Genetics

Mitosis & Meiosis

• Review p 45-47• A. The Cell Cycle• 1. The dividing cell goes through a cycle of events

known as the cell cycle• 2. Cycle divided into interphase and mitosis• B. Interphase• 1. Period of DNA replication in preparation of

nucleus dividing• 2. Divided into 3 periods, G1 (1st growth or gap

period), S (synthesis), and G2 (2nd growth or gap period)

Mitosis• C. Mitosis• 1. Terms• a. Mitosis = division of the nucleus• b. Cytokinesis = division of the cytoplasm• c. Meristem = regions in plants where mitosis occurs (growing site)• 2. Prophase• a. Chromosomes become shorter, thicker and double move toward

equatorial plate• 1) Chromatids• • double threads of chromosomes• 2) Centromeres• • hold chromatids together• 3) Kinetochore• • near centromere, spindle fibers attach here• 4) Other constrictions may occur on individual chromosomes• • called satellites• b. Nucleolus and nuclear envelope disappear• c. Centrioles and asters form (only in algae, fungi, and animal cells)

• 3. Metaphase• a. Spindle forms• b. Chromosomes become aligned at the equatorial plate and connect to

spindle fiber at the kinetochore (part of the centromere)• 4. Anaphase• a. sister chromatids separate• b. once separated, daughter chromosomes are pulled by the kinetochore

along spindle fibers to opposite poles • 5. Telophase• a. Each group of daughter chromosomes becomes surrounded by a new

nuclear envelope• b. Daughter chromosomes become longer and thinner• c. New nucleoli appear• d. Phragmoplast appears• e. Cell plate forms• 1) Vesicles from the Golgi fuse to form the cell plate• 2) Plasmodesmata form as ER becomes trapped in cell plate

Phragmoplast

Alternation of Generations review • A. Terminology: n and x• B. Sporophyte Phase (2n)• 1. Diploid (2x)• 2. Meiosis takes place in special spore mother cells (meiocytes)• C. Gametophyte Phase (n)• 1. Haploid (x)• 2. Meiospores begin this phase• 3. Meiospores develop by mitosis into multicellular

gametophyte• 4. Gametes are produced which upon fertilization produce the

zygote, the first cell of the new sporophyte (2n) phase• D. Many Plant Species Are Polyploid• E. Six Rules Pertaining to Alternation of Generations

Six Rules Pertaining to Alternation of Generations (from the book)

• 1. the first cell of any gametophyte generation is normally a spore (sexual spore or meiospore), and the last cell is normally a gamete

• 2. any cell of a gametophyte generation (n) is usually haploid (x)

• 3. the first cell of any sporophyte generation is normally a zygote, and the last cell is normally a sporocyte (meiocyte)

• 4. any cell of a sporophyte generation (2n) is usually diploid (2x)

• 5. the change from a sporophyte to a gametophyte generation occurs as a result of meiosis

• 6. the change from a gametophyte to a sporophyte generation occurs as a result of fertilization (fusion of gametes) which is also called syngamy

Meiosis

• First meiosis• Interphase: replication of DNA and

chromosomes• Prophase I: chromosomes of diploid nucleus

becomes visible as long thin threads (each consists of 2 chromosomes)– Example: 2 chromosomes with 4 chromotids– 2 homologous pairs of chromosomes

Prophase cont.

– Prophase I cont: 2 chromosomes with 4 chromotids

– 2 homologous pairs of chromosomes – N = 2

Prophase cont.

• Homologues chromosomes pair up (synapses); each chromosome now with 4 chromatids (tetrad) and there is ½ the original number of chromosomes

• ========o==========• ========o==========• Above is a tetrad

Prophase cont.• Chromosomes condense as homologous

chromatids exchange parts (crossing over)

Prophase cont.

• Nuclear membrane disappears• Nucleolus disappears• Homologous chromatids separate at the

centramers but remain attached by schismatic

Metaphase I• Paired chromosomes move to equatorial

plane• Centromeres of paired chromosomes line up

on opposite sides of the equatorial plate

Anaphase I

• See above• Homologues chromosomes separate and

move toward poles• Two sister chromatids comprise each

chromosome• Homologues separate (not the sister

chromatids)• Homologs differ because of crossing over

Telophase I

• Chromosomes at each pole relax and become elongate and indistinct

• Nuclear envelope forms• Nucleoli appear• Each of the two new nuclei have ½ of the

original chromosome number• Reduction completed• Cytokinesis may occur or nuclei may proceed

immediately to the second division of meiosis

Meiosis II

• No additional duplication• Prophase II: nuclear envelope disappears• nucleolus disappears• Metaphase II: chromosomes line up at equator

plate• Anaphase II chromatids separate and move to

opposite poles• Telophase II: new nuclear envelope and

nucleolus reappear

Review

• Stop here Wednesday, 7 November 2012

Mendelian Genetics

• A. Mendel’s Studies• 1. Austrian monk, born 1822• 2. Scientific and mathematical studies• 3. Experiments with pea plants• a. Crosses between tall and

short plants• b. Crosses between plants with

smooth seeds and wrinkled seeds• c. Determination of factors

4. Law of Unit Characters

• factors which always occur in pairs, control the inheritance of various characteristics

• Paired factors now known as gene "alleles"

5. Law of Dominance

• in any given pair of factors (alleles), one may suppress or mask the expression of the other

• Dominant allele: the expressed factor• Recessive allele: the factor not expressed• Phenotype: what something looks like (pink flowers, wide

leaves)• Genotype:• 1) Homozygous: both alleles of a pair are identical (e.g.,

YY or yy)• 2) Heterozygous: allele pair is composed of contrasting

alleles (e.g., Yy)• Incomplete dominance:

• 6. Law of Segregation• • members of allele pairs become separated

during meiosis• 7. The Monohybrid Cross• a. F2 genotype ratio is 1:2:1

• b. F2 phenotype ratio is 3:1• 8. The Dihybrid Cross• a. Law of independent assortment• b. Punnett square• c. F2 genotype ratio is 1:2:2:4:1:2:1:2:1

• d. F2 phenotype ratio is 9:3:3:1

• 9. The Backcross• • between homozygous recessive

parent and F1 offspring• 10. Incomplete Dominance• 11. Interactions Among Genes• 12. How Genotype Controls Phenotype• 13. Polyploidy in plants

Incomplete Dominance

How Genotype Controls Phenotype

Polyploidy in plants

Polyploidy in agricultural plants

• Science 25 April 2008: Vol. 320 no. 5875 pp. 481-483

• AT SCHOOL: go to Google and put the above into the search line. Print out the article and bring it to class.

• We will make plans

Hardy-Weinberg Law

• We will go into this in lab.

DNA and RNA

• Two kinds of nucleic acids– 1. RNA = ribose nucleic acid– 2. DNA = deoxy ribo nucleic acid

nucleotides

Sugars

D-Ribose 5-phosphate

DNA & RNA

• A. Structure of DNA• 1. Nucleotides• a. Nitrogenous base• b. 5-carbon sugar• c. Phosphate group• 2. Nitrogenous bases• a. Purines, adenine and guanine (2 linked

rings)• b. Pyrimidines, cytosine and thymine

(single ring)

DNA Structure

• B. DNA Functions• 1. Storage of Genetic Information• a. A “Gene” molecular unit of of heredity of a living

organism• b. Codons and amino acids (see table)• 2. Replication (Duplication) of Information• a. Semi-conservative replication would produce two copies

that each contained one of the original strands and one new strand.• b. DNA polymerase• 3. Expression of Information• a. Transcription• b. Translation• 4. Mutation

Codons

DNA polymerase

Transcription

• 1. DNA unzips and RNA polymerase enzyme binds to one strand of DNA.

• 2. RNA polymerase makes an elongating chain of RNA nucleotides, each new RNA nucleotide is complementary to the DNA nucleotide.

• 3. The completed mRNA molecule is released from RNA polymerase - DNA complex and can begin translation. In eukaryotic cells this means first moving from the nucleus into the cytoplasm.

Translation

• 1. the ribosome binds to mRNA at a specific area (promoter region)

• 2. the ribosome starts matching tRNA anticodon sequences to the mRNA codon sequence

• 3. each time a new tRNA comes into the ribosome, the amino acid that it was carrying gets added to the elongating polypeptide chain

• 4. the ribosome continues until it hits a stop sequence, then it releases the polypeptide and the mRNA

• 5. the polypeptide forms into its native shape and starts acting as a functional protein in the cell

Mutations

• Somatic: occurs in a body cell and will occur in all cells produced by mitosis

• 1. source of new types of horticultural plants• Germ line: occurs in tissue that will produce

gametes.• 1. passed on to future generations• 2. important for the genetic improvement of

plants (example very sweet corn)

Little Review

• DNA– 1. must carry genetic material from cell to cell and

form generation to generation. Must carry a great deal of information

– 2. must be able to self replicate with great precision– 3. must be able to be modified or changed (mutation)– 4. must have mechanism to read stored information

and transfer it into the living system to be used to carry out activities

• DNA replication – semi conservative– 1. each strand has ½ of the original strand– 2. after replication both strands are the same 3.

occurs in s phase of mitosis • DNA strand is very long to have all of the

information– 1. carried on triplicate sequences of DNA– 2. begin reading on a specific sequence– 3 There are 16 combinations of those same 4 bases

Little Review

• RNA• Three kinds of RNA– 1. tRNA– 2. rRNA – structural portion, rough ER– 3. mRNA – carries sequences of triplet codes

• Codon: triplet sequence on mRNA• Anticodon sequence on tRNA

DNA mRNA tRNA

ATG

UAC

AUG

END