded for an amino acid, we would only have 4 amino acids
• if two bases coded for each amino acid, we would only have 16 (4^2) amino acids
• if three bases code for each amino acid, we have 64 (4^3) amino acids, more than enough to support 20
• therefore amino acids are coded for by DNA base triplets

FEATURES OF THE GENETIC CODE

• DEGENERATE: amino acids coded for by multiple triplets
• NON-OVERLAPPING: each triplet read in particular direction once due to stop/start codes
• UNIVERSAL: each triplet codes for same amino acids in an all organisms

PROKARYOTIC DNA: DNA molecules shorter, circular and not associated with proteins - therefore, no chromosomes present

EUKARYOTIC DNA: DNA molecules longer, linear, associated with proteins (hustones) to form chromosomes (mitochondria and chloroplasts also contain DNA like that of prokaryotic cells)

CHROMOSOMES.

• only visible when cell is dividing
• chromosomes appear as two threats, chromatids, joined at single point, centromere
• DNA exists as double helix which is wound around histones
• DNA-histone complex is coiled
• coil is looped and further coiled and DNA packed into condensed chromosome

HOMOLOGOUS: one maternal and one paternal chromosome in a pair with the same genes at the same loci

ALLELE: one of multiple forms of a gene with a different base sequence and therefore different amino acid sequence

RNA STRUCTURE.

• polymer of repeating mononucleotide sub-units in a single strand
• pentose sugar ribose
• organic base - adenine, uracil, cytosine or guanine
• phosphate group

MESSENGER RNA (mRNA).

• long strand in a single helix with a base sequence determined by that of DNA through transcription
• leaves nucleus via pores and associates w/ ribosomes, acts as template for protein synthesis
• possesses information in terms of codons - three bases coding for one amino acid

TRANSFER RNA (tRNA).

• single stranded chain folded into clover-leaf shape with one end extending beyond other
• specific amino acid attaches to extended portion
• anticodon at opposite end that binds to RNA during translation

PROCESS.

1. enzyme acts on DNA causing two strands to separate, exposing nucleotide bases

2. nucleotide bases on template strand pair with complementary nucleotides in nucleus

3. RNA polymerase enzyme joins adjacent nucleotides together to form pre-mRNA

4. DNA strand rejoins behind the RNA polyemerase

5. RNA polymerase detaches when it reaches 'stop' codon

SPLICING.

• DNA of eukaryotic cells is made up of exons - code for proteins - and introns - do not code for proteins
• introns would prevent synthesis of polypeptide if not removed
• pre-mRNA contains introns which are removed during splicing to join exons and form mRNA

1. ribosome attaches to start codon of mRNA

2. tRNA with complementary anticodon pairs with codon on mRNA, carrying specific amino acid

3. tRNA with complementary anticodon to next codon pairs, also carrying an amino acid

4. ribosome moves along RNA, pulling together two tRNA molecules

5. amino acids on tRNA joined by peptide bond using enzyme + ATP

6. ribosome moves to third codon, linking second and third amino acids

7. proceess continues with ribosomes passing behind the first to synthesise many identical polypeptides

8. ribosome reaches stop codon, all molecules detatch leaving polypeptide behind

1. DNA provides instructions - long sequence of bases

2. complementary section made in form of pre-mRNA through transcription

3. pre-mRNA spliced to form mRNA

4. mRNA used as template to which tRNA molecules attach, carrying amino acids to be linked to form polypeptides in translation

5. polypeptide is coiled, producing secondary structure of a-helix through hydrogen bonding

6. secondary structure is folded, producing specific 3D structure through hydrogen, ionic and disulfide bonds

7. different polypeptide chains + non-protein groups are linked to form quaternary structure

GENE MUTATION: any change to one or more nucleotide bases or change to sequence of bases in DNA

SUBSTITUTION.

• one nucleotide is replaced with another, causing the polypeptide to potentially differ by one amino acid
• depends on degenerate nature of code AND importance of amino acids - may affect bond formation and thus tertiary structure + function

DELETION.

• nucleotide is lost from normal DNA sequence
• causes a frame shift meaning each triplet is read differently

CHROMOSOME MUTATIONS

• whole sets - organisms have multiple sets of chromosomes (polyploidy), occurs in plants
• non-disjunction - chromosomes fail to separate in meiosis so diploid is not restored in the offspring from the gamete

• during sexual reproduction, two gametes fuse
• if gametes had diploid number, offspring would have twice that over each generation
• meiosis halves diploid in gametes (haploid) so that diploid is restored upon fertilisation
• maintains diploid number across all generations of species

PROCESS.

• MEIOSIS 1 - homologous chromosomes pair and wrap around one another for exchange in crossing over. One chromosome from each pair goes to one of two daughter cells.
• MEIOSIS 2 - chromosomes separate into chromatids with one chromatid going to one of two daughter cells, producing 4 daughter cells in total

CHROMOSOME COMBINATIONS.

• number of possible combinations of daughter cells = 2^n (n = number of pairs)
• number of possible combinations in offspring = (2^n)^2

INDEPENDENT SEGREGATION.

• meiosis 1 involves homologous chromosomes aligning in their pairs
• this occurs randomly 
• one of each goes to each daughter cell - results in new combinations of maternal and paternal chromosomes

NEW COMBINATIONS.

• homologous chromosomes have same genes but different alleles
• independent assortment produces new combinations of alleles
• random fusion of gametes leads to random combinations of alleles in offspring

CROSSING OVER.

• equivalent portions of chromatids twist around one another, portions break off and are exchanged between homologous chromosomes through recombination 
• leads to new combinations of maternal and paternal alleles

the total number of different alleles within a population.

• more alleles = more genetic diversity
• more genetic diversity = more likely to survive environmental change (different characteristics)

ALLELE FREQUENCY.

• gene pool - wide variety of alleles
• random mutation of alleles can producing chracteristics which are advantageous in environment
• those with allele for chracteristic more likely to survive and reproduce
• allele passed onto offspring
• process continues over several generations
• frequency of advantageous allele increases in frequency throughout population over time 

better adapted organisms survive and breed in the place of less adapted organisms.

DIRECTIONAL SELECTION.

• occurs in situations of environmental change
• favours individuals that vary in one direction from the mean, changing the characteristics of the population
• selects for extremes, selects against averages
• e.g. antibiotic resistance

STABILISING SELECTION.

• occurs in situations of environmental stability
• favours average individuals, preserving the characteristics of the population
• selects for averages, selects against extremes
• e.g. birth weight

ADAPTIONS MAY BE...

anatomical (body parts - fur, ear size), physiological (oxidising fat to produce water in dry environments), behavioural (migration patterns to cope with food shortages)