Chapter 3: Biological basis of life: genetics, cells, DNA
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Constraints on Darwin's theory of natural selection
Constraint 1: Didn't know there's biological variation in all species.
Constraint 2: Didn't know how traits were inherited.
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Cells: the basic units of life
3.5 bya (billion years ago) primitive cellular forms emerged-similar to modern day algae
1.7 bya earliest modern, eukaryotic cells emerged
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Cells: the basic units of life
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*Somatic cells
*Sex cells
DNA-chains of stacked nucleotides
-nucleotide structure
Sides: a sugar and a phosphate
Rungs: bases pairs
Adenine
Thymine
Guanine
Cytosine
Bases form complementary bonds5
DNA compared to RNA
RNA-single-stranded-Uracil base instead of thymine
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Proteins-complex molecules able to bind with other molecules-composed of amino acid chains
Amino acids -basic building blocks of proteins-sequence and number determine protein function
Triplets: sequence of 3 DNA bases specifies amino acids
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DNA replication
1. DNA unzips
2. Exposed bases attract
complementary nucleotides
3. Two original strands are
templates for new strands
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DNA function: protein synthesis
Part 1 (transcription) in the nucleus...-mRNA copies DNA bases three at a time (triplets) into codons
Part 2 (translation) in the cytoplasm with ribosomes...-tRNA binds amino acids with complementary codons
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Proteins
Structural: form an organism's physical attributes (keratin, collagen)
Functional: regulate chemical activities (enzymes, hormones)
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Proteins examples
E.g., Hormones -produced by specialized cells -influence chemical reactions -regulates various cell functions
E.g., Hemoglobin is a protein molecule found on red blood cells. Hb binds with oxygen and carries it throughout the body.
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Humans have 46 Chromosomes-coiled DNA visible during cell replication-each human somatic cell has 46 (23 pairs)
Autosomes -carry genetic information that governs all physical characteristics
Sex chromosomes-determines primary sex
X chromosome = femaleY chromosome = male
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Human Genome Project-complete sequence of DNA bases that specifies the order of amino acids in an entire protein, part of protein, or any functional product
Human Genome-25,000 genes-90,000 proteins
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Genes
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Mitosis: somatic cell division
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-occurs during growth, repair, and replacement of tissues
Result: 2 genetically identical daughter cells-46 chromosomes
Meiosis: sex cell division
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Two cell divisions 1st division: reduction2d division: similar to mitosis
Result: 4 genetically different daughter cells -23 chromosomes
Recombination
-paired chromosomes exchange genes during meiosis
-new combination of genetic material is inherited
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Recombination
-exchange of genetic material between paired chromosomes at the beginning of meiosis.
-New combinations of existing genes arise at the beginning of meiosis when homologous pairs of chromosomes from each parent exchange genetic material.
-new combination of genetic material is inherited20