Introduction to Genetics -the DNA molecule -nucleotides -chromosomes vs genes -DNA replication -cell...
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Introduction to Genetics
-the DNA molecule
-nucleotides
-chromosomes vs genes
-DNA replication
-cell division
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The DNA molecule
• Composed of 2 polymers of nucleotides
• Polymers are oriented in antiparallel
• Molecule resembles a spiral staircase of complementary base pairs
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Nucleotide structure of DNA
• Each nucleotide of DNA contains:– Deoxyribose
– Phosphate
– Nitrogen base (either A, G, C, T)
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Nucleotide structure of RNA
• Each nucleotide of RNA contains:– Ribose
– Phosphate
– Nitrogen base (either A, G, C, U*)
*contains Uracil instead of Thymine
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DNA structure
• “Double helix” propsed by Watson and Crick (1953)
• Antiparallel backbones
• Complementary base pairing:– Adenine to Thymine
– Cytosine to Guanine
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DNA structure
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Chromosomes vs Genes
• A chromosome constitutes an entire DNA molecule + protein– Protein = histones– Supercoiled DNA in
nucleosomes– Humans contain 46 such
molecules (23 pairs)• 44 somatic chromosomes• 2 sex chromosomes (X +Y)
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Chromosomes vs Genes
• Genes constitute distinct regions on the chromosome
• Each gene codes for a protein product
• DNA -> RNA-> protein• Differences in proteins
brings about differences between individuals and species
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How do chromosomes become double stranded?Answer: DNA replication
• During the life of the cell, each chromosome of DNA makes a copy of itself
• This must occur prior to cell division to insure each daughter cell gets a complete set
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Therefore, prior to dividing, any cell must first replicate DNA
• Each single-stranded (SS) chromosome duplicates to become a double-stranded (DS) chromosome
• Example:– A human cell is formed
with 46 SS chromosomes– Each chromosome
replicates to produce 46 DS chromosomes
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DNA replication
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DNA replication occurs during the life of a cell = the Cell Cycle
• DNA replicates (makes a copy of itself) to produce DS chromosomes
• During this time, the cytoplasmic contents also duplicate
• Spindle tubules form to aid in the process of cell division– Mitosis in body cells– Meiosis in sex cells
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The cell cycle