Meiosis to Mendel Chapter 9 Read first few sections.
Reproduction • Asexual reproduction – mitosis • Produces clones – genetically identical individuals • What would happen if the environment changed? • Genetic variability helps the population as a whole survive changes
WHY SEX ? • Sexual reproduction combines the DNA from two different individuals • A gene is a section of a chromosome that carries instructions for a specific trait (protein) • An allele is a different version of a gene • The greater the number of different combinations of genes the more variation among individuals, and the greater the chance of survival of the species.
Sex cell formation • Sexually reproducing organisms need to produce specialized reproductive cells or gametes. • Produced from germ cells in organs called gonads • In females ovaries produce eggs or ova • In males testes produce spermatozoa • A mutation in one of your cells won’t be passed on unless it is in the germ line cells
Diploid vs. Haploid In animals, most plants, and many other organisms, each cell contains 2 sets of chromosomes. This is the diploid (2n) number of chromosomes. A pair of homologous chromosomes, one from each parent. Gametes contain only one member of each pair or the haploid (n) number of chromosomes
When a sperm and egg combine in the process of fertilization, or syngamy, a new diploid cell or zygote is formed. • The gametes must be haploid, that is, have n number of chromosomes. If the gametes were diploid, every generation would have twice the number of chromosomes. • To form haploid gametes, there needs to be a process other than mitosis – this is called meiosis.
Meiosis • This is a two part process : meiosis I and meiosis II, features TWO cell divisions. • However, the DNA is only replicated once • Meiosis I and II both use the same four stages of mitosis: prophase, metaphase, anaphase and telophase
Major differences between mitosis and meiosis • Meiosis has two cell divisions. • Meiosis has a step where homologous chromosomes swap pieces • Increases genetic variation, new combos. • In the first division, sister chromatids don’t split apart! Homologs separate instead. • In the second division, sister chromatids separate like in mitosis.
During prophase I the homologous chromosomes pair up in synapsis. This is the longest phase of meiosis. Crossing over may occur further increasing genetic variation.
Homologs don’t hang together • In metaphase I the tetrads migrate to the center of the cell. • In Anaphase I the centromeres do not break and one member of each homologous pair (2 sister chromatids) move to opposite ends of the cell • Which homolog goes to which end of the cell occurs at random. • Telophase occurs as in mitosis.
Meiosis II • There is no replication of chromosomes between telophase I and prophase II • Meiosis II proceeds just like mitosis – during anaphase the centromeres break and the two sister chromatids go to opposite poles.
With only 2 pairs of chromosomes, a gamete could have any one of 4 different combinations
Cytokinesis • Varies by which type of cell is being made • If we are producing sperm, each of the four cells produced by meiosis II become sperm.
If we are making ova, cytokinesis is uneven and one cell takes nearly all the cytoplasm, leaving the other cell merely a package of discarded DNA called a polar body. • In humans, the cell again divides unevenly, so at the end of meiosis II we have formed one ovum (egg) and three polar bodies. The polar bodies disintegrate.
The average woman produces one ovum every 28 days • Males produce 300 million sperm/ day • If less than 20 million / ml, a man is considered infertile. • Fertilization is a group effort, but only one sperm penetrates the ovum.
Sometimes things go wrong. • In anaphase I the separation of homologous chromosomes is called disjunction. • When they do not separate it is called nondisjunction and the resulting gametes contain one too many or one too few chromosomes. • Fertilization results in a zygote with 45 or 47 chromosomes. This is an aneuploid (vs. euploid) number of chromosomes
Chromosomal Abnormalities per 100,000 recognized human pregnancies 15,000 85,000 spontaneous abortions live births --------------------------------------------------------------------------------------------------------- Trisomy A Group 1 0 0 2 159 0 3 53 0 B Group 4 95 0 5 0 0 C Group 6-12 561 0 G group 21 350 113 22 424 0
Three copies of a chromosome is called trisomy – Down syndrome is trisomy 21 • A zygote with one too few chromosomes does not usually develop. (monoploid) • Extra copies of the sex chromosomes (vs. the autosomes) do not cause as much of a problem • XX is female in humans (male in birds) • XY is male in humans • XXY , XYY, XXX, XO all happen • Plants do much better with multiple chromosomes
15,000 85,000 spontaneous abortions live births sex chromosomes XYY 4 46 XXY 4 44 XO 1350 8 XXX 21 44 Translocations Balanced 14 164 Unbalanced 225 52 Polyploid Triploid 1275 0 Tetraploid 450 0 Other (mosaics, etc) 280 49 Total 7500 550
Genetics • A gene is a section of DNA that codes for : • Proteins – for structures such as muscles - or for enzymes • RNA molecules like rRNA and tRNA • Genes have sections of DNA next to them that control whether they get used or not: regulatory regions
DNA is like magnetic recording tape; song information is encoded in the structure of the tape magnetically. Many different songs are recorded and each one can be played individually (or not played). DNA is very long and thin like recording tape. DNA is like digital media also, because each song can be specifically accessed at the beginning.
Each cell of an organism that reproduces sexually has two copies of each chromosome, and therefore has two copies of every gene – one on each member of each pair of chromosomes(exception is the Y chromosome, which is smaller than the X). • The two versions of each gene are called alleles. Alleles may be the same or different, depending on the traits of the parents.
All the genes that are contained on all the chromosomes of an individual make up that individual’s Genome. • The Genotype is all the genetic information the individual has for a particular trait.
Not all genes are expressed. • Those traits that are expressed: can be seen (physical traits) or measured (chemical traits) are the individual’s phenotype. • The phenotype is influenced by both the genotype and the environment. • In diploid organisms (like us) an allele from one parent can mask the appearance of the other. So often genotype ≠ phenotype. • We can still pass on a hidden trait and see it in our children.