Lecture topics – Meiosis and Mitosis Basic vocabulary Basic vocabulary Structure of chromosomes Structure of chromosomes DNA structure, replication and mutation DNA structure, replication and mutation Mitosis – a conservative division Mitosis – a conservative division Meiosis – a reduction division Meiosis – a reduction division –Meiosis and sex

Genome The complete set of genetic information in an individual The complete set of genetic information in an individual In humans, includes 23 pairs of chromosomes with ~30,000 genes, plus mitochondrial DNA In humans, includes 23 pairs of chromosomes with ~30,000 genes, plus mitochondrial DNA

Chromosome A long strand of DNA containing many genes or loci A long strand of DNA containing many genes or loci Autosomes are numbered from longest to shortest Autosomes are numbered from longest to shortest Number varies among species Number varies among species Sex chromosomes, e.g., X and Y, are partially homologous and segregate as homologs Sex chromosomes, e.g., X and Y, are partially homologous and segregate as homologs Long arm Short arm Centromeres Sex chromosomes

DNA – DeoxyriboNucleic Acid is a large molecule composed of four complementary nucleotides or bases: Adenine, Thymine, Guanine, Cytosine (A, T, G, C) Molecule is in the form of a double helixDNA – DeoxyriboNucleic Acid is a large molecule composed of four complementary nucleotides or bases: Adenine, Thymine, Guanine, Cytosine (A, T, G, C) Molecule is in the form of a double helix (b) Deoxyribose GuanineCytosine Deoxyribose AdenineThymine

Homologs & Alleles Diploid (2n) organisms have two sets of chromosomes – one set from each parent. Diploid (2n) organisms have two sets of chromosomes – one set from each parent. Haploid (1n) has just one set Haploid (1n) has just one set Homologous chromosomes have the same genes, but may have different alleles Homologous chromosomes have the same genes, but may have different alleles Illustrated chromosomes have alleles of A, B, C & D loci. Illustrated chromosomes have alleles of A, B, C & D loci. –A & a, B & b, C & c, D & d B is NOT an allele to A; nor is C or D B is NOT an allele to A; nor is C or D

Codons – “Three letter words" each refer to one of 20 amino acids that can go into a protein Codons – “Three letter words" each refer to one of 20 amino acids that can go into a protein There is redundancy in the genetic code There is redundancy in the genetic code But single base substitution can alter the amino acid sequence in a protein But single base substitution can alter the amino acid sequence in a protein –CAC  valine –CTC  glutamic acid Single AA substitution can alter protein function Single AA substitution can alter protein function –Valine for glutamic acid in hemoglobin gene  sickle cell CAC CTC

Page 282

Fig top Normal DNA sequence Normal mRNA sequence Normal protein sequence BASE SUBSTITUTION MUTATIONS Missense mutation Nonsense mutation (Stop) Mutation are errors in copying DNA

FRAMESHIFT MUTATIONS Deletion  nonsense (Stop) Deletion  altered amino acid sequence Fig b

New genetic jargon Homologous chromosomes – chromosomes with the same sets of genes from different parents Homologous chromosomes – chromosomes with the same sets of genes from different parents Sister chromatids – replicate copies of a chromosome from a single parent Sister chromatids – replicate copies of a chromosome from a single parent Nucleotide or base – one of four basic units of information in DNA Nucleotide or base – one of four basic units of information in DNA –A hydrogen bonds to T, C hydrogen bonds with G Codon – a sequence of 3 bases that codes for a particular amino acid Codon – a sequence of 3 bases that codes for a particular amino acid Amino acid – one of 20 basic units of a protein Amino acid – one of 20 basic units of a protein Mutation – an error in copying DNA that alters the base pair sequence; the source of new alleles Mutation – an error in copying DNA that alters the base pair sequence; the source of new alleles

Two types of cell division in eukaryotes Eukaryotes use mitosis to produce identical daughter cells by means of asexual reproduction Eukaryotes use mitosis to produce identical daughter cells by means of asexual reproduction –Mitosis is a conservative division, i.e., ploidy level is maintained Eukaryotes use meiosis to produce gametes for sexual reproduction. Eukaryotes use meiosis to produce gametes for sexual reproduction. –Meiosis similar to mitosis but is a reduction division, i.e., ploidy level is halved (usu. diploid  haploid)

The Eukaryotic Cell Cycle Interphase (blue arrows): Interphase (blue arrows): –G 1 : Gap 1 Normal housekeeping Normal housekeeping Basic cell functions Basic cell functions DNA is actively used to make proteins DNA is actively used to make proteins –S: Synthesis (replication) of DNA Note: Cells retain same ploidy level before and after synthesis Note: Cells retain same ploidy level before and after synthesis –G 2 : Gap 2 Preparation to divide Preparation to divide See Fig. 9-5

DNA replication simplified Strands of double helix open and each serves as the template for a new complementary strand Strands of double helix open and each serves as the template for a new complementary strand Complementary nucleotides are added to the growing new chain by DNA polymerase enzyme Complementary nucleotides are added to the growing new chain by DNA polymerase enzyme Complementary base pairing (A-T, C-G) determines next nucleotide added to chain

Semi-conservative replication: new double helix consists of one old and one new strand Parental DNA Replicated DNA

“M” Phase –Prophase Chromosomes condense Chromosomes condense Nuclear membranes degrade Nuclear membranes degrade –Metaphase Chromosomes line up Chromosomes line up –Anaphase Chromosomes separate Chromosomes separate –Telophase Nuclei reform Nuclei reform Cytokinesis (yellow) Cytokinesis (yellow) –when daughter cells separate Mitosis is a nuclear division

Chromosomal organization during the cell cycle In G 1 the chromosomes are singlet because they are single DNA / histone complexes (they may still be diploid, however) In G 1 the chromosomes are singlet because they are single DNA / histone complexes (they may still be diploid, however) In S the chromosomes duplicate and become duplex (they may still be haploid, however) In S the chromosomes duplicate and become duplex (they may still be haploid, however) Duplex chromosomes are held together by the centromere Duplex chromosomes are held together by the centromere In G 2 chromosomes continue to be duplex In G 2 chromosomes continue to be duplex In M they condense and individual sister chromatids become evident In M they condense and individual sister chromatids become evident These sister chromatids separate during mitosis to return to the singlet state. These sister chromatids separate during mitosis to return to the singlet state.

Fig left The cell is carrying out its normal life activities. The chromosomes are duplicated. Nuclear envelope disappears. Chromosomes condense into visible chromosomes. Chromosomes continue to shorten and migrate toward equatorial plane Chromosomes line up along the equatorial plane of the cell. (a) INTERPHASE EARLY PROPHASE LATE PROPHASE METAPHASE

Fig right ANAPHASE Sister chromatids separate at their centromeres, and one group of chromosomes moves toward each pole. TELOPHASE Two sets of chromosomes are at opposite poles of cell. Nuclear envelope reforms. Cytokinesis usually follows. INTERPHASE The daughter cells are genetically (and usually physically) identical to the parent cell, except for size.

Where does mitosis occur? Everywhere (in eukaryotes) Everywhere (in eukaryotes) Growth and development of most of body takes place by mitotic divisions Growth and development of most of body takes place by mitotic divisions Only exception is in germ line cells, where meiosis can take place for production of gametes (note: germ line cells can still undergo mitosis prior to meiotic division) Only exception is in germ line cells, where meiosis can take place for production of gametes (note: germ line cells can still undergo mitosis prior to meiotic division)

Sex and cell division Sexual eukaryotic organisms combine their DNA to produce progeny Sexual eukaryotic organisms combine their DNA to produce progeny To do this they must prepare their DNA for combination with the DNA of another individual To do this they must prepare their DNA for combination with the DNA of another individual The process used to prepare the DNA is called meiosis (contrast with mitosis) The process used to prepare the DNA is called meiosis (contrast with mitosis) –Halves the ploidy level in gametes (fertilization restores original ploidy) –Generates new chromosomal genotypes via recombination  increased genetic variation in offspring

My Kids: A tribute to meiosis!

Meiosis does NOT happen the "easy" way you might think it does (Just skip synthesis, right?) (Just skip synthesis, right?) WRONG!  WRONG!  Two cell divisions Two cell divisions –Meiosis I is the reduction division (2n  n) –Meiosis II separates sister chromatids

Premeiotic interphase: DNA replicates Premeiotic interphase: DNA replicates Prophase I: Chromosomes condense; synapsis and crossing over take place Prophase I: Chromosomes condense; synapsis and crossing over take place Metaphase I: Tetrads move to the equator Metaphase I: Tetrads move to the equator Anaphase I: Homologs separate Anaphase I: Homologs separate Telophase I: Nuclear envelope reforms; cytokinesis Telophase I: Nuclear envelope reforms; cytokinesis Interkinesis: No DNA synthesis; otherwise like interphase Interkinesis: No DNA synthesis; otherwise like interphase Meiosis I

Fig Tetrads: Paired replicated homologs held together by a synaptonemal complex of cohesin proteins Still unknown how tetrads form and dissociate

Second Meiotic Division The second meiotic division (Meiosis II) proceeds exactly as if it were mitosis: The second meiotic division (Meiosis II) proceeds exactly as if it were mitosis: –Prophase II Chromosomes condense again Chromosomes condense again –Metaphase II Chromosomes move to equator, with centromeres lined up on equator Chromosomes move to equator, with centromeres lined up on equator –Anaphase II Sister chromatids separate Sister chromatids separate –Telophase II Nuclear envelopes reform Nuclear envelopes reform –Haploid (1n) daughter cells – gametes Now ready for fertilization process Now ready for fertilization process

Tetrad Fig 9-12 (in part)

Crossover & Chiasmata The region of crossover forms an X- shaped structure. The region of crossover forms an X- shaped structure. This X-shape is called a chiasma (from the Greek; plural = chiasmata) This X-shape is called a chiasma (from the Greek; plural = chiasmata)

Homologs Sister chromatids

Recombinants Parentals Figure 9-12, Meiosis II

Mitosis & Meiosis Compared Mitosis: 1 replication, 1 nuclear division Mitosis: 1 replication, 1 nuclear division 2 daughter cells 2 daughter cells Chromatids separate Chromatids separate Singlet chromosomes produced Singlet chromosomes produced Ploidy retained Ploidy retained Meiosis: 1 replication, 2 nuclear divisions Meiosis: 1 replication, 2 nuclear divisions 4 daughter cells4 daughter cells Synapsis, tetrads;Synapsis, tetrads; crossing over  recombination Homologs separate in meiosis I, halving ploidyHomologs separate in meiosis I, halving ploidy Chromatids separate in meiosis II, which is similar to mitosisChromatids separate in meiosis II, which is similar to mitosis HHHHHHHHHHHHHHHHHHHHH

Review – Meiosis and Mitosis New and review terminology New and review terminology Structure of chromosomes Structure of chromosomes DNA structure, replication and mutation DNA structure, replication and mutation Mitosis – a conservative division Mitosis – a conservative division Meiosis – a reduction division Meiosis – a reduction division –Meiosis and sex –Meiosis and Mendel’s laws Linkage and sex linkage Linkage and sex linkage