Textbook Notes (368,316)
Canada (161,798)
Biology (657)
BIOLOGY 1A03 (168)
Xudong Zhu (12)
Chapter 12

Textbook and Class Notes Collaborated - Week 5+6 - Unit 3 - Chapter 12 Bio 1A03
Premium

9 Pages
79 Views
Unlock Document

Department
Biology
Course
BIOLOGY 1A03
Professor
Xudong Zhu
Semester
Fall

Description
Unit Three: Gene Structure and Expression Chapter 12: Meiosis  Ultimate explanation – explains why something happens; evolutionary nature  Proximate explanation – explains how something happens; mechanic in nature  Sexual reproduction allows for genetic variation  A sperm (male reproductive cell) and an egg (female reproductive cell) unite in a process called fertilization  Meiosis – nuclear division that leads to a halving of chromosome number o Formation of gametes (egg and sperm cells) o Occurs in sexually reproducing individuals 12.1 How Does Meiosis Occur?  Gamete – a haploid (n) reproductive cell that has one set of chromosomes  Fertilization – occurs during the fusion of two gametes (one from each parent)  Sexual Reproduction in Humans o Brings 2 gametes (one from the male and one from the female) together o 2 parents produce offspring (progeny) o parents have 2n (diploid) nuclei o gametes are n (haploid) and differ one from another in gene composition o gametes have half the DNA content of each parent nucleus (n is ½ of 2n) o each parent passes on have of its genes to its offspring o offspring inherit a unique combination (mixture of genes from both parents  genetic variation  Chromosomes o Composed of DNA and proteins o Carry genes (hereditary information) o Homologs/Homologous Chromosomes – chromosomes of the same type size and with the same genes at identical locations  Carry the same genes but may carry different alleles  Genes o Units of heredity o Made of DNA (nucleic acid) located on chromosomes o Have specific sequences of nucleotides (monomers) o Most genes program cells to synthesize proteins o The actions of these proteins produce the organisms inherited traits  Alleles – are the different versions of a specific gene  Sex Chromosomes – are chromosomes that determine an individuals sex o Female (human) have XX o Male (human) have XY  Autosomes – are non-sex chromosomes  Haploid Number and Ploidy o Haploid number “n” – the number of distinct types of chromosomes in a given cell o Ploidy – the combination of the number and n o Diploid (2n) – when an organisms cells contain two versions of each chromosome o Haploid (n) – when an organisms cells contain one version of each chromosome  Do not contain homologous chromosomes; have one allele of each gene  Karyotype – the number and type of chromosomes present; a display of an individuals chromosomes that is organized in terms of chromosomes and type o Used to organize chromosomes by length o Currently organize chromosomes by number o Chromosomes are only readily visible when they are condensed during mitosis and meiosis  Obtain cells during cell division o Collect cells and grow the cells in culture o Treat cells with colchicine, which disrupts the formation of the mitotic spindle, stopping mitosis at metaphase  Chromosomes are easily studied because they are condensed and consist of sister chromatids o Chromosomes are then stained and examined with a light microscope o Karyotypes are useful for genetic screening to identify specific chromosomal defects in their number, size and type o Researchers can distinguish condensed chromosomes by size, position of the centromere and by striping or banding patterns that appear in response to some stains  Banding Patterns – Locus – positions of a gene alone a chromosome (or along a DNA double helix) o Spectral Karyotyping (SKY) or Chromosome Painting  Done with fluorescent dyes that are attached to short DNA molecules  The dyed pieces of DNA bind to particular regions of particular chromosomes  By using a combination of dyes, technicians can give each pair of homologous chromosomes a distinctive suite of colors  Allows physicians to diagnose an array of chromosomal abnormalities  Figure 12.6a, page 266; Karyotype of Human Chromosome o Real World Examples  CML (Chronic Myelogenous Leukemia)  Translocation- swapping of chromosome segments  A large portion of Chromosome 22 switches with a small piece from Chromosome 9, easily recognizable shortened chromosome 22 (called the Philadelphia chromosome)  Is a cancer, which affects the cells that give rise to white blood cells  Figure 12.6b, page 266  Klinefelter syndrome  Males have an abnormal of sex chromosomes, XXY  Males have very small testes and are sterile  Frontal baldness absent  Fewer chest hairs  Female-type pubic hair pattern  Poor beard growth  Long legs  Males display feminine body contours (enlargement of breast tissue, narrow shoulders, wide hips)  Have normal intelligence  Occur in about 1 in 1000 male births  Figure 12.6c, page 266  Cri Du Chat Syndrome  Deletion on chromosome 5  Age range > 10 years old  1 in 50000  Abnormal glottis and larynx  Size of deletion on chromosome 5 influences physical and mental skills  Anatomical malformations, G1 and cardiac complications  Severly mentally challenged o Requires special care and schooling An Overview of Meiosis  Summary Table 12.2, page 260; Vocabulary  Before undergoing meiosis, cells replicate each of their chromosomes – at the start of the process, chromosomes are in the same state they are in prior to mitosis o Each chromosome contains two identical sister chromatids – contain the same genetic information and are physically joined at the centromere and their entire length o Unreplicated chromosomes consist of a single DNA molecule and its associated protein – single chromosome o Replicated chromosome consists of two sister chromatids –single chromosome  Meiosis consists of two cell divisions o Meiosis I – Diploid (2n) parent cell produces two haploid (n) daughter cells  Homologs from each pair of homologous chromosomes, separate and move to 2 different daughter cells  End result – daughter cells have one of each type of chromosomes instead of two – half as many chromosomes as parent cell o Meiosis II – equivalent to mitosis occurring in a haploid cell  Sister chromatids from each chromosome separate  Sister chromatids of each chromosome separate and move to each of the daughter cells  At end of meiosis II and cytokinesis, there are 4 haploid gametes each with one copy of each chromosome produces  End result – have one of each type of chromosomes, but not eh chromosomes are unreplicated o Figure 12.2 a and b, page 261; Meiosis  Two gametes fuse during fertilization forming a zygote, full complement of chromosomes is restored o Figure 12.2c, page 261; Meiosis  Gametogenesis – formation of egg or sperm cells  Maternal or paternal chromosomes – homologous chromosomes from mother or father (respectively)  Figure 12.3, page 262; Ploidy Changes during the Life Cycle of a Dog  Reduction Division o Reduces chromosome number from 2n (diploid) to n (haploid) o Daughter cells have half the number of parent chromosomes o Union of two haploid (n) gametes during fertilization restores the diploid (2n) number Phases of Meiosis I  The Phases of Meiosis I o Prophase I o Metaphase I o Anaphase I o Telophase I o Cytokinesis 1. Prior to Meiosis  During interphase, each chromosome in the 2n parent cell replicates 2. Early Prophase I o Nuclear envelope breaks down o Chromosomes condense o Spindle apparatus forms o Synapsis – replicated homologous chromosomes pair to form tetrads o Tetrad – consists of two homologous chromosomes, each consisting of two sister chromatids o Non-sister chromatids – chromatids from different homologs (one from mother, one from father) o Is precise o Homologous chromosomes align gene by gene o The exact mechanism which causes synapsis is still unknown, but it may involve a protein complex called the synaptonemal complex  When homologs synapse, two pairs of non-sister chromatids are brought close together and are held there by the synaptonemal complex (network of proteins)  Figure 12.7, page 267; Replication, Synapses and Crossing Over o Regions of homologous chromosomes that are similar at the molecular level attract one another via mechanisms that are being researched o Spindle fibers attach to the kinetochores at the centromeres of chromosomes 3. Late Prophase I o Non-sister chromatids begin to separate but remain joined at certain locations o Crossing Over (recombination) – maternal and paternal chromatids break and rejoin each chiasma, producing chromatids that have both paternal and maternal segments o Occurs between two non-sister chromatids o Chiasma (sing. abbreviated Xma) – one crossing over o Chiasmata (pl. abbreviated Xta) – several crossing over o Evidence for Crossing Over (Creighton and McClintocks Experiments)  Harriet Creighton and Barbara McClintock studied Zea mays (corn  Strain 1  Long chromosome 9, with knob  Kernels are colorless and waxy  Strain 2  Short chromosome 9, no knob  Kernels are colorless and starchy  Strain 1 x Strain 2 (strain 1 is crossed with strain 2)  Chiasmata form between non-sister chromatids  Some offspring have 1. Long chromosomes 9, no knob Kernels are colorless and waxy 2. Short chromosome 9, no knob Kernels are colored and starchy 3. Same as parental types  Conclusion o Physical exchange of DNA (knob) took place o They are displayed in phenotypes o Crossing over has occurred 4. Metaphase I o Tetrads line up along the metaphase plate by spindle fibres o Each tetrad moves to the metaphase plate independently of the other tetrads o The alignment of maternal and paternal homologs from each chromosome is random 5. Anaphase I o Homologs separate and start migrating to the opposite poles of the cell o Disjunctional segregation occurs during Anaphase I 6. Telophase I and Cytokinesis o Chromosomes finish their movement to the opposite sides of the cell o Cytokinesis divides the cytoplasm to form 2 haploid daughter cells  End Of Meiosis I o One chromosome of each homologous pair is distributed to a different dau
More Less

Related notes for BIOLOGY 1A03

Log In


OR

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


OR

By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.


Submit