Mitosis and Asexual Reproduction
Asexual reproduction – reproduction where there is only one parent and the result
is two offspring
No specialized reproductive organs are used
Unicellular organisms – results in the creation of another individual
organism
Multicellular organisms – results in growth and replacement of worn out
tissue – it can form a separate individual
Accomplished by the process of Mitosis
Mitosis is the division of cells that have a distinct nucleus
1) there are two processes
2) the first is the division of the nucleus to form two distinct nuclei
3) identical sets of the hereditary material are produced
The second process is called cytokinesis – cyto= cell, kinesis= division
1) cytokinesis is the division of the cytoplasm – occurs the same time
mitosis does
2) each cell gets one copy of the hereditary material and a nucleus
Sexual reproduction – reproduction that is accomplished with two parents and the
result is one or more offspring
Unicellular organisms – there are none that use sexual reproduction
Multicellular – requires the production of specialized reproductive
cells or organs
Meiosis – later part of the chapter
Make up of the nucleus:
Chromosomes – rodlike structures that contain the heredity information of an
organism. Each organism has a different number - humans have 46 fruit
flies have 8
Centrioles – tiny cylinders positioned at right angles – associated with spindle of
mitosis – still a mystery
Phases of mitosis in animals – I peeled more apples today
Interphase – called the resting stage, the stage that is between mitotic
phases
1) the cell grows in size and duplicate sets of every organelle is produced
2) the nucleus is contained within the nuclear membrane – nucleoli are
oresent
3) the DNA appears to be thread like in structure
Prophase – the start of the mitotic cycle – usually divided further into 3 stages
1) early – doubled chromosomes become visible and move toward the
“poles”
2) middle – doubled chromosomes begin to move toward the “equator” of
the cell
3) late – the nuclear membrane and nucleolus have disappeared
Metaphase – meta or middle stage
1) centromeres of doubled chromosomes are lined up at the “equator”
2) each chromatid separates and becomes their own chromosomes – single
stranded and identical
Anaphase – duplicate chromosomes move to opposite poles
1) one complete set is now at each of two poles
Telophase – the last stage – begins when the chromosomes reach the poles
1) chromosomes uncoil and become threadlike structures called chromatin
2) the spindles and asters disappear
3) a nuclear membrane is formed around each daughter cell and the
nucleoli reappear
Cytokinesis and “pinching” takes place during late anaphase and ends in telophase
Mitosis in plant cells: differs from plant cells in two ways
1) plant cells do not have centrioles and thus do not form asters
2) plant cells do not pinch because of the cell wall – so a cell plate forms which then
divides the cell - cell wall material is then secreted on each side of the plate
Time span of mitosis – some bacterium it takes about 20 minutes – some human
cells take about 1 hour to divide and then remain in interphase for about 16 – 20
hours. Some cells never go through mitosis
Types of Asexual reproduction
Binary Fission – the simplest form of asexual reproduction
1) the parent cell divides into two equal daughter cells – no parent is left
2) used in one celled organisms
3) in bacteria – lack an organized nucleus – some divide but do not separate
4) in protozoa – paramecium has two nucleii – micro (controls fission) and a
macro – each daughter cell gets one of each
Budding – a type of asexual reproduction in which the result is two unequal parts
Unicellular – in yeast the nucleus moves to the side of the cell and divides with one
going to the bud and the other staying with the parent cell. A cell wall is
produced – they may stay together but each is independent
Multicellular – in the hydra a small mound of cells is produced which go on dividing
and eventually forma complete hydra that separates from the parent
Regeneration – the ability of an organism to regrow lost body parts
1) relatively simple organisms such as starfish, earthworms, hydras and
planarian
2) as organisms become more complex the power of regeneration disappears
3) Some organisms can regenerate lost parts – like a crab’s claw but not the
whole organism
4) Organisms that can regenerate usually do not reproduce in this manner
Artificial Reproduction
1) Cutting – any vegetative part used to create a new individual
2) Layering – stem is bent over and covered with soil and forms a new plant
which is then cut from the parent
3) Grafting – a stem or bud is cut away and permanently joined to another
similar plant
Advantages: We can continue to grow the type of fruit and vegetables that we want
– such as seedless oranges
Meiosis and sexual reproduction
Sexual reproduction – involves the production of specialized sex cells and the
fusion of their nuclei
Gametes – specialized sex cells produced in the gonads by males and females
Sperm – male gamete
egg – female gamete
Gonads – specialized organs where sex cells are produced
Testes – male gonads
ovaries – female gonads
Gametogenesis – the process by which gametes (sperm/egg) develop in the gonads
Spermatogenesis – the production of sperm in the testes
Produces 4 haploid cells
Each cell usually matures into a sperm cell
Sperm – smaller than the egg and highly mobile
Oogenesis – the production of eggs (AKA ovum and/or oocyte)
One cell becomes haploid egg and the other 3 become smaller polar
bodies which eventually are reabsorbed into the body and die
They are unequal in size due to unequal to unequal cytoplasmic
division
Ovum – is much larger and non-mobile
Nutrients are stored in the yolk of the egg
Hermaphrodites – individuals that have both testes and ovaries
They are neither male or female
Self fertilization is rare – a mate is needed
Common to slow moving animals such as earthworms, hydra and snails
Both individuals can leave the encounter impregnated
Chromosome numbers
In humans the body cells contain 46 chromosomes
The sex cells (gametes – sperm and egg) contain 23 chromosomes
Homologous chromosomes
Pairs of chromosomes that are similar in size and shape and have similar
genetic content – humans have 23 of these
Diploid cells (2N) cells that have all the homologous chromosomes
Therefore 46 total chromosomes or 23 homologous pairs
Haploid cells (1N) (aka monoploid or N cells) – cells that have only one chromosome
from each homologous pair
Therefore 23 chromosomes – in either the egg or sperm
Haploid + haploid = diploid (N+N = 2N) – fertilization
Meiosis – also known as - reduction division
Cell division in diploid cells that produce haploid cells
Takes place in the gonads of males and females and produces gametes
Each primary sex cell divides twice
Chromosomes only replicate once
Each daughter cell divides from a diploid cell but contains the haploid
number
Increases the genetic variation within a species
Stages of Meiotic division
Prophase 1 – chromosomes have already replicated
Synapsis – each homologous chromosome pairs up and aligns with
chromatids and they fasten at the centromere
Tetrad – each group of four chromatids – each tetrad moves toward
the equator of the cell
Crossing over – the exchange of segments between chromatids
Occurs about 20% of the time and further increases genetic
variation
Occurs only during prophase 1 when chromatids are in the
tetrad form
Metaphase 1 – centromeres of the tetrad line up at the equator
Spindle microtubules fasten at the centromers
Anaphase 1 – disjunction – homologous chromosomes separate and move to
Opposite ends of the cell or poles
Half as many chromosomes as the original cell however – each set is
doubled (46 in each cell)
Telophase – end of meiotic division
Cytoplasm divides (cytokenesis) forming two daughter cells each with
the diploid number
Each cell has half the parent cell chromosomes but in replicated form
2nd meiotic division
Prophase 2 – each daughter cell forms spindles chromosomes move toward
the middle
Metaphase 2 – chromosomes become fastened to spindles at the centromeres
and line up at the Equator
Anaphase 2 – centromeres divide and chromatids separate
Results in single stranded chromosomes which move toward
the poles
Telophase 2 – both daughter cells divide further forming 4 haploid cells
Nuclear membrane reforms
Summary:
First meiotic division produces 2 diploid cells containing double
chromosomes or the 2n number
Second meiotic division produces 4 haploid containing single strnded
chromosomes or the 1n number
Fertilization – the union of the haploid sperm and the haploid egg – N+N=2N
Zygote – the diploid cell that results from the fusion of two gametes
Only one sperm can fertilize an egg
After that a fertilization membrane forms preventing a second sperm from
penetrating and entering the egg
External fertilization – when gametes are fertilized outside the body
Common to aquatic vertebrates such as fish and amphibians
Large numbers of eggs are required to insure the species survival
Developing organisms have less parental care and thus less protection than
those that develop internally
Hormones control behavior so animals know when and where to release
gametes
Internal fertilization
Gametes fuse in the moist reproductive tract of the female
Common to most land living or terrestrial vertebrates
An adaptation that helped insure survival
Egg Shell
A method of protection for zygotes as they undergo internal fertilization and
external development
Parthenogenesis – formation of an adult without fertilization
Common to insects that where the queen bee produces drone workers
PZP is a protein that all mammals have and it that stimulates sperm attachment to
eggs. It is taken from another animal, usually a pig and injected into a healthy
animal, for example a deer or any other target specie. Because it is a foreign
substance being from another animal antibodies are built up against it. Then when
the protein is produced naturally it is destroyed. As a result sperm cells now
literally bounce off an egg cell instead of sticking to it thus decreasing the birth rate.
Only 1 problem is that it reverses “mother nature” because it only works in healthy
individuals. The weaker ones that didn't build up antibodies to the injected PZP are
the ones reproducing. Only the strong are supposed to survive!! In the short term
it will decrease a population but in the long term man will have gone a long way to
create a population of weaker individuals that run the risk of crashing to extinction.
Fecundity rate: the rate at which a species reproduces over the course of its lifetime
– humans 2.5 kids
As parental nurturing increases the fecundity rate decreases
Ex. Salmon – no parental care thousands of eggs vs humans that have
lifelong parental care and have only 2.5 children on average
Three Patterns
Viviparous – egg cell develops within the female and offspring are born as live
Juveniles - Mammals, some reptiles, some fishes
Oviparous – eggs are released by the female and are fertilized and develop outside
the female birds
Ovoviviparous – eggs develop in the female without additional nutrition from the
mother. Females bear live young – live young or are born just after
hatching – ex. Sharks
Chromosome numbers by species:
Human – 46
Sheep – 54
Dog –78
Horse – 64
Rat – 42
Dove – 16
Guinea Pig – 16
Woodchuck – 46
Camel – 70
Squirrel – 40
Goat - 60
Onion - 16
Tomato - 24
Spinach - 12
Apple - 34
Rice - 24
Alfalfa - 16
Potato - 48
Pig – 38
Alligator – 32
Bat – 44
Although humans do not have the most chromosomes – they are larger than most
species and contain more genetic information making us the most advanced species
on the planet