1 UNIT 4 PART 1: MODERN GENETICS In sexual reproduction the new individual develops from the zygote formed by the union of two gametes, one from each parent. Because hereditary material comes from two different parents, the offspring is both similar to, and different from, each parent.

2 GENETICS is the branch of biology that studies the ways in which hereditary information is passed on from parents to offspring. Gregor Mendel, an Austrian monk, was the first to scientifically study heredity in the 1800s. He studied the inheritance of certain traits in peas using 1000’s of pea plants.

3 Mendel then thought that each trait was controlled by a pair of “factors”, and that each factor could be one of two kinds. For example, one factor for green pod color and one for yellow pod color. In a cross, the offspring receives one factor from each parent. In a hybrid one factor may be hidden, but show itself again in later generations when fertilization brings together two of these recessive factors. Ex: yellow pod color

4 When more was learned about cells, it was noted that the movements of the chromosomes during meiosis and fertilization would account for the separation and recombination of Mendel’s factors, so it was thought that the chromosomes might carry the factors. The factors were then called genes. THE GENE- CHROMOSOME THEORY

5 ALLELES It is now known that genes do carry the hereditary information, and that they are on chromosomes in a definite order. Each body cell has two copies of the gene for each trait. These two copies can be the same or different. Each form of a gene that controls a trait is called an allele. Pea pod color has two alleles: green and yellow.

6 If the two alleles an organism has for a trait are the same, like GG, then the individual is said to be homozygous for that trait. If the alleles are different, Gg, the individual is said to be heterozygous. The genetic makeup of an individual, the alleles it has, is called its genotype. The way the individual looks as a result of its genotype, its physical trait, is called its phenotype. GG = green; Gg = green; gg = yellow

7 SEX DETERMINATION Scientists noticed that all the chromosome pairs were the same in both males and females except for one pair. This pair determines the sex of the individual. These are the sex chromosomes. The other pairs are called autosomes. In fruit flies, as well as humans, the two sex chromosomes look the same in the female (XX), but are different in the male (XY).

8 GENE LINKAGE The traits Mendel studied were all on different chromosomes. Most chromosomes have 100’s of genes so many are inherited together. Genes on the same chromosome are said to be Linked. abcde are linked, so are ABCDE.

9 LINKED TRAITS Linked genes are inherited together. Ex: –Blond hair & Blue eyes –Red hair & Freckles –White cats & deafness

10 CROSSING OVER Linked traits may be separated by crossing- over of homologous chromosomes. Genes A an a have crossed over and formed new linkage groups: –Abcde and aBCDE Ex: –Brown hair & Blue eyes –Red hair & dark skin

11 Genes determine the characteristics of an individual and are carried on chromosomes. It still needed to be known what a gene was and how it worked. Through a number of experiments it was determined that chromosomes were made of DNA and that DNA was the hereditary material.

12 DNA STRUCTURE A DNA molecule is made up of thousands of subunits called nucleotides. Each nucleotide has three parts: 1.phosphate group 2.five carbon sugar – deoxyribose 3.nitrogenous base 4 kinds of bases: adenine (A), guanine (G), cytosine (C), and thymine (T)

13 DNA STRUCTURE Watson and Crick discovered the shape of DNA. Two chains of sugar- phosphate groups running parallel to each other with pairs of bases joining the chains like rungs of a ladder. Twisting this ladder forms the double helix.

14 DNA STRUCTURE Every DNA molecule has a different sequence of bases. The order of bases along one strand of the double helix determines the matching bases on the other side: –A always pairs with T; C always with G –So if one strand is AGGTAC the other will be: TCCATG The two strands are said to be complementary. One gene = a sequence of hundreds of bases.

15 DNA REPLICATION DNA can make copies of itself. The two strands unzip at the weak bonds between the bases. Two new molecules are built by attaching new nucleotides to each original strand which acts as a template, or pattern.

16 DNA REPLICATION Each “new” strand will actually be half old and half new. The copies of the DNA molecule can be passed on to the daughter cells during cell division. This is how the chromosomes replicate.

17 DNA FUNCTION How do the genes control all body traits and functions? One Gene  One Polypeptide –Each gene codes for the production of a different polypeptide chain.

18 The work of the cell is carried out by the proteins it assembles. Proteins are made up of chains of amino acids –i.e. polypeptide chains. There are 20 different types of amino acids. The sequence of amino acids determines the protein created and the shape it takes on. Examples of proteins include: enzymes, hormones, and structural proteins.

19 PROTEIN SYNTHESIS BASICS One gene makes one polypeptide. The order of bases of three adjacent nucleotides codes for a particular amino acid. These 3 bases are called a codon. –e.g. CAG is the DNA codon for glutamine – AAA is the DNA codon for phenylalanine Some codons indicate where a polypeptide begins or ends. (Similar to punctuation in a sentence.)

20 Protein Synthesis Helper: RNA Polypeptides are synthesized outside the nucleus, but the genes are inside the nucleus. RNA transcribes the genes from DNA & takes them to ribosomes outside the nucleus. RNA is similar to DNA, but it has 3 main differences: 1.the sugar is ribose 2.Uracil (U) takes the place of the Thymine (T) base 3.RNA is single stranded RNA DNA

21 THREE TYPES OF RNA Messenger (mRNA) copies the genetic code for a protein from DNA carries the instructions outside the nucleus Transfer (tRNA) carries amino acids to ribosomes Amino acid is attached to chain if anticodon pairs with codon Ribosomal (rRNA) in the nucleoli, protein and rRNA join together to form a ribosome site of the assembly line in the cytoplasm of all cells

22 Summary of Protein Synthesis mRNA copies a gene (transcription) Translation: mRNA attaches to a ribosome tRNA brings amino acids to the ribosome, base pairing its anticodon to the mRNA codon Amino acids connect to each other When finished, a polypeptide is released

23 INSIDE THE NUCLEUS OUTSIDE THE NUCLEUS

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25 GENE EXPRESSION All your body cells originated from a single cell, so they all contain the same genes. However, each cell only turns on the genes it needs; it uses only some of the genetic material it contains. This is called expression. Expression causes differentiation- the formation of special types of cells.

26 DIFFERENTIATION Chemical signals from within the cell or from other cells may activate certain genes. Ex: hormones The proteins that are assembled in each type of cell are different so the cells themselves become different.

27 GENE EXPRESSION CAN BE AFFECTED BY THE ENVIRONMENT The Himalayan rabbit is white with some black fur on some cooler parts. If the skin on the back is shaved & an ice pack applied, the new fur will also be black. Alligators sex is deter- mined by the egg temp.: >34 o C for males, <30 o C for females.