Presentation on theme: "Human Genetics Mitosis and Meiosis. Chromosomes and Cell Division How are Chromosomes replicated? Cell Division: Why are there two types: mitosis."— Presentation transcript:
Chromosomes and Cell Division How are Chromosomes replicated? Cell Division: Why are there two types: mitosis and meiosis? How are they different? How does meiosis produce new combinations of genes?
What is a Chromosome? It is a linear strand of DNA in combination with nuclear proteins. We call this complex of DNA and proteins chromatin. It is a linear array of genes As a set - they are our genome
Chromosomes exist in different states As visible structures seen during cell division. Most of the time they are not visible because they exist as uncoiled threads of chromatin distributed through out the nucleus.
Chromosome Basics The number of chromosomes present within the nucleus is a characteristic of the species. Chromosomes of humans and most other eukaryotic species occurs in pairs. Members of a chromosome pair are known as homologs. Chromosomes in a pair are homologous because they have the same genes (coding for the same proteins) but not always the same alleles.
Haploid vs. Diploid The state in which there are two copies of each chromosome present is known as diploid. Haploid – only one copy of a chromosome is present (half of the diploid). This number is referred to as n. All of the somatic cells in our bodies are diploid cells. Therefore, they are 2n. The only haploid cells in our bodies are our gametes – eggs and sperm.
Different organisms have different chromosome numbers
Human somatic cells have 46 chromosomes Diploid number (2n) = 46 23 from mom - 23 from dad 22 pairs are autosomes –true homologous pairs 1 pair is not necessarily homologous - Sex chromosomes Females are XX Males are XY – a non homologous pair.
Mitosis Division of cytoplasm or cytokinesis occurs. Purpose: to make copies of cells and their DNA Sister chromatids separate and move to opposite poles. Nuclear membranes form around each new nucleus. Replicated chromosomes align at the metaphase plate.
Prophase Replicated chromosomes condense. Microtubules organize into a spindle
Metaphase Chromosomes line up on the metaphase plate. Spindle microtubules are attached to centromeres of chromosomes
Anaphase Centromeres of sister chromatids separate Chromosomes move to opposite ends of the cell
Telophase Nuclear membranes form Spindle disappears Division of cytoplasm occurs (cytokinesis)
Cytokinesis Cytoplasmic division occurs after nuclear division is complete. Two cells are formed. Cells enter G 1
Cell Cycle Control Checkpoint proteins monitor progression through the cell cycle.
Cell death Apoptosis is the process of cell death in which cells signal their own demise in a programmed way Development Response to abnormality
How Apoptosis Works Signal molecule binds to death receptor. Caspase enzymes destroy proteins. Cells fragment. Phagocytes engulf cell remnants..
Apoptosis is critical for regulation of development in embryos. Different amounts of apoptosis create feet with digits versus webbing.
Mitosis A form of cell division that produces two daughter cells, each having the same genetic complement as the parent cell. 2n --- briefly 4n (anaphase) ------2n Identical chromatids (sister chromatids)are separated and then segregated to different daughter cells. Occurs in the somatic cells of our bodies
Meiosis Specialized form of cell division with two successive rounds of cell division following DNA replication Produces haploid cells (n) Start with 46 double stranded chromosomes (2n) After 1 division - 23 double stranded chromosomes (n) After 2nd division - 23 single stranded chromosomes (n) Occurs in our germ cells cells that produce our gametes egg and sperm
Why do we need meiosis? - to bring two haploid gametes together to form a diploid zygote. n (mom) + n (dad) = 2n (offspring) It is the fundamental basis of sex. What is the purpose of sex?
Chromosome Replication Replication is the process of duplicating a chromosome Occurs prior to division Replicated copies are called sister chromatids Held together at centromere
Why form gametes? Goal: Reduce genetic material by half from momfrom dadchild meiosis reduces genetic content too much!
Meiosis:cell division in two parts Result: one copy of each chromosome in a gamete.
Meiosis I : the reduction division Prophase I (early) (diploid) Prophase I (late) (diploid) Metaphase I (diploid) Anaphase I (diploid) Telophase I (diploid) Nucleus Spindle fibers Nuclear envelope
Homologs separate in meiosis I and therefore different alleles separate. homologs same genes maybe different alleles sister chromatids same genes same alleles Gene X Result: Reduction in genetics of cell from two genomes from parents to one random genome from both parents
Prophase I Early prophase Homologs pair. Crossing over occurs. Late prophase Chromosomes condense. Spindle forms. Nuclear envelope fragments.
Metaphase I Homolog pairs align along the equator of the cell.
Anaphase I Homologs separate and move to opposite poles. Sister chromatids remain Attached at their centromeres.
Telophase I Nuclear envelopes reassemble. Spindle disappears. Cytokinesis divides cell into two.
Meiosis II:Only one homolog of each chromosome is present in the cell. Meiosis II produces gametes with one copy of each chromosome and thus one copy of each gene. Sister chromatids carry identical genetic Information for gene X Gene X
Meiosis II : the equational division Prophase II (haploid) Metaphase II (haploid) Anaphase II (haploid) Telophase II (haploid) Four nonidentical haploid daughter cells
Prophase II Nuclear envelope fragments. Spindle forms.
Metaphase II Chromosomes align along equator of cell.
Anaphase II Sister chromatids separate and move to opposite poles.
Telophase II Nuclear envelope assembles. Chromosomes decondense. Spindle disappears. Cytokinesis divides cell into two cells.
Results of meiosis Gametes Four haploid cells One copy of each chromosome One allele of each gene Different combinations of alleles for different genes along the chromosome
Meioisis vs. Mitosis MitosisMeiosis Number of divisions 1 2 Number of daughter cells 24 Genetically identical? YesNo Chromosome #Same as parentHalf of parent WhereSomatic cellsGermline cells WhenThroughout lifeAt sexual maturity RoleGrowth and repairSexual reproduction
Why do we have meiosis? To generate haploid gametes! To make new combinations of genes. How? Random (independent) assortment Recombination
Independent assortment The homolog of one chromosome can be inherited with either homolog of a second chromosome.
Maternal and Paternal chromosomes shuffle in your children
Independent assortment generates variation in offspring Since the combination of maternal and parental chromosomes received by a gamete is random; And we have 23 pairs of chromosomes; The possible combinations in an egg or a sperm are: 2 23 = 8,388,608 combinations in an offspring 2 23 X 2 23 = 70,368,744,177,664 Result: New combinations of genes (alleles) are generated when the genes are located on different chromosomes.
Crossing-over is: The physical exchange of chromosomal material between chromatids of homologous chromosomes. Result: Generation of new combinations of genes (alleles) if the genes are located on the same chromosome. Recombination generates variation