Evolutionary Genetics

How Genes Work

How Genes Work

– We now understand that the genetic code is a set of instructions for the production (or synthesis) of proteins from amino acids

– The genetic code is found in the nucleus of cells, on long strands called chromosomes.

• Chromosomes: strands of DNA in he nucleus of a cell.

• A chromosome is made up of a protein core and strands of nucleic acid.

• This nucleic acid is called deoxyribonucleic acid (DNA) and carries the code.

–DNA is like a ladder with the ends twisted in opposite directions. • The rungs of the ladder carry the code.

• They are made up of pairs of bases joined or bonded in the middle.

• Each base, along with the portion of the side of the ladder to which it is connected is called a nucleotide.

• Nucleotide: the basic building block of DNA and RNA, made up of a sugar, a phosphate, and one of four bases.

• Only 4 bases are involved:– Adenine (A)– Thymine (T)– Cytosine (C)– Guanine (G)

• They are only paired in A-T or T-A and G-C or C-G.

– Therefore, if we know the sequence of bases on one side of the DNA helix, we can correctly predict the sequence on the other side.

–The first function of this paring is to enable the DNA molecule to make copies of itself during cell division.• During cell division, the helix unwinds, and each strand, with its now unpaired bases, picks up the proper complementary bases, which are in solution in the cell.

• This is called replication.–Replication: the copying of the

genetic code during cell division.

• We refer to each consecutive sequence of three DNA bases as a codon.– Codon: the 3-base sequence that codes for a

specific amino acid.– Codons are like words made up of three letters– A string of words—or a genetic sentence—stands

for a chain of amino acids. – Therefore, a sequence of condons codes for the

linking together of amino acids to make a pattern. This process is called protein synthesis.

• Protein synthesis: the process by which the genetic code puts together proteins in the cell.

• During protein synthesis only a small portion of the DNA molecule is unwound. – Messenger ribonucleic acid (mRNA) is

assembled against one strand of the unwound DNA.

– mRNA: the molecule that carries the genetic code out of the nucleus for translation into proteins.

– The mRNA transcribes the gene by matching complementary based to those exposed in the coding strand of DNA, except that uracil (U) replaces thymine (T).

• After the mRNA has transcribed the code, it leaves the nucleus of the cell and moves to specialized structures in the cell called ribosomes, where the messenger is decided and translated into an actual protein. – Another type of RNA called transfer RNA (tRNA)

reads the three-letter codes, or codons, as instructions for assembling a chain of amino acids.

– tRNA: RNA that lines up amino acids along mRNA to make proteins.

– Now, this process is long and complex and it beyond the scope of this class to get in to.

– Suffice it to say, it is still a long way from a protein to a trait or the measurable, physical or chemical characteristics of an organism.

• It does this by unwinding and splitting at the bond between the bases A, T, G, and C, as in the first step in protein synthesis.– Each strand now picks up nucleotides with the

proper complementary bases and thus replaces its other half.

– There are now two pair of each chromosome.– When the cell divides, each daughter cell receives a

full set of chromosome pairs.– This process is known as mitosis.

• Mitosis: the process of cell division that results in two copies of the original cell.

• Some single-celled organisms reproduce in the same fashion, but other single-celled organisms and nearly all multicellular organisms reproduce sexually, that is by the joining of specialized cells from two parents.– Such specialized sex cells are called gametes

(sperm and eggs).• Gametes: the cells of sexual reproduction, which

contain only half the chromosomes of a normal cell.

– Zygote is the fertilized egg before cell division begins.

• Zygote: the fertilized egg before cell division.

• The process of producing gametes is called meiosis. – Meiosis: the process of cell division in

which gametes are produced.

– When the male fertilizes the gamete from the female, the zygote once again had 2 of each chromosome and 2 of each gene.

– But, because the members of each pair come from different parents, the combination of alleles of each gene may well be different from that of either parent.