biology4alevel600

1 Meiosis consists of two divisions. The first division, meiosis I , separates the homologous chromosomes, so that each cell now has only one of each pair. The second division, meiosis II, separates the chromatids of each chromosome. Meiotic division therefore produces four cells, each with one complete set of chromosomes. 2 Diploid organisms contain two copies of each gene in each of their cells. In sexual reproduction, gametes are formed containing one copy of each gene. Each off spring receives two copies of each gene, one from each of its parents. 3 The cells produced by meiosis are genetically different from each other and from their parent cell. This results from independent assortment of the chromosomes as the bivalents line up on the equator during metaphase I, and also from crossing over between the chromatids of homologous chromosomes during prophase I.  4 Genetic variation also results from random fertilisation, as gametes containing diff erent varieties of genes fuse to...

Some genes are transcribed all the time to produce constituitive proteins; others are only ' switched on ' when their protein products are required. Transcription factors bind to the promoter region of a gene. This may increase or decrease the transcription of the gene. Their roles are to make sure that genes are expressed in the correct cell at the correct time and to the correct extent . Effects of transcription factors: form part of the protein complex that binds to the promoter region of the gene activate appropriate genes in sequence determination of sex in animals allow responses to environmental stimuli regulate cell cycle , growth and apoptosis give hormones their effect Gibberellin (a plant hormone) controls seed germination in plants such as wheat and barley by stimulating the synthesis of amylase .  - DELLA protein inhibits the binding of a transcription factor to the gene promoter PIF  - gibberellin causes the breakdown of DELLA protein - gibberellin ...

In both prokaryotes and eukaryotes, transcription of a gene is controlled by transcription factors . Transcription factors are: proteins that bind to a specific DNA sequence control the formation of mRNA --> control the flow of information from DNA to RNA  Structural genes code for proteins required by a cell. Regulatory genes code for proteins that regulate the expression of other genes. The synthesis of repressible enzymes can be prevented by binding a repressor protein to the operator (a specific site) on the bacterium's DNA. The synthesis of inducible enzymes occur when its substrate is present. Prokaryotic lac operon operon: a length of DNA making up a unit of gene expression in a bacterium. The lac operon consists of a length of DNA with operator and promoter regions and a cluster of 3 structural genes : lacZ - coding for β-galactosidase (hydrolyses lactose to glucose + galactose) lacY - coding for permease (allows lactose to enter cell) lacA - coding for...

A mutation is an unpredictable change in the genetic material of an organism. Gene mutation : change in the structure of a DNA molecule, producing a different allele of a gene. Chromosome mutation : changes in the structure or number of whole chromosomes in a cell. Sources/ mutagens : increase chances of a mutation occuring -  random - environmental factors (ionising radiations alpha, beta, gamma; UV radiation; chemicals) Gene mutations Types: base substitution base addition base deletion Consequences: silent : same amino acid; a mutation that has no apparent effect on the organism  missense : different amino acid; no apparent effect nonsense : introduce a 'stop' triplet frame shift : protein that is made becomes totally useless Base additions and deletions usually have a very significant effect on the structure and therefore the function of the polypeptide that the allele codes for. 1. Sickle cell anaemia Base substitution in the gene that codes for the amino acid sequenc...

Patterns of inheritance are explained by using genetic diagrams . Genetic diagrams are the standard way of showing the genotypes of offspring that might be expected from 2 parents. In sexual reproduction, haploid gametes are made following meiosis. Each gamete contains 1 pair or chromosomes = one copy of each gene. Monohybrid inheritance : inheritance of 1 gene Codominance : alleles both have an effect on the phenotype of a heterozygous organism. Multiple alleles : As the name suggests, when a gene has more than 2 alleles Sex linkage - a sex linked gene is a gene that is present on the X chromosome and not the Y chromosome e.g.: colour blindness Dihybrid inheritance: inheritance of 2 genes Independent assortment of homologous chromosomes during meiosis I results in a variety of genotypes in the gametes formed. ratio     9 : 3 : 3 : 1 of dihybrid cross between 2 heterozygotes alleles of both genes show complete dominance genes are on different chromosomes  Interac...

Studies of human genetic conditions have revealed the links between genes, enzymes and the phenotypes. gene : length of DNA that codes for a particular protein/polypeptide locus : position at which a particular gene is found on a particular chromosome; same gene on same locus allele : particular variety of a gene dominant : the allele whose effect on the phenotype of a heterozygote is identical to its effect on a homozygote recessive : the allele that is only expressed when no dominant allele is present codominant : alleles that both have an effect on the phenotype of a heterozygous organism linkage : the presence of 2 genes on the same chromosome so that they tend to be inherited together and do not assort independently test cross : a genetic cross in which an organsim showing a characteristic caused by a dominant allele is corssed with an organism that is homozygous recessive --> phenotype of offspring is a guide to whether the 1st organism is homozygous or heterozygous F...

Meiosis involves two divisions , called meiosis I and meiosis II. Meiosis I : reduction division  resulting in 2 daughter nuclei (haploid)  half the number of chromosomes of the parent nucleus Meiosis II : behaves like mitosis results in total of 4 haploid nuclei Meiosis I 1. Prophase I centrosomes divide and move to opposite poles of nucleus --> spindle formation homologous chromosomes pair up --> crossing over and form bivalents nucleus disappears nuclear envelope breaks down 2. Metaphase I bivalents line up at equator spindles attach to centromere 3. Anaphase I whole chromosomes move to opposite poles of spindle pulled by microtubules 4. Telophase I nuclear envelope reforms nucleolus reform Meiosis II 5. Prophase II nuclear envelope breaks down nucleolus disappears centrosomes and centrioles replicate ; move to opposite poles 6. Metaphase II chromosomes line up separately across equator of spindle 7. Anaphase II centromeres divide spindle microtubules pull chr...

Diploid organisms contain pairs of homologous chromosomes. Homologous chromosomes: a pair of chromosomes in a diploid cell same structure, genes, loci pair together to form bivalent during the first division of meiosis Haploid: possesses 1 complete set of chromosomes: n Diploid: possesses 2 complete sets of chromosomes: 2n Meiosis: reduction division number of chromosomes would double without meiosis introduces genetic variation   à  mutation Gametogenesis in mammals Gametogenesis in plants Development of pollen grain Development of embryo sac 16.1 Passage of information from parent to offspring  Diploid organisms contain pairs of homologous chromosomes. The behaviour of maternal and paternal chromosomes during meiosis generates much variation amongst individuals of the next generation.  a) explain what is meant by homologous pairs of chromosomes  b) explain the meanings of the terms haploid and diploid and the need for a reduction division (meiosis) prior ...

16.1 Passage of information from parent to offspring  16.2 The roles of genes in determining the phenotype 16.3 Gene control Genetic information is transmitted from generation to generation to maintain the continuity of life. In sexual reproduction, meiosis introduces genetic variation so that offspring resemble their parents but are not identical to them. Genetic crosses reveal how some features are inherited. The phenotype of organisms is determined partly by the genes they have inherited and partly by the effect of the environment. Genes determine how organisms develop and gene control in bacteria gives us a glimpse of this process in action.  Candidates will be expected to use the knowledge gained in this section to solve problems in familiar and unfamiliar contexts. Learning outcomes Candidates should  be able to: 16.1 Passage of information from parent to offspring  Diploid organisms contain pairs of homologous chromosomes. The behaviour of maternal and paterna...