Chapter 3

Genetics: Reproducing Life and Producing Variation

• Eukaryote: an organism whose cells have nuclei. Both the palm tree and the grasshopper are eukaryotes

• In Eukaryotic organisms

  • Somatic cells: Cells that form most parts of an organism’s body (Diploid)

  • Gametes: Sex cells (sperm, eggs, pollen, ovules). These cells pass DNA to the next generation (Haploid)

  • All carry DNA molecules, DNA molecules are chromosomes. 1 DNA molecule = 1 chromosome

• The DNA Molecule

  • Nuclear DNA

    • Contained within the nucleus of a cell

    • The complete set (plus the mitochondrial DNA) is called the genome

    • Autosomes (non-sex chromosomes). In humans these are the first 22 pairs

    • Sex chromosomes, the 23rd pair

      • X, Y

      • In mammals, females carry two X chromosomes, while males have one X and one Y chromosome

      • The Father determines the sex of the offspring

    • Polymorphisms

      • Each gene has a specific physical location (locus), it is riding on a particular spot on a particular chromosome

      • Loci (plural) are valuable to understand genetic variation

      • Alleles at the same locus can be chemically alternative versions of the same gene. This is polymorphism

      • Some alleles at the same locus are identical

  • Mitochondrial DNA

    • Contained in the mitochondria. Mitochondrion is in cell’s cytoplasm

    • Inherited from the mother

  • DNA: the blueprint of life

    • Chemical template for every aspect of organisms

    • Double helix, ladder-like structure

      • Ladder verticals are phosphates and sugars

      • Ladder rungs made up of 4 types of bases

        • Adenine, thymine, guanine, cytosine

        • Complementary pairs (A&T, C&G)

• Replicating the Code

  • DNA is a self-replicating molecule

  • DNA makes identical copies of itself any time a cell is preparing to divide

• Mitosis: Production of Identical Stomatic Cells

  • DNA replication (of course) followed by one cell division

  • Happens in a diploid cell (cell containing a full set of chromosomes)

  • Rapid mitosis is responsible for the growth of an individual who started from 1 juicy egg + one little sperm into a baby, in just 9 months

  • The pairs could have different alleles for the same traits Hh

• Meiosis: Production of Gametes

  • One DNA replication followed by two cell divisions

  • Gametes are haploid (half the number of chromosomes)

  • Does NOT result in identical cell copies

  • Errors can occur during meiosis

    • Nondisjunction, translocation

  • Homologous pairs have found each other. Now crossing over happens. Then each chromosome becomes a hybrid

• Polymorphisms

  • some genes have one allele while others have more. Some genes have dozens of alleles (Dozens of possibilities)

  • BUT REMEMBER: Genes come in pairs. Each person gets 2 alleles at a particular locust

  • Pairs of chemically identical alleles are termed homozygous

  • Pairs of chemically different alleles are heterozygous

    • Dominant allele is expressed in the pair, even though there is only one

    • For recessive allele to be expressed, there must be two copies

    • “Codominance” is sometimes possible

  • Punnett Square

    • Dominant (capital)

    • recessive (lower case)

• Mendel’s Law of Segregation: a parent passes one allele of its pair to each gamete

  • recessive is segregated, no blending

• Mendel’s Law of Independent Assortment: Alleles of a gene separate independently from alleles of another gene. The inheritance pattern of one trait will not affect the inheritance of another.

  • Hair and eye color

• Producing Proteins

  • Some proteins are chemicals that make up tissues

  • Some regulate functions, repair, and growth of tissues

  • Proteins are made up of amino acids

    • Twenty different types

  • Structural proteins responsible for physical characteristics

  • Regulatory proteins responsible for functions: enzymes, hormones, antibodies

  • Protein synthesis involves two steps

    • Transcription (unzipping, template for RNA)

    • Translation (template attaches to ribosomes)

• Genes: Structural and Regulatory

  • Structural genes are responsible for body structures

  • Regulatory genes turn other genes on and off

• Protein Synthesis: transcription takes place in the nucleus. In translation…

• Differences between DNA and RNA

  • The four bases in DNA are C, G, A, and T. The four bases in RNA are C, G, A, and U. Nucleotides with these various bases are floating around in the nucleus

  • DNA is a double stranded molecule and RNA is a single strand

  • The sugar molecules of each are different. DNA’s sugar is deoxyribose. RNA’s sugar is ribose

• In protein synthesis the while gene gets transcribed

  • But inly certain sections of the gene actually code for the protein. These are called exons

  • Some sections of the gene don’t code for the protein. (They don’t code for anything) These are called introns

  • The mRNA transcript must be edited. The non-coding parts must be cut out. Exons get to exit the nucleus and get translated. Introns stay in the nucleus

  • Introns account for a large portion of the DNA. About 75 - 90% is non-coding!

• Homeotic (Hox) Genes

  • Big changes in animal can happen just by tweaking a homeotic gene

  • dramatic

  • Sex determination in males:

    • Y chromosomes carries the SRY gene, especially important for sex determination. Turns the gonad into a testis and stimulates the SOX9 gene

    • SOX9 produces a hormone called Mullerian Inhibiting Substance (MIS). MIS makes the female tubular reproductive structures regress

    • If mutation in SRY, it fails to switch on SOX9. No SOX9 in an XY fetus? Baby born looking female because female structure persist. Gonadal digenesis

• ABO Blood Group System

  • Each of us has 2 alleles for this trait

  • From one parent you might inherit an A, B, or O allele

  • From the other parent you might inherit an A, B, or O allele

  • Terms:

    • Genotype: the particular pair of alleles you have at a locus

    • Phenotype: how the alleles are expressed (in this case what “blood type” the Red Cross determines you are)

    • Genotypes for this trait include: AO, AA, BO, BB, AB or OO

  • In the ABO blood system A and B are each dominant over O. A and B are co'-dominant

  • What you see expressed in the person (phenotype, their ‘blood type’) is due to the dominance/ recessive nature of the alleles

  • You have ‘type A’ blood if your genotype is either AA or AO

  • You have ‘type B’ blood if your genotype is BB or BO

  • The A and B alleles are both expressed if they end up in the same person, and you will have ‘type AB’ blood if your genotype is AB

  • You have ‘type O’ if and only if your genotype is OO

  • 6 genotypes with 4 phenotypes

• Polygenic Variation

  • Much of genetics is based on the “one gene, one protein” model

  • However, many traits are polygenic and are determined by genes at more than one locus

Additional Information

• Mutations: a creative force in evolution

  • Usually neutral

  • Depends on the situation

    • Sometimes harmful

    • Sometimes beneficial

• Intersex conditions

  • First: two syndromes that have different (from the usually XX and XY) number of sex chromosomes

    • Klinefelter’s Syndrome = XXY, or even XXXY

      • Reduced muscle mass

      • Less facial and body hair

      • Broad hips

      • Enlarged breasts

      • Increased belly fat

      • Infertility

      • Caroline Cossey is an example

    • Turner’s Syndrome = X…

      • Missing all or part of the second X chromosome

  • Other Intersex Conditions

    • Congenital Adrenal Hyperplasia

      • Enzyme defect that stops the biochemical pathway leading to cortisol production. Baby boy looks normal, Baby girl has genitalia that looks somewhat masculine

      • Lady Colin Campbell

    • Alpha 5 Reductase Deficiency

      • An enzyme defect that stops the biochemical pathway leading from testosterone to dihydrotestosterone. An XY child born looking female, but there are testes hidden in the abdomen. At puberty extra testosterone causes clitoris to enlarge into what appears to be a small penis

      • Can father children

      • Caster Semenya

    • Androgen Insensitivity Syndrome

      • A mutation that prevents normal testosterone receptors to form on the surface of cells. The XY fetus has testes in his abdomen. ‘Target’ tissues cannot respond to the testosterone that is circulating in the bloodstream, so the XY baby is born looking female

      • Hanne Gaby Odiele

Protein Synthesis

• DNA

  • Double stranded

  • Inside nucleus

  • ATCG

• RNA

  • Single stranded

  • Can move in and out of nucleus

  • AUCG

    • Uracil replaces Thymine

  • mRNA

    • Transcription

    • Mirror image of DNA strand

    • A codon is made up of 3 bases. It specifies a particular amino acid that will eventually be brought to the ribosome

  • tRNA

    • Translation

      • An anti-codon is also made up of 3 bases and matches the codon in a complementary fashion

• Ribosome

  • Found outside of the nucleus

  • Protein factory

• DNA: 64 3-letter words

  • Each 3-letter word codes for an amino acid

• EX

  • DNA triplet: TCA

  • mRNA codon: AGU

  • tRNA Anti-codon: UCA=Ser

• When a cell receives the signal to make a protein:

  • DNA will unzip to reveal the gene

• Transcription: Making the mRNA chain

  • The mRNA chain travels OUTSIDE the nucleus to get to the RIBOSOME

  • This is where TRANSLATION will take place

• Translation:

  • The ribosome will ‘read’ 3 bases of mRNA (one codon) at a time

  • Matches the mRNA codon with the tRNA Anti-codon (+Amino Acid)

  • Amino acids are connected by polypeptide bonds = a protein