Chapter I: Cellular Basis of Heredity Lecture 1 Chapter I: Cellular Basis of Heredity 

The cell The cell is the functional basic unit of life There are two types of cells: eukaryotic and prokaryotic. Prokaryotic cells are usually independent, while eukaryotic cells are often found in multicellular organisms. La cellule est l'unité fonctionnelle de base de la vie Anatomie des cellules Il existe deux types de cellules: les eucaryotes et procaryotes. Les cellules procaryotes sont généralement indépendants, tandis que les cellules eucaryotes sont souvent trouvés dans les organismes multicellulaires.

Organisms that reproduce Sexually are made up of two different types of cells. Somatic Cells are “body” cells and contain the normal number of chromosomes ….called the “Diploid” number (the symbol is 2n). Examples would be … skin cells, brain cells, etc. Gametes are the “sex” cells and contain only ½ the normal number of chromosomes…. called the “Haploid” number (the symbol is n)….. Sperm cells and ova are gametes.

Fertilization n=23 egg 2n=46 zygote The fusion of a sperm and egg to form a zygote. A zygote is a fertilized egg n=23 egg sperm n=23 2n=46 zygote

Chromosomes If an organism has the Diploid number (2n) it has two matching homologues per set. One of the homologues comes from the mother.… the other homologue comes from the father. Most organisms are diploid. Humans have 23 sets of chromosomes… therefore humans have 46 total chromosomes….. The diploid number for humans is 46 (46 chromosomes per cell).

Homologous Chromosomes (because a homologous pair consists of 4 chromatids it is called a “Tetrad”) eye color locus hair color Paternal Maternal

Humans have 23 Sets of Homologous Chromosomes Each Homologous set is made up of 2 Homologues.

In Humans the “Sex Chromosomes” are the 23rd set Sex Chromosomes The Sex Chromosomes code for the sex of the offspring. ** If the offspring has two “X” chromosomes it will be a female. ** If the offspring has one “X” chromosome and one “Y” chromosome it will be a male. In Humans the “Sex Chromosomes” are the 23rd set XX chromosome - female XY chromosome - male

Types of Chromosome Chromosomes are placed into broad categories depending on the position of the centromere. Metacentric: centromere in the middle, with arms of equal length. (A) Telocentric: centromere at one end, with only 1 arm. (D) Acrocentric: centromere near one end, with arms of very different lengths. (C) Sub-metacentric: centromere near the middle, with arms of slightly different lengths. (B)

Cell Division Mitosis & Meiosis

Phases of Mitosis

The Basic Phases of a Cell’s Life: Mitosis The Basic Phases of a Cell’s Life: Interphase Prophase Metaphase Anaphase Telophase Cytokinesis

Interphase Interphase – The Cell spends the majority of its life here, growing and functioning. During the S Phase of the Cell Cycle, the DNA replicates, in anticipation of Mitosis

The Cell begins the division process Prophase Prophase The Cell begins the division process The nucleolus disappears, The nuclear membrane breaks apart

Prophase 3. The chromosomes become visible 4. The spindle apparatus forms and attaches to the centromeres of the chromosomes

Metaphase During Metaphase the Chromosomes line up across center of the cell, also called the equator, or Metaphase plate. Spindle Fibers Equator, or Metaphase Plate Chromosomes

Anaphase In Anaphase the Chromatids that make up each Chromosome move apart and travel to opposite ends of cellular spindle Daughter Chromosomes Chromatid Chromosome

Telophase In Telophase an envelope surrounds each set of Chromatids to form new Nucleus and the Cytoplasm starts to divide Cleavage Furrow

Cytokinesis Cytokinesis takes place when the Cytoplasm divides and two cells with identical genetic material are formed Daughter Cells

Quick Review – Place Cells in Mitosis Order A B C D E

Interphase Quick Review: Identify What happens in each phase of Mitosis: Prophase Metaphase Anaphase Telophase

Meiosis Why We Are Who We Are

Diploid (2n)  Haploid (n) Meiosis is the process by which ”gametes” (sex cells) , with half the number of chromosomes, are produced. During Meiosis diploid cells are reduced to haploid cells Diploid (2n)  Haploid (n) If Meiosis did not occur the chromosome number in each new generation would double…. The offspring would die.

Meiosis Meiosis in males is called spermatogenesis and produces sperm. Meiosis is Two cell divisions (called meiosis I and meiosis II) with only one duplication of chromosomes. Meiosis in males is called spermatogenesis and produces sperm. Meiosis in females is called oogenesis and produces ova.

4 sperm cells are produced from each primary meiosis Spermatogenesis n=23 sperm haploid (n) meiosis II Secondary Spermatocyte 2n=46 human sex cell diploid (2n) n=23 meiosis I Primary Spermatocyte Secondary Spermatocyte 4 sperm cells are produced from each primary meiosis

Oogenesis *** The polar bodies die… only one ovum (egg) is produced from each primary oocyte.

Phases of Meiosis

Interphase I Similar to mitosis interphase. Chromosomes replicate (S phase). Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres. Centriole pairs also replicate.

Meiosis I (four phases) Cell division that reduces the chromosome number by one-half. four phases: a. prophase I b. metaphase I c. anaphase I d. telophase I

Prophase I Longest and most complex phase. 90% of the meiotic process is spent in Prophase I Chromosomes condense. Synapsis occurs: homologous chromosomes come together to form a tetrad. Tetrad is two chromosomes or four chromatids (sister and nonsister chromatids).

Prophase I centrioles spindle fiber aster fibers

Question 1: A cell containing 20 chromosomes (diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes? 10 chromosomes (haploid)

Question 2: A cell containing 40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes? 10 chromosomes (haploid)

Metaphase I Shortest phase Tetrads align on the metaphase plate. INDEPENDENT ASSORTMENT OCCURS: 1. Orientation of homologous pair to poles is random. 2. Variation 3. Formula: 2n Example: 2n = 4 then n = 2 thus 22 = 4 combinations

Metaphase I metaphase plate OR metaphase plate

Anaphase I Homologous chromosomes separate and move towards the poles. Sister chromatids remain attached at their centromeres.

Anaphase I

Telophase I Each pole now has haploid set of chromosomes. Cytokinesis occurs and two haploid daughter cells are formed.

Telophase I

Meiosis II No interphase II (or very short - no more DNA replication) Remember: Meiosis II is similar to mitosis

Prophase II same as prophase in mitosis

Metaphase II same as metaphase in mitosis metaphase plate

Anaphase II same as anaphase in mitosis sister chromatids separate

Telophase II

Meiosis Original Gamete Metaphase 1 Anaphase 1 Telophase 1 Cytokinesis 2 Daughter Cells Metaphase 2 Anaphase 2 Telophase 2 Cytokinesis – 4 Gametes

Mitosis vs. Meiosis Mitosis Meiosis Results in Cells are Occurs in 2 diploid cells (2n) 4 haploid cells (1n) Cells are Genetically identical Genetically different Occurs in Somatic (body) Cells Sex Cells