Sexual Cell Reproduction Meiosis Sexual Cell Reproduction

Sexual Cell Reproduction All living cells arise from pre-existing cells, tracing the lineage of every living things back to some primordial ancestor. If mitosis were the only process involved the production of new cells, then all cells would be exactly the same. Instead, there exists an incredible variety among organisms

Gametes and the Life Cycle of a Sexual Organism Egg cell The life cycle of a multicellular organism is the sequence of stages leading from the adults of one generation to the adults of the next Sperm cell Meiosis Fertilization Multicellular diploid adults (2n = 46) Diploid zygote (2n = 46) Mitosis and development Figure 8.13

DNA Sequence It is the sequence of the bases in a gene that holds the genetic code And therefore holds the code for all cellular processes and the variety of inherited traits we see in organisms

The cellular bases for sexual reproduction Meiosis The cellular bases for sexual reproduction -the mixing of genetic traits produces offspring different from each parent - Fertilization requires 2 gametes (egg and sperm) These gametes contain ½ the genetic information Each sperm or egg produced carries 1 of over 8 million possible combinations of parental chromosomes 2n 46 chrom 2n (46) 2n (46) n 23 n 23 n 23 n 23

Meiosis versus Mitosis 3 Important Differences: 1. Takes place in 2 stages involving 2 successive divisions 2. The chromosomes arrange themselves in homologous pairs (pair up with another chromosome of the same size and shape) 3. Therefore, the four daughter cells are not necessarily identical and have only ½ the number of chromosomes present in the original parent cell.

Meiosis The production of gametes with half the number of chromosomes as the original “parent” cell During this process specialized cells in the gonads (ovary & testes), produce sex cells that contain only one set of chromosomes. A human germ cell with 46 chromosomes will undergo meiosis and produce gametes that have 23 chromosomes

Meiosis The 46 chromosomes number is referred to as diploid and is written as 2n The 23 chromosomes number is referred to as haploid and is written as n Fertilization occurs when 2 gametes (sperm & egg) fuse, forming a diploid zygote (46 chromosomes) Zygote 46 chromosomes (diploid) Egg 23 chromosomes (haploid) Sperm 23 Chromosomes (haploid)

Each chromosome has a partner Keep in mind that the 23 chromosomes are not just any 23, but one member from each pair. Each of the 23 chromosomes that you receive from your father is matched by 23 chromosomes from your mother Example: Your father gives you a chromosome with genes that code for eye colour and so does your mother

Homologous Chromosomes The paired chromosomes are called homologous chromosomes Homologus Pairs = a pair of chromosomes that have similar lengths, shapes and carry genes controlling the same traits However each chromosome does not necessarily carry the same genetic information For example: both chromosomes in a homologous pair may carry a gene for eye colour but one may code for blue and the other for brown eyes Each cell produced during meiosis contains one member from each pair of homologous chromosomes After DNA replication

Phases of Meiosis Meiosis I Interphase Prophase I Metaphase I Anaphase I Telophase I Meiosis II Prophase II Metaphase II Anaphase II Telophase II

Meiosis Meiosis involves 2 cell divisions that produces 4 haploid cells To keep things simple, in our example we will use cells that contain 2n=4 chromosomes. Therefore, the cell will contain two pairs of homologous chromosomes.

Interphase As in mitosis, interphase (cell growth and DNA replication), must occur before cell can replicate Very important = interphase occurs before prophase I but not before prophase II Chromatin Centrioles

Interphase: DNA replication of homologous pairs

Prophase 1 Chromatin coils tightly to form chromosomes - DNA has already replicated Homologous chromosomes pair up side by side in a process called synapsis. - When 2 homologous chromosomes are paired, the structure is called a bivalent ( = 2 chromosomes) Paternal Homolog Maternal homolog

Prophase 1 3. Chromosomes shorten & thicken - the homologous chromosomes (bivalents) pair up to form tetrads. Each tetrad contains 2 homologous chromosomes and 4 chromatids Tetrad

Crossing Over As the homologous chromosomes come close together, they often intertwine Sometimes chromatids break & exchange segments. This process is called crossing over The area(s) where the chromatids overlap is called a chiasma Here homologous chromosomes exchange genetic material

Crossing Over This leads to enormous genetic variation even between siblings There are over 8 million possible combinations of parental chromosomes 5. The chiasma begins to disappear The nuclear membrane disintegrates **In females this is where meiosis stops until puberty (Prophase I)

Prophase I : 2n = 6 or n=3 Draw a germ cell in prophase I Label: Homologous chromosomes & tetrad Nuclear membrane Centrioles, astral rays Homologous pair Tetrad

Metaphase I The tetrads line up on the equatorial plate Metaphase plate The tetrads line up on the equatorial plate The homologous pairs line up randomly There is a 50-50 chance for the daughter cells to get either pair from each chromosome. Centriole You could get this one or that one

Spindle fibres attach to the centromeres of the chromosomes Each pair of sister chromatids from each homologous chromosome is ready to move to opposite poles of the cell Draw a diagram

Anaphase I Homologous chromosomes separate and move to opposite poles This process is known as segregation – the separation of paired genes At this point, reduction division has occurred! Each chromosome remains double stranded

Draw a diagram

Telophase I Cytoplasm divides = Cytokinesis A nuclear membrane begins to form Chromosomes unwind into chromatin The two new daughter cells contain 2 chromosomes each. Two haploid cells (cells with half the chromosome number) start to form. Chromosomes are half the number, but still double stranded.

Draw a diagram Label the following: 1. Cleavage furrow 2. Daughter cells 3. Nuclear membrane 4. Chromatin

Meiosis I versus Mitosis The first division is different from mitosis because the daughter cells are not identical Each daughter cell contains 1 member of the chromosome pair Remember homologous pairs are similar but they are not identical

Meiosis II ****The short phase between Meiosis I and II is referred to as interkinesis There may or may not be an interphase II depending on species The next set of cell divisions will separate the chromatids Begin with the 2 daughter cells from meiosis I DNA replication does NOT occur Nuclear membrane dissolves, spindle fibres form Prophase II

Draw a diagram

Metaphase II The chromosomes , each with two sister chromatids, align on the equatorial plate The centromeres attach to spindle fibres

Anaphase II The centromeres separate and chromatids move towards opposite poles

Telophase II 4 new cells will be formed Each of the new cells will contain only one member from each homologous pair = haploid (n) The parent cell had 6 chromosomes; the daughter cells have 3 chromosomes each

Meiosis Summary The haploid cells complete the meiotic cycle and differentiate to produce gametes (egg & sperm) Important to remember: -Meiosis I is referred to as reductional division because the chromosome number is reduced by half - Meiosis II is called equational division and is similar to mitosis as centromeres on sister chromatids separate and chromosome number remains unchanged

MEIOSIS I: Homologous chromosomes separate INTERPHASE PROPHASE I METAPHASE I ANAPHASE I Centrosomes (with centriole pairs) Sites of crossing over Metaphase plate Sister chromatids remain attached Spindle Nuclear envelope Chromatin Sister chromatids Tetrad Homologous chromosomes separate

Meiosis II TELOPHASE I & CYTOKINESIS TELOPHASE II & CYTOKINESIS MEIOSIS II: Sister chromatids separate TELOPHASE I & CYTOKINESIS TELOPHASE II & CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II Sister chromatids separate Haploid daughter cells forming