honors biology chapter 9 just the facts!. cell reproduction = the process by which cells divide to...
TRANSCRIPT
Honors BiologyChapter 9
JUST THE FACTS!
Cell Reproduction = the process by which cells divide to create new cells = cell division
Why Most Cells Divide:
• 1. Surface Area to Volume Ratio:- As cells grow their volume increases
faster than their surface area.- This decreases the surface area to volume
ratio.- So, diffusion and other forms of transport
cannot happen fast enough to keep the cell alive.
What would the volume of a cell be if each side measured 4 cm?
Why Most Cells Divide: (cont.)
• 2. DNA Overload:– Genetic information cannot be processed
fast enough in a really large cell to keep the cell functioning.
– Giant amebas have 2 nuclei with 2 sets of chromosomes to overcome this problem!
Cell Cycle = the orderly sequence of growth and division for a cell• Events of the Cell Cycle:
• G1 Phase: Period of growth before DNA is duplicated
• S Phase: DNA is replicated (duplicated)
Cell Cycle
• Events of the Cell Cycle: (Cont.)
• G2 Phase: More growth, and organelles and materials needed for cell division are made.
• M Phase: Cell division – 2 Parts:– Nuclear Division (Mitosis) – Cytoplasm Division (Cytokinesis)
Cell Cycle
B. The Cell Cycle- Series of events that cells go through as they grow and divide
1. Consists of 4 phases (interphase= G1, S, and G2) a. M phase (mitosis)-
division of cell nucleus and cytokinesis
b. G1 phase (gap)-periods of growth and activityc. S phase (synthesis)- DNA synthesized (duplicated)
d. G2 phase (gap)- period of growth and acitivity. Organelles produced.
Two Types of Nuclear Division
TYPE WHO?
(Type of Cell)
WHY?
(Purpose)
Mitosis
Somatic (Body) Cells = any cell but a sex cell
Make identical copies of the original cell for growth, repair and maintenance = clones.
Meiosis
Sexual reproductive cells (sex cells)
To reduce the chromosome number by half.
To increase genetic variation
D. Cytokinesis- division of cytoplasm and organelles. Follows mitosis or meiosis (division of the nucleus)
1. Animal cells- cell membrane drawn inward until cytoplasm is pinched into two nearly equal parts
2. Plant cells- cell plate forms midway between divided nuclei. Gradually develops into separating membrane. Eventually cell wall begins to appear.
Asexual Reproduction
• Single parent produces offspring
• All offspring are genetically identical to one another and to parent
Chromosome = thin thread of DNA wrapped around histone proteins. • Structure:
•
Chromosome
A. Chromosomes- genetic information carried on chromosomes
1. Before cell division each chromosome is replicated (copied)
2. Each chromosome consists of two identical “sister” chromatids
3. Each pair of chromosomes attached to area called centromere
Chromosome Number
• Chromosome number is expressed as either:
• 1. Diploid = 2n = the number of chromosomes found in somatic cells, where n = the number of homologous pairs.
Chromosome Number (cont.)
• Haploid Number = n = the number of chromosomes in a gamete of that organism. (Half the diploid number)
• Examples:– Humans: n = 23– Gorillas: n = 24– Pea Plants: n = 7– Why is it necessary for gametes to have half
the chromosome number of somatic cells?
CHROMOSOMES
• Homologous chromosomes = a matched set; each chromosome has a partner chromosome that looks just like it and contains genes for the same things.
Homologous Chromosomes Carry Different Alleles
• Cell has two of each chromosome
• One chromosome in each pair from mother, other from
father
• Paternal and maternal chromosomes carry different
alleles (allele = different form of a gene, but for the same
inherited trait. Ex. We have genes for eye color, but you
could have an allele for brown eyes, or an allele for blue
eyes.
Mitosis Details: Interphase
• Usually longest part of the cycle
• Cell increases in mass
• Number of cytoplasmic components doubles
• DNA is duplicated
Mitosis Details: Prophase
Duplicated chromosomes begin to condense
Mitosis Details: Late Prophase • New microtubules are
assembled
• One centriole pair is moved toward opposite pole of spindle
• Nuclear envelope starts to break up
Mitosis Details: Transition to Metaphase
• Spindle forms
• Spindle microtubules become attached to the two sister chromatids of each chromosome
Mitosis Details: Metaphase
• All chromosomes are lined up at the spindle equator
• Chromosomes are maximally condensed
Mitosis Details: Anaphase
• Sister chromatids of each chromosome are pulled apart
• Once separated, each chromatid is a chromosome
Telophase• Chromosomes de-condense
• Two nuclear membranes form, one around each set of unduplicated chromosomes
Results of Mitosis
• Two daughter nuclei • Each with same
chromosome number as parent cell
• Chromosomes in unduplicated form
Mitosis Details - Result
• Mitosis is then followed by Cytokinesis.
• Final Result:– 2 identical offspring cells that have the diploid
chromosome number and the same genetic information as the original parent cell.
Mitosis Details (cont.)
• Abnormalities:– Cancer: uncontrolled cell division
Meiosis Details
Chapter 9
Sexual Reproduction
• Involves
–Meiosis
–Gamete production
–Fertilization• Produces genetic variation among
offspring
Sexual Reproduction Shuffles Alleles
• Through sexual reproduction, offspring inherit new combinations of alleles, which leads to variations in traits
• This variation in traits is the basis for evolutionary change
Factors Contributing to Variation Among Offspring
• Crossing over during prophase I
• Random alignment of chromosomes
at metaphase I
• Random combination of gametes at
fertilization
Gamete Formation
• Gametes are sex cells (sperm, eggs)
• Arise from germ cells
testes
ovaries
anther ovary
Meiosis: Phases and Events• Two consecutive nuclear divisions
–Meiosis I
–Meiosis II
• DNA is NOT duplicated between
divisions
• Four haploid nuclei are formed
Stages of Meiosis
Meiosis I
• Prophase I
• Metaphase I
• Anaphase I
• Telophase I
Meiosis II
• Prophase II
• Metaphase II
• Anaphase II
• Telophase II
Meiosis I - Stages
Prophase I Metaphase I Anaphase I Telophase I
Prophase I• Each duplicated, condensed
chromosome pairs with its homologue• Homologues swap segments =
Crossing Over• Each chromosome becomes attached
to microtubules of newly forming spindle
Crossing Over
•Each chromosome
becomes zippered to its
homologue
•All four chromatids are
closely aligned = tetrad
•Non-sister chromosomes
exchange segments
Meiosis I
Each homologue in the cell pairs with its partner,
then the partners separate
Effect of Crossing Over
• After crossing over, each chromosome
contains both maternal and paternal
segments
• Creates new allele combinations in
offspring
Metaphase I• Chromosomes are pushed and
pulled into the middle of cell• Sister chromatids of one homologue
orient toward one pole, and those of other homologue toward opposite pole
• The spindle is now fully formed
Anaphase I
• Homologous chromosomes
segregate(separate) from each
other
• The sister chromatids of each
chromosome remain attached
Telophase I & Cytokinesis
• The chromosomes arrive at opposite
poles
• The cytoplasm divides
• There are now two haploid cells
• This completes Meiosis I
Meiosis II - Stages
Prophase II Metaphase II Anaphase II Telophase II
Prophase II
• Microtubules attach to the kinetochores of the duplicated chromosomes
• Motor proteins drive the movement of chromosomes toward the spindle’s equator
Metaphase II
• All of the duplicated chromosomes are lined up at the spindle equator, midway between the poles
Anaphase II
• Sister chromatids separate to become independent chromosomes
• Motor proteins interact with microtubules to move the separated chromosomes to opposite poles
Telophase II & Cytokinesis
• The chromosomes arrive at opposite ends of the cell
• A nuclear envelope forms around each set of chromosomes
• The cytoplasm divides
• There are now four haploid cells
Random Alignment
• Either the maternal or paternal member of a homologous pair can end up at either pole
• The chromosomes in a gamete are a mix of chromosomes from the two parents
Possible Chromosome Combinations
As a result of random alignment, the number
of possible combinations of chromosomes in a gamete is:
2n
(Humans = 8,388,608!!!)
(n is number of chromosome types)
Possible Chromosome
Combinations
or
or
or
1 2 3
Results of Meiosis
• Meiosis
– Four haploid cells produced
– Differ from parent and one another
Abnormalities
• Nondisjunction: Chromosome pairs fail to separate during Meiosis
• Causes Chromosomal abnormalities – extra or missing chromosmes
• Ex. Down Syndrome
Spermatogenesis
GrowthMitosis I,
Cytoplasmic divisionMeiosis II,
Cytoplasmic division
spermatids (haploid)
secondary spermatocytes
(haploid)
primary spermatocyte
(diploid)
spermato-gonium
(diploid male reproductive
cell)
Oogenesis
Growth Mitosis I,Cytoplasmic division
Meiosis II,Cytoplasmic division
ovum (haploid)
primary oocyte (diploid)
oogonium (diploid
reproductive cell) secondary
oocyte haploid)
first polar body
haploid)
three polar bodies
haploid)
Fertilization
• Male and female gametes unite and nuclei
fuse
• Fusion of two haploid nuclei produces
diploid nucleus in the zygote
• Which two gametes unite is random
– Adds to variation among offspring
Mitosis• Functions
– Asexual reproduction– Growth, repair
• Occurs in somatic cells
• Produces clones
Mitosis & Meiosis Compared
Meiosis• Function
– Sexual reproduction
• Occurs in germ cells
• Produces variable offspring