Lecture 12: Cell Division & Intro to Genetics PDF

Behavioural Biology Lecture 12: Cell Division & Intro to Genetics Today’s Lecture Get familiar with terminology Importance of Cell Division Human Lifecycle Mitosis Reading: Meiosis Chapter 19 Chromosomal disorders Introduct...

Behavioural Biology Lecture 12: Cell Division & Intro to Genetics Today’s Lecture Get familiar with terminology Importance of Cell Division Human Lifecycle Mitosis Reading: Meiosis Chapter 19 Chromosomal disorders Introduction to genetics  Nucleus Chromosomes are individual DNA molecules which are stored within the nucleus of our cells. Specific regions on chromosomes are called genes. Genes are instructions which determine all of  How many chromosomes do we have in our cells? Tissue Cells (ex. Muscle, bone, skin, fat...) Have 46 chromosomes (Two sets of 23 unique chromosomes, one set from each parent) Gametes (sperm and egg cells) Have only 23 chromosomes (all unique in kind) Karyotype A profile of a person’s chromosomes found within their cells. This karyotype is of a tissue cell, not a gamete, how did I determine this? Chromosomes are arranged from largest to smallest in size. Autosomal & Sex Chromosomes There are 22 unique Autosomal chromosomes and the Sex chromosomes. A tissue cell has 2 copies of each autosomal chromosome and 2 sex chromosomes which may be the same (female) or different (Male). A chromosom e pair Copy from mom Copy from dad Tissue cells have Homologous Chromosomes – 23 unique kinds of chromosomes – 2 sets (1 inherited from each parent) 1 2 3 4 5 6 7 8 ………………………..23 1 2 3 4 5 6 7 8……………………….. 23 Copies of the same chromosome are called homologous chromosomes because they share the same genes. Is this karyotype from a male or female? A tissue cell or gamete? How can you tell? Tissue cell of a male, because there is one X and one Y sex chromosome and 2 copies of all autosomal chromosomes. A Female Karyotype No Y chromosome 2 X chromosomes Cell Division The continuity of human life is dependent on cell division. Why do we need it? 1.Reproduction: Having children 2.Growth & Development: Get bigger and mature into an adult. 3.Renew & Repair Tissues: old and damaged cells need to be replaced. Human Life Cycle The human life cycle requires 2 different types of cell division: 1. Mitosis: responsible for growth, development, renewal & repair. 2. Meiosis: responsible for creating our gametes (sperm and eggs) for reproduction.  Conception Egg cell Sperm cell with 23 chromosomes nucleus One sperm cell Egg cell with 23 chromosomes enters the egg cell and their nuclei fuse together forming Zygote with fused nucleus one nucleus including 46 chromosomes containing both sets of chromosomes. - 23 from the sperm cell and 23 from the egg cell. This new cell is called a Zygote. Growth of the Zygote The zygote grows by increasing its number of cells through repeated cell divisions. The initial zygote cell divides creating 2 cells, these 2 cells divide to create 4 cells, these 4 cells divide to create 8 cells……. All of the chromosomes in the nucleus must be copied exactly before each cell division, ensuring that all new cells have a complete set of all the chromosomes. Chromosome Replication Each of the 46 chromosomes in your cells must be copied before the cell can divide. The identical copies of a chromosome remain attached at a midpoint. Together they are called a replicated chromosome. These replicated chromosomes will be Mitosis Creates 2 daughter cells that are genetically identical to the original parent cell What types cells undergo mitosis when we are adults? Stem Cells Example: skin stem cells undergo repeated mitotic cell divisions to replace cells that get sloughed off on the surface of our skin. Whereas, once brain development is complete a mature neuron will never undergo a mitotic cell division. Meiosis = Making Gametes Gametes (sperm and egg) are required for sexual reproduction. Sexual Reproduction Contribution of genetic material from 2 individuals is required. Although a child shares genetic traits with their parents and siblings each one has a unique assortment of chromosomes. Why is meiosis necessary? Tissue cells contain 46 chromosomes If eggs and sperm also had 46 chromosomes: egg with 46 chromosomes + sperm with 46 chromosomes = child with 92 chromosomes/cell Number of chromosomes / cell would double each generation! Job of Meiosis: Convert stem cells that have 2 sets of chromosomes (46) in the ovaries or testes into gametes (egg or sperm) with only 1 set of chromosomes (23). Stem Cell Sperm or Egg Cell Meiosis At the end of meiosis, 4 daughter cells have been produced, each with half the number of chromosomes of the original mother cell. Meiosis in males vs females In females, there is unequal division of cytoplasm. Only 1 viable egg is produced instead of 4. Fill in the Summary of Differences Mitosis Meiosis 1. Involves __ cell 1. Involves __ cell division(s). division(s). 2. Produces ___ 2. Produces ____ daughter daughter cells each with cells each with ______ ____ chromosomes. chromosomes. 3. Daughter cells 3. Daughter cells genetically ___________. genetically __________. 4. Purpose: 4. Purpose: ___________________________ ___________________________ ___________________________ ___________________________ Chromosomal Disorders may result from errors occurring during meiosis Nondisjunction errors are commonly the cause. Essentially chromosomes do not separate properly during the process of meiosis which results in the When these abnormal gametes are formation of involved in fertilization it results in gametes with the a zygote with an abnormal number wrong number of of chromosomes. Down Syndrome Individuals with down syndrome have an extra copy of chromosome 21 due to a nondisjunction error that occurred during egg or sperm production (meiosis). Most common genetic disorder (around 1 in every 800 births) Individuals have distinct features and experience both physical and mental developmental delays. Females born with the wrong number of X chromosomes XXX- Triple X. Healthy. XO - Turner syndrome Only known viable monosomy in humans. Female sexual development occurs, but internal organs are underdeveloped resulting in sterility. Males with the wrong number of X or Y chromosomes OY- Not viable XYY- Male sexual development. Taller than usual. XXY - Klinefelter syndrome Born with male sex organs but testes small and individual is often sterile. At puberty may develop secondary female sex characteristics. What is Genetics? The Study of Heredity The chromosomes we inherit from our parents contain genes that determine our traits.  Recall: Genes The basic unit of heredity We have approximately 25 000 genes. A gene is a section of chromosome that carries the instructions to make specific proteins. Each protein will have a structural or functional effect within our bodies. Some genes play a role in determining our physical appearance, traits like eye colour, Recall: Homologous Chromosomes Human somatic cells have: – 23 unique chromosomes – 2 sets 1 2 3 4 5 6 7 8 ………………………..23 1 2 3 4 5 6 7 8……………………….. 23 Copies of the same chromosome are called homologous chromosomes because they share the same genes. Homologous Chromosomes are not genetically identical A pair of alleles They have the same genes, but may have variations in those genes. Different variations of genes are called alleles. Allele for blue eyes Allele for E.g. We all have a gene for eye colour, but brown variations in that gene give different eyes Dominance of Alleles We have 2 alleles for every gene because we have 2 copies of each A pair of alleles chromosome. So what happens if the 2 alleles for a particular gene are not the same? E.g. One allele for brown eye colour and one allele for blue eye colour.  Dominant and Recessive Alleles Dominant alleles for a gene are always expressed as a trait when present. Dominant alleles are always denoted with a capitol letter. E.g. Brown eye colour; E Recessive alleles for a gene are only expressed when an individual has 2 copies of it. Recessive alleles are denoted with a lower case letter. E.g. Blue eye colour; e  Genotype The genotype is the combination of alleles an individual has for a particular gene. here are 3 possible genotypes for a gene that has only 2 allel e.g. If eye colour is denoted by the letter E. Brown eye colour is dominant (E) and blue eye colour is recessive (e). Possible Genotypes: EE Ee ee Homozygous or Heterozygous When an individual’s genotype has 2 alleles the same it is called homozygous. E.g. Genotypes EE or ee for eye colour. When an individual’s genotype has 2 different alleles it is called heterozygous. E.g. Genotype Ee for eye colour.  Phenotype The phenotype for a gene is the trait that is expressed in the individual. E.g. Blue eyes or brown eyes. Genotype: EE Ee ee Phenotype: Which genotypes are homozygous and heterozygous Example: The dominant allele (F) for a gene causes pigment to be deposited in the skin in the form of freckles while the recessive allele (f) does not. More examples: