• Cell Division: reproduction of cells

  • a highly regulated process that distributes and monitors the integrity of genetic material

• Two different sorting processes

  • Mitosis: allows two daughter cells to receive same amount of genetic material

  • Meiosis: cells have two chromosomes, but daughter cells have one set

14.1 Eukaryotic cell cycle

• Cell Cycle: a series of events that leads to cell division

  • highly regulated process

Chromosomes are inherited in sets and occur in homogenous pairs

• Karyotype: photographic representation of chromosomes

  • reveal number size & form of chromosomes

• Sister Chromatids: two copies that are still joined together

• Chromosomes appear in pairs and sets

  • each set is composed of several chromosomes

    • humans have 23 types of chromosomes

• Chromosomes are numbered by size

  • does not apply to sex chromosomes

    • those are referred to as X and Y

• all other chromosomes are called autosomes

• Most cells have two sets of chromosomes

  • humans have 46 chromosomes

• Diploid: cells that carry two sets of chromosomes

• Gametes: sperm and egg cells

  • called haploid

• When an organism is a diploid, pair of chromosomes are called homologs

  • nearly identical in size and composition of genetic material

Cell Cycle is a series of phases that lead to cell division

• Cell cycle involves growth, replication, and division

• Four phases

  • G₁ phase

  • S Phase

  • G₂ phase

  • M Phase

• G₁ and G₂ phase indicate periods between DNA synthesis (S phase) and mitosis (M phase)

• Interphase: G₁, S, and G₂ phase where cells grow and copies chromosomes in preparation for cell division

• G₀: when cells exit the cell cycle

G₁ Phase

• When a cell is committed to divide

  • Cell Growth occurs during this stage

  • Longest phase of Mitosis

S Phase

• Chromosomes are replicated forming pairs of sister chromatids

  • cells have twice as many chromatids

G₂ Phase

• Cell synthesizes proteins necessary for chromosome sorting & cell division

M Phase

• First part is called Mitosis

  • cell nucleus divides into two nuclei

  • pairs of chromatids are separated and sorted, so each daughter cells receive 46 chromosomes

• Second part is cytokinesis

  • division of cytoplasm to produce two distinct daughter cells

• Shortest phase of mitosis

Cyclins and Cyclin dependent kinesis

• Proteins that are responsible for advancing a cell through cell cycle

• cdk must bind to cyclins in order to work

• Four steps

  • during G₁, cyclin increases in response to sufficient nutrients and growth factor

  • G₁ Cyclin binds to cdk, to form activated G₁/cdk complex

    • cdk acts as a protein kinase, creating proteins needed to advance to next cell phase

  • After cell passes into S phase, G₁ Cyclin degrades

  • Mitosis cyclin accumulates late in G₂ Phase

• Checkpoints: critical regulatory points

  • G₁ Checkpoint: checks if conditions are favorable for cell division, also senses in DNA has incurred damage

  • G₂ Checkpoint: Checks for DNA damage and ensures that all SNA has been replicated

  • Metaphase Checkpoint: monitors integrity of spindle apparatus which aids in chromosome sorting

14.2 Mitotic Cell Division

• When cell divides into two identical cells

• Asexual Reproduction: offspring from a single parent

Chromosome are replicated & compacted to produce sister chromatids

• Two sister chromatids are associated at a region called the centromere

  • attachment site for Kinetochore

    • structure needed for sorting chromosomes

Spindle Apparatus

• Composed of microtubules

  • growth starts at two centrosomes

• Each centrosomes defines a pole

  • has a pair of centrioles

Transmission of Chromosomes

• Mitosis occurs in phases known as prophase, prometaphase, metaphase, anaphase, telophase

• Interphase:

  • chromosomes are replicated, but not condensed

• Prophase:

  • chromosomes are replicated to produce 12 chromatids

    • as it proceeds, nuclear envelope dissolves into vesicles

  • Chromosomes are now condensed

• Prometaphase:

  • Spindle Apparatus is fully formed

    • centrosomes move apart

      • spindle fibers interact with sister chromatids

  • Microtubules are rapidly formed, making contact with kinetochore

• Metaphase

  • pairs of sister chromatids are aligned on metaphase plate

• Anaphase

  • connections between pairs of sister chromatids are broken

    • chromatids are now chromosomes liked to one of the two poles

• Telophase

  • Chromosomes reach their respected poles and decondense

    • nuclear envelope reforms to produce two separate nuclei

• Cytokinesis

  • Follows mitosis

  • involves formation of cleavage furrow

    • most apparent in telophase

      

14.3 Meiosis

• The process of a diploid cell forming haploid cell

  • formation of gametes

Meiosis I separates Homologous Chromosomes

• Prophase I

  • sister chromatids associate with each other, lying side by side to form bivalents

• Prometaphase I

  • nuclear envelope becomes vesicles, spindle apparatus is formed

    • chromatids attach to one pole, not two

• Metaphase I

  • bivalents are organized onto metaphase plate

• Anaphase I

  • segregation of homologs

    • connection between bivalents break, but sister chromatids are still together

    • chromatid migrates to one pole

• Telophase I

  • Sister chromatids reach respective poles and decondense

    • nuclear envelope reforms to create two nuclei

Meiosis II Separates sister chromosomes

• Occurs after cytokinesis

• Exactly the same process of Mitosis

  

14.4 Sexual Reproduction

• The process in which two haploid gametes unite to form diploid cell called zygote

  • zygote grows and divides by mitotic cell divisions

• Asexual reproduction is when offspring are produced from a single parent

Promotes greater genetic diversity in offspring

• has some disadvantages, costs more energy than asexual reproduction

• But has some advantages

  • allows for genetic variation

  • Allows for more rapid adaptation

  • Disease resistance

Sexually Reproducing species produce haploid and diploid

• the sequence of events that produces another generation of organisms known as life cycle

• Diploid Dominant species

  • most species are diploid, their haploid gametes are considered specialized

• Haploid-Diploid species

  • most fungi and some protists

    • only their zygote is a diploid

• Alternation of generations

  • when organisms switch between diploid dominant and haploid dominant

  • Usually plants and some protists

14.5 Variation Chromosome structure and number

• Generates know that variations in chromosome structures and number has major effects on organisms

Natural Variation exists in chromosomes structure and number

• chromosomes composition within given species is relatively constant

• Chromosomes vary in size and shape

  • identifiable during metaphase

• depending on size, location of centromere, and banding patterns

• Four classification types

  • Metacentric

    • centromere near center

  • Submetacentric

    • centromere off center

  • Acrocentric

    • centromere near one end

  • Telocentric

    • centromere at the end

• Since chromosomes often have similar sizes and centromere location, they are further identified via banding

Mutations can alter chromosome structure

• chromosomal mutations are characterized deletion, duplications, inversions, and translocation

• deletion and duplication alter total amount of genetic material

  • Detection: segment of chromosomal material is removed

  • Duplication: section of chromosome is repeated

    • detrimental

• Inversions and translocation rearrange chromosomes

  • Inversions: change in direction of genetic material

  • Translocation: when one segment of chromosome attaches to another chromosome

    • Simple translocation: single piece attaches to another chromosome

    • Reciprocal Translocation: two different types exchange pieces

Variation occurs in number sets and number of individual chromosomes

• Organisms that are euploid have chromosomes that occur in one or more complete sets

• Polyploid: organisms with three or more sets of chromosomes

• Aneuploidy: alteration in an number of particular chromosome

  • variations in chromosome number can have a significant effect on characteristics of plants and animals

• Nondisjunction: an event in which chromosomes do not separate properly during cell division

  • can occur during Meiosis I or Meiosis II

Consequences of Chromosome Change

• Changes in number of animals

  • usually severe consequences

    • Polyploidy in mammals lead to lethal conditions

  • Alterations in aneuploidy can lead to human disorders

    • early miscarriages

  • several human disorders are due to abnormalities in chromosome number

    • occurs in trisomy of chromosomes

  • Most well known is Down Syndrome

    • Has three copies chromosome 21

    • More likely occur when woman are older

Changes in Plants

• Plants commonly exhibit polyploidy which is necessary

• aneuploidy is bad for all eukaryotic species