Transcript Document

Introduction
Stages of an Organism’s Life Cycle:
Development: All changes that occur from a fertilized
egg or an initial cell to an adult organism.
Reproduction: Production of offspring that carry
genetic information in the form of DNA, from their
parents.
Two types of reproduction:
1. Sexual Reproduction
2. Asexual Reproduction
Lecture 10
The Cellular Basis of Reproduction and
Inheritance
1. Sexual Reproduction
• Most common type of animal reproduction.
• Male and female gametes or sex cells (sperm and egg cell) join
together to create a fertilized egg or zygote.
• The offspring has genetic information from both parents.
• Offspring are genetically different from each parents and their
siblings.
Advantages:
Ensures genetic diversity of offspring.
Population more likely to survive changing environment.
Disadvantages:
Cannot reproduce without a partner of opposite sex.
Considerable time, energy, and resources spent to find a suitable mate.
Parents only pass on 1/2 (50%) of their genetic information to each
offspring.
2. Asexual Reproduction
• Production of offspring by a single parent through:
Splitting: Binary fission in bacteria.
Budding: Yeasts, plants
Offspring inherit DNA form one parent only.
Offspring are genetically identical to parent and siblings, unless
mutations occur.
Advantages:
• Can reproduce without a partner of opposite sex.
• Don’t spend time, energy, and resources to find a suitable
mate.
• Parents pass on 100% of their genetic information to each
offspring.
Disadvantage:
• No genetic diversity of offspring.
• Population less likely to survive changing environment.
Cells Only Arise from Preexisting
Cells
• New cells are made through cell division
• Unicellular organisms (Bacteria, protozoa): Division of
one cell into two new organisms through binary fission
or mitosis.
• Multicellular organisms (Plants, animals):
1. Growth and development from zygote or fertilized
egg. Original cell divides by mitosis to produce many
cells, that are genetically identical to first cell. Cells
later develop specific functions (differentiation).
2. Reproduction requires: Meiosis: Special type of cell
division that will generate gametes or sex cells, with
50% of individual’s genetic material.
Bacteria (Prokarytoes) Reproduce
Asexually by Binary Fission
• Features of Bacterial DNA
Single, relatively small circular chromosome
About 3-5 million nucleotide base pairs Contains only about
5-10,000 genes
• Binary fission
Single circular DNA is replicated
Bacterium grows to twice normal size
Cell divides into two daughter cells
Each daughter cell with an identical copy of DNA Rapid
process, as little as 20 minutes.
Bacteria Reproduce Asexually by Binary Fission
Eukaryotic Cell Division
• Eukaryotic cell division is more complex and time
consuming process than binary fission
• Features of Eukaryotic DNA
1. DNA is in multiple linear chromosomes. Unique
number for each species: Humans have 46
chromosomes. Cabbage has 20, mosquito 6, and fern
over 1000.
2. Large Genome: Up to 3 billion base pairs (humans)
Contains up to 50,000-150,000 genes Human genome
project is determining the sequence of entire human
DNA.
3. DNA is enclosed by nuclear membrane. Correct
distribution of multiple chromosomes in each daughter
cell requires a much more elaborate process than
binary fission.
Human Body Cells Have 46 Chromosomes
DNA: Found as Chromosomes or Chromatin
Chromosomes
Found only during cell division
Tightly packaged DNA
DNA is not being used
for macromolecule synthesis.
Chromatin
Found throughout cell cycle
Unwound DNA
DNA is being used
for macromolecule synthesis.
Eucaryotic Chromosomes Duplicate Before
Each Cell Division
Cell Cycle of Eucaryotic Cells
• The Cell Cycle – is the sequence of events from the time
a cell is formed, until the cell divides once again.
• Before cell division, the cell must:
a) precisely copy genetic material (DNA)
b) roughly double its cytoplasm
c) synthesize organelles, membranes, proteins, and
other molecules.
• The Cell cycle is divided into two main phases:
Interphase: Stage between cell divisions
Mitotic Phase: Stage when cell is dividing
Eucaryotic Cell Cycle:
Interphase + Mitotic Phase
The Life Cycle of a Eucaryotic Cell:
• Interphase: Time between cell divisions. Most cells
spend about 90% of their time in interphase. Cells
actively synthesize materials they need to grow.
Chromosomes are duplicated.
• Interphase can be divided into three stages:
1. G1 phase: Just after cell division. Cell grows in size,
increases number of organelles, and makes proteins
needed for DNA synthesis.
2. S phase: DNA replication. Single chromosomes are
duplicated so they contain two sister chromatids.
3. G2 phase: Just before cell division. Protein
synthesis increases in preparation for cell division.
Duplication of Chromosomes During S
stage of Interphase
DNA replication during
S stage of Interphase
Single chromosome
Two identical sister chromatids
joined by a centromere ( )
The Life Cycle of a Eukaryotic Cell
• Mitosis: The process of eucaryotic cell division. Most
cells spend less than 10% of time in mitosis.
• Mitosis is divided into four stages:
• 1. Prophase: Cell prepares for division.
• 2. Metaphase: Chromosomes line up in “middle” of
cell.
• 3. Anaphase: Sister chromatids split and migrate to
opposite sides of the cell.
• 4. Telophase: DNA is equally divided into two new
daughter cells. Cytokinesis usually occurs.
Cytokinesis: Division of cytoplasm.
• Mitotic Phase: Mitosis + Cytokinesis
Mitotic Phase: Mitosis + Cytokinesis
Mitotic Phase: Mitosis + Cytokinesis
• Cytokinesis The division of cytoplasm to
produce two daughter cells.
• Usually begins during telophase.
• In animal cells: Division is accomplished by a
cleavage furrow that encircles the cell like a
ring in the equator region.
• In plant cells: Division is accomplished by the
formation of a cell plate between the daughter
cells. Each cell produces a plasma membrane
and a cell wall on its side of the plate.
Mitosis
Mitotic Phase: Mitosis + Cytokinesis
• Cytokinesis The division of cytoplasm to
produce two daughter cells.
• Usually begins during telophase.
• In animal cells: Division is accomplished by a
cleavage furrow that encircles the cell like a
ring in the equator region.
• In plant cells: Division is accomplished by the
formation of a cell plate between the daughter
cells. Each cell produces a plasma membrane
and a cell wall on its side of the plate.
Cytokinesis in Animal and Plant Cells
Animal Cell
Plant Cell
External Factors Control Mitosis
1. Anchorage Most cells cannot divide unless they are
attached to a solid surface. May prevent
inappropriate growth of detached cells
2. Nutrients and growth factors Lack of nutrients can
limit mitosis Growth factors: Proteins that stimulate
cell division.
3. Cell density Density-dependent inhibition: Cultured
cells will stop dividing after a single layer covers the
petri dish. Mitosis is inhibited by high cell density.
Cancer cells do not demonstrate density inhibition
Cell-Cycle Control System
• There are three critical points at which the cell cycle is
controlled*:
1. G1 Checkpoint: Prevents cell from entering S phase
and duplicating DNA. Most important checkpoint.
Amitotic cells (muscle and nerve cells) are frozen here.
2. G2 Checkpoint: Prevents cell from entering mitosis.
3. M Checkpoint: Prevents cell from entering
cytokinesis.
*Cells must have proper growth factors to get through
each checkpoint.
Cell Division is Controlled at Three Key Stages
Growth factors are
required to pass
each checkpoint
Cancer is a Disease of the Cell Cycle
• Cancer kills 1 in 5 people in the United States.
• Cancer cells divide excessively and invade other body
tissues.
• Tumor: Abnormal mass of cells that originates from
uncontrolled mitosis of a single cell.
Benign tumor: Cancer cells remain in original site. Can
easily be removed or treated .
Malignant tumor: Cancer cells have ability to “detach”
from tumor and spread to other organs or tissues
Metastasis: Spread of cancer cells form site of origin to
another organ or tissue. Tumor cells travel through
blood vessels or lymph nodes.
Metastasis: Cancer Cells Spread
Throughout Body
Functions of Mitosis in Eukaryotes
1. Growth: All cells that originate after a new
individual is created are made by mitosis.
2. Cell replacement: Cells that are damaged or
destroyed due to disease or injury are
replaced through mitosis.
3. Asexual Reproduction: Mitosis is used by
organisms that reproduce asexually to make
offspring.
Mitosis Replaces Dead Skin Cells
Chromosome -1
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Chromosomes are structures that contain information
Chromosomes come in pairs.
Normal humans have 46 chromosomes in 23 pairs.
One chromosome of each pair comes from an
individual’s mother, the other comes from the father.
• Homologous chromosomes carry genes that control the
same characteristics. Examples: Eye color, blood type,
flower color, or height
• Locus: Physical site on a chromosomes where a given
gene is located.
• Allele: Different forms of the same gene. Example:
Alleles for blood types A, B, or O.
Homologous Chromosomes: Code for the Same Genetic
Traits, but Have Different Alleles
Chromosomes-2
• There are two types of chromosomes
• Autosomes - Found in both males and females. In
humans there are 22 pairs of autosomes. Autosomes are
of the same size and are homologous.
• Sex Chromosomes (X and Y)- Determine an
individual’s gender.
• The X and Y chromosomes are not homologous.
• The X chromosome is much larger than the Y
chromosome and contains many genes.
• The Y chromosome has a small number of genes. In
Humans and other mammals females are XX and males
are XY.
Normal Genetic Complement of
Humans
Females: 44 autosomes (22 pairs) + XX
Males: 44 autosomes (22 pairs) + XY
Note: In most cases, having additional or missing
chromosomes is usually fatal or causes serious
defects.
E.g Down’s Syndrome: Trisomy 21. Individual’s
with an extra chromosome 21. Most common
chromosomal defect (1 in 700 births in U.S.).
Mental retardation, mongoloid facial features,
heart defects, etc.
Chromosomes-3
• Humans have two sets of chromosomes.
• One inherited from each parent.
• Diploid Cells: Cells whose nuclei contain two
homologous sets of chromosomes (2n). Almost all cells
in our body are diploid . In humans the diploid number
(2n) is 46.
• Haploid Cells: Cells whose nuclei contain a single set of
chromosomes (n). Egg and sperm cells are haploid. In
humans the haploid number (n) is 23.
• Fertilization: Haploid egg fuses with a haploid sperm to
form a diploid zygote (fertilized egg).
Mitosis versus Meiosis
Mitosis
Meiosis
One cell division
Produces two (2) cells
Produces diploid cells
Daughter cells are genetically
Two successive cell divisions
Produces four (4) cells
Produces haploid gametes
Cells are genetically
identical to mother cell
different from mother cell and each other
No crossing over
Crossing over*
Functions: Growth,
Functions: Reproduction
cell replacement
*Crossing over: Exchange of DNA between homologous
chromosomes.
Meiosis: Generates haploid gametes
• Reduces the number of chromosomes by half,
producing haploid cells from diploid cells.
• Also produces genetic variability, each gamete is
different, ensuring that two offspring from the
same parents are never identical.
• Two divisions: Meiosis I and meiosis II.
Chromosomes are duplicated in interphase prior to
Meiosis I.
• Meiosis I: Separates the members of each
homologous pair of chromosomes. Reductive
division.
• Meiosis II: Separates chromatids into individual
chromosomes.
STAGES OF MEIOSIS
Interphase:
Chromosomes
replicate
Meiosis I:
Reductive division.
Homologous
chromosomes separate
Meiosis II:
Sister chromatids
separate
Meiosis I: Separation of Homologous
Chromosomes
1.
2.
Prophase I: (90% of time) Chromatin condenses into
chromosomes. Nuclear membrane and nucleoli
disappear. Centrosomes move to opposite poles of cell
and microtubules attach to chromatids. Synapsis:
Homologous chromosomes pair up and form a tetrad
of 4 sister chromatids. Crossing over: DNA is
exchanged between homologous chromosomes,
resulting in genetic recombination. Unique to meiosis.
Chiasmata: Sites of DNA exchange.
Metaphase I: Chromosome tetrads (homologous
chromosomes) line up in the middle of the cell. Each
homologous chromosome faces opposite poles of the
cell.
Meiosis I
Significance of Meiosis
• Sexual Reproduction – if meiosis did not occur the
fusion of gametes would lead to double the
number if chromosomes, each generation.
• Genetic Variation – Opportunity for new
combination of genes in gametes.