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Chromosomes, genes and DNA, and cell division (Protein Synthesis…
Chromosomes, genes and DNA, and cell division
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DNA Structure
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Four bases: Adenine, Thymine, Cytosine and Guanine
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=> Amounts of A and T, and C and G are equal
DNA Replication
- The polynucleotide strands of DNA separate
- Each strand acts as a template for the formation of a new strand of DNA
- DNA polymerase assembles nucleotides into two new strands according to the base-pairing rule
- Two identical DNA molecules are formed – each contains a strand from the parent DNA and a new complementary strand
The genetic code
Only one DNA strand actually codes for the manufacture of proteins in a cell, KA the template strand. The other strand is KA the non-template strand
Proteins
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Some are structural, eg. keratin in skin, myosin in muscles
Some are functional, eg. haemoglobin and some hormones
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Protein Synthesis
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Takes place in two stages, KA transcription and translation
Transcription
- Part of the DNA 'unzips', so that the two strands separate, exposing the bases along the template strand
- The template strand forms a framework upon which a molecule of mRNA is formed
- The RNA nucleotides link up to form an mRNA molecule. They form bonds between their ribose and phosphate groups, forming the sugar-phosphate backbone of the molecule
- When a gene has been transcribed, the molecule leaves the nucleus and passes into the cytoplasm through pores in the nuclear membrane
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Translation
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The code consists of a triplet of bases in the mRNA, KA codons
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- The mRNA molecule attaches to a ribosome
- Each tRNA molecule has an anticodon of complementary bases to a codon
- the tRNA molecule carries its amino acid to the ribosome, where its specific anticodon pairs up with the three corresponding bases
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Chromosomes
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Quantities
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Gametes only have 23 chromosomes, KA haploid cells
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Cell Division
Mitosis
- The DNA replicates and the chromosomes form two exact copies KA chromatids
- The chromosomes line up in single file in the middle of the cell
- The chromatids separate and move to opposite ends of the cell
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Meiosis
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- Chromosomes pair up in homologous pairs
- Members of each pair separate
- There are now two nuclei each with half the original chromosome number
- The two chromatids of each chromosome separate
- Four haploid gametes remain
Mutations
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wrong nucleotide => gene mutation => alter base sequence => gene codes for wrong amino acid => wrong protein
Types:
Duplication: the nucleotide is inserted twice instead of once
=> entire base sequence altered
=> entirely different protein
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Substitution: different nucleotide is used
=> one base triplet is altered
=> may code for a different amino acid
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Regulating genes
Operon: a group of structural genes headed by a non-coding length of DNA KA an operator, along with another sequence of DNA KA a promoter
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Regulatory gene: close to promoter, codes for a protein KA a repressor
–> binds with the operator
=> prevents promoter binding with RNA polymerase
=> stops transcription