Ib Biology Hl Notes
Genes and Genetics 1.2.each chromosome exists as two genetically identical chromatids attached to their centromere.
Each chromosome appears as two chromatids attached to a centromere. In the first meiotic division chromosomes align in homologous pairs. Points of contact form between members of the same homologous pair. The points of contact or crossing over between members of a homologous pair are the chiasmata. 3. The homologous pairs move to the equator of the cell. Equal lengths of the chromatids of the same homologous pair have broken off and crossed over. The chromosomes align at random and independantly.
Only one possible arrangement is shown. 4. Homologous pairs align at random at the equator of the cell. This shows the early separation of the chromosomes of each of the homologous pairs. 5. Homologous pairs are separated. This shows the cell at an early stage of meiotic division. The cell membrane is starting to pinch inwards. 6. The cell divides to form two cells each with a haploid set of chromosomes. Cell division is complete and two cells containing a haploid set of chromosomes are formed. 7. In the second meiotic division the chromosomes in each cell align independently and randomly at the equator of the cell. . The chromatids in each cell are separated. The chromatids are pulled towards the opposite poles of the cell. 9. The cells divide. The cell membrane is starting to pinch inwards. 10. Cell division is complete, resulting in the formation of four cells each with the haploid number. 11. Meiosis – Functions ? ? Halving the chromosome number – meiosis consists of two nuclear divisions (meiosis I and meiosis II) but the chromosomes replicate once. Producing four daughter cells. Each are haploid Producing Genetic Variety – through prophase I and through random assortment during metaphase I.
In addition, random fertilisation also produces variety since any gamete has an equal chance on combining. Crossing Over – during prophase one, homologous chromosomes combine to form bivalent so maternal and paternal chromosomes are adjacent. Homologous pairs contain same genes but different alleles. They line up and non-sister chromatids break then they rejoin in corresponding position on other chromatid. Region this occurs is called the chiasma (chiasmata). Point where this is in random but not all chromatids form a chiasma and some dont happen at all. Therefore, chromatids keep alleles.
Monohybrid and Dihybrid Inheritance0 There Codominance – both alleles have an effect on a phenotype neither allele is recessive multiple allels – 3 alleles in blood typed these are Ia , Ib and ‘i’ (Io) these 3 alleles make up the phenotype and genotypes in the blood gene. Sex Link Genes Autosome (normal chromosome) pairs are said to be linked. Meaning they contain the same locus on a gene. HOMEWORK 2 b) Genotypes Gametes F1 Genotypes Phenotypes – AB BO A B B O AB AO BB BO AB, A, -B, B 4 b) Bw Tl Male Gametes- B,w T,l ww ll Female Gametes – w,w,l,l B w w Bw Bw w ww ww ) Polygenic inheritance of skin colour – Depends on the amount of pigment that is produced in the skin. Melanin synthesis is controlled by genes. The degree of pigmentation can range from very dark to very pale. Several genes are involved in skin colour determination and these produce continuous variation. Each gene has two alleles so there are numerous outcomes for example when three genes are involved there are seven outcomes. One allele M contributes to melanin. 7. 4. 6 ? ? ? ? ? ? ? ? ? ? ? Ribosomes are made of two subunits, large and small Small subunits binds to the first codon on the mRNA.
This is the initiation part of translation. The first codon is usually AUG and codes for the amino acid methionine. Large Sub-units will now bind and two binding sites for tRNA sit over the first two codons Transfer RNA with the correct with the correct anticodon will lock into the first tRNA binding site and form hydrodgen bonds to the first codon. The second tRNA will now enter into the second binding site as it has the correct anti-codon Each tRNA has its own specific amino acid, and now a peptide bond forms between these amino acids.
This is the beginning of elongation ( joining amino acids to make a polypeptide) the first tRNA now breaks away and moves into the exit site and now the next codon has to be read. the small subunit shifts down the mRNA in a 5′ 3′ direction followed by the large subunit the second tRNA now moves down into the neighbouring binding site and the third tRNA can now move onto the mRNA and locks them to the exposed binding site another peptide bond is formed this process of the ribosome moving along the mRNA is called translocation eventually the ribosome reaches the last codon and it has no corresponding anti-codon so no tRNA will bind. This is a biological full-stop and this is termination.
The ribosome sub units break apart, releasing the polypeptide chain, which will now coil into its secondary and tertiary structures to form a protein. CroCrossing Over (p237) ? ? ? ? Crossing over occurs during prophase one. Chromosomes shorten and coil and homologous pairs of chromosomes come together form a bivalent Chromosomes ontain the same genes but have different alleles. Two segments then rejoin from broken chromatids at corresponding positions. Mendel’s law of Independent assortment When gametes are formed, the separation of one pair of alleles into the new cells is independent of the separation of any other pairs. or; either pair of alleles is equally likely to be inherited with either of another pair. production but m does not making MMMMMM the darkest and mmmmmmm the palest. Polymerase chain reaction is used to copy and amplify minute quantities of DNA.
It can be useful when only a small amount of DNA is available but a large amount is required to undergo testing. We can use DNA from blood, semen, tissues and so on from crime scenes for example. The PCR requires high temperature and a DNA polymerase enzyme from Thermus aquaticus (a bacterium that lives in hot springs). Sample is heated which denatures DNA and separates into two strands. Thermophlils Aquaticus is a bacterium which Polymerase increases rate of base pair creation. Replication in a few hours 4. 4. In gel electrophoresis, fragments of DNA move in an electrical field and are separated according to their size. Electric charge separatees DNA and allows to see natural variation in DNA Restriction Enzymes cut to produce varying fragment sizes. magnetic field so all DNA (-ve charged) move different distances and then are stained. this leaves a DNA profile Chamber, power supply, gel it doesn’t mater 4. 4. 4 Organisms have short sequences of bases which are repeated many times. These are called satellite DNA. These repeated sequences vary in length from person to person.
The DNA is copied using PCRand then cut up into small fragments using restriction enzymes. Gel electrophoresis separates fragmented pieces of DNA according to their size and charge. This gives a pattern of bands on a gel which is unlikely to be the same for two individuals. This is called DNA profiling. DNA profiling can be used to determine paternity and also in forensic investigations to get evidence to be used in a court case for example. Human Genome ? It is now easier to study how genes influence human development. ? It helps identify genetic iseases. ? It allows the production of new drugs based on DNA base sequences of genes or the structure of proteins coded for by these genes. ? It will give us more information on the origins, evolution and migration of humans. ? 20,000 – 25,000 ? Carried out in 1990 to map entire DNA base sequence the whole genome. 3 billion pairs in 2003. ? Spots added to a solid surface. Can determine levels of thousands of genes by measuring mRNA spots on microarray. Generating profile of expression on a cell Pharmacogenomics – drugs which target specific genomes.