BBC Bitesize - GCSE Combined Science - Cell division - AQA - Revision 1
How do chromosomes, DNA and genes all fit together? We can use the analogy of a city to better understand the relationship between DNA molecules, genes. Your genes are part of what makes you the person you are. You are different from everyone alive now and everyone who has ever lived. DNA. But your genes. The fundamentals of mutation, genetic linkage, and the relationships between genes and chromosomes were largely.
Amino acids are made up of a set of 3 nucleotides called triplets or codons. There are 20 amino acids that are used to form a variety of proteins. Here we see the formation of 3 amino acids: This replication takes place during interphase of mitosis.
The process can be listed as follows: Nucleotides are made in huge quantity in the cytoplasm. An enzyme unzips the two complementary strands of DNA.
Heredity (Ordinary Level)
New complementary nucleotides link to the exposed bases on the separated strands. A can only join with T G can only join with C 4.
Two DNAs, identical to the original and each other, are now present. As a result of DNA replication the 2 cells formed by mitosis are exact duplicates. Extremely variable regions of non-coding DNA are used.
Heredity (Ordinary Level)
Genetically different individuals produce different profiles. Inhe published his observations regarding patients whose urine turned black. This condition known as alkaptonuria happens when there is a buildup of the chemical homogentisate, which causes the darkening of urine.
In most situations, excess amounts of amino acid phenylalanine are metabolised by the body. This led Garrod to surmise that the enzyme responsible for its breakdown must be defective in these patients. In addition, since the black urine phenotype was passed from generation to generation in a regular pattern, Garrod reasoned that a gene had to be responsible for the production of the defective enzyme.
He attributed a defective enzyme to a defective gene, suggesting a direct link between genes and proteins. The Relationship Between Genes and Proteins Most genes contain the information require to make proteins. The journey from gene to protein is one that is complex and controlled within each cell and it consists of two major steps — transcription and translation.
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For Neurospora, minimal media consist only of salts, glucose, and biotin; rich media are supplemented with amino acids, vitamins, purines, and pyrimidines. Beadle and Tatum isolated mutants of Neurospora that grew normally on rich media but could not grow on minimal media.Alleles and Genes
Each mutant was found to require a specific nutritional supplement, such as a particular amino acidfor growth. Furthermore, the requirement for a specific nutritional supplement correlated with the failure of the mutant to synthesize that particular compound.
Thus, each mutation resulted in a deficiency in a specific metabolic pathway.
Cell division - AQA
Since such metabolic pathways were known to be governed by enzymes, the conclusion from these experiments was that each gene specified the structure of a single enzyme—the one gene-one enzyme hypothesis. Many en-zymes are now known to consist of multiple polypeptides, so the currently accepted statement of this hypothesis is that each gene specifies the structure of a single polypeptide chain.
Identification of DNA as the Genetic Material Understanding the chromosomal basis of heredity and the relationship between genes and enzymes did not in itself provide a molecular explanation of the gene.
Chromosomes contain proteins as well as DNAand it was initially thought that genes were proteins. The first evidence leading to the identification of DNA as the genetic material came from studies in bacteria. These experiments represent a prototype for current approaches to defining the function of genes by introducing new DNA sequences into cells, as discussed later in this chapter.
The experiments that defined the role of DNA were derived from studies of the bacterium that causes pneumonia Pneumococcus. Virulent strains of Pneumococcus are surrounded by a polysaccharide capsule that protects the bacteria from attack by the immune system of the host.
Because the capsule gives bacterial colonies a smooth appearance in culture, encapsulated strains are denoted S. Mutant strains that have lost the ability to make a capsule denoted R form rough-edged colonies in culture and are no longer lethal when inoculated into mice. In it was observed that mice inoculated with nonencapsulated R bacteria plus heat-killed encapsulated S bacteria developed pneumonia and died. Importantly, the bacteria that were then isolated from these mice were of the S type.
Subsequent experiments showed that a cell-free extract of S bacteria was similarly capable of converting or transforming R bacteria to the S state.