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DNA and chromosomes
Biochem and medical genetics
| Question | Answer |
|---|---|
| What did Gregor Mendel do | Explained how characteristics were inherited. Breeding pure peas produced no phenotype change. Breeding pures of different kinds resulted in a loss of a phenotype. Breeding this returned the phenotype |
| What controls characteristics | Alleles - two alternative forms, one dominant and one recessive. These randomly segregate into gametes and randomly unite at fertilisation |
| What did Friedrich Miescher do | Isolated a substance from white blood cell nuclei consisting of phosphoric acid and nitrogen. He called this nuclein (now DNA) |
| What did Theodore Boveri do | Established that sperm and egg both contribute an equal number of chromosomes Suggested chromosomes contain the characters described by Mendel (carry alleles) |
| What did Walter Flemming do | Created the term chromatin to describe a substance which becomes stringy on cell division This is coloured under a microscope Condensation of chromosomes at cell division |
| Chromosomes | A strand of DNA packaged in protein Double chromatids appear at cell division and are linked by a centromere Telomeres at the end of each chromatid - identify end of chromosome On staining develop characteristic bands - show pattern of bases |
| Two parts of a chromosome | The short arm (p) The long arm (q) |
| Types of chromosomes | Depends on length of p arm p<<<q - acrocentric e.g. 13, 14, 15 p<q - sub-metacentric p=q - metacentric e.g. 1, 3, 19, 20 |
| Karyotype | The entire set of chromosomes in a species Each species has a specific set of chromosomes E.g chinese and indian muntjac have different karyotypes but look similar |
| Human karyotype | 22 autosomal pairs, 2 sex chromosomes Organised according to length Banding is used to identify chromosomes Genetic info in chromosomes is passed on to daughter cells. Cell division is highly controlled with specific stages before division |
| Chromosomes and the cell cycle | For a cell to divide it needs to duplicate its DNA during synthesis. Chromosomes exist as single entities in G1, are replicated in synthesis, are double in G2 and are divided in mitosis |
| How do chromosomes exist during interphase | Chromosomes are not condensed but are diffused in the nucleus You cannot identify individual chromosomes, but they do occupy distinct areas known as chromosome territories |
| How do chromosomes contain DNA | Basic units are 4 nucleotides: Cytosine, Thymine, Adenine and Guanine DNA is double stranded and adopts a helical structure Two strands hybridise via H bonds between G and C (3) and A and T (2) |
| Structure of DNA | A long polymer of alternating phosphates and sugar residues Basic units are nucleotides P sugar backbone is negatively charged If sugar is deoxyribose, polymer is DNA If sugar is ribose, polymer is RNA (has an additional OH reactive group) |
| Nitrogenous bases | Purines - adenine and guanine Pyrimidines - thymine, cytosine and uracil (in RNA) Purines have 2 rings whilst pyrimidines have 1 ring |
| What is a nucleoside | A base attached to a sugar E.g. adenine on ribose is adenosine |
| What is a nucleotide | A base attached to a sugar attached to a phosphate E.g. ADP |
| Phosphodiester bond | Sugar residues can be linked by a 3' to 5' phosphodiester bond Nucleophilic attack of a phosphate by the lone pair on oxygen of carbon 3 on the sugar forms this bond. This is catalysed by polymerases (only 5' to 3') Releases pyrophosphate |
| Directionality of the DNA strand | The 5' end is typically phosphorylated Synthesis occurs in the 5' to 3' direction only The 3' end contains a free OH Chain grows at the 3' end |
| Nucleotide base pairs | DNA is double stranded in a double helix. The two strands are joined by hydrogen bonds between A T and G C. Amounts of A and T and G and C are equal The G-C bond is stronger as they form 3 H bonds whilst A T only form 2 |
| DNA double helix | Two DNA strands run antiparallel The two base paired strands are complementary. The nucleotide composition of one strand can be inferred from the nucleotide content of the other strand |
| Use of single stranded DNA as templates | In DNA replication the double helix breaks. (can be done artificially with heat) Each single strand can then have a new DNA strand built on it by complementary base pairing to form 2 identical strands |
| The double helix structure | Two DNA strands are wound around each other resulting in major and minor grooves. These play an important role in specific interactions of DNA binding proteins such as transcription factors with the DNA. |
| Nucleosome - basic unit of chromatin | Made up of 8 histone proteins on which 146 nucleotides are wound up twice. Linker DNA connects nucleosomes into bead structures. Histones are +ve while DNA is -ve, so they associate to offset charges and reduce the length of DNA |
| 3D architecture | Basic unit of chromatin is the nucleosome 3D architecture includes the organisation into complex dynamic structures including chromatin loops that are formed by shaped protein e.g. cohesin and CTCF (these can bind together etc) |
| Basic 3D architecture of DNA | Double helix Nucleosomes Chromatin loops TADs A (active)/B (inactive) compartments of separate chromosomes Chromosome territory Nucleus |
| Modification of N-terminals | N-terminals of histones are post-transcriptionally modified by acetylation, methylation and phosphorylation This regulated chromatin structure, architecture and gene expression |
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