Glossary of Biology - Reproduction

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What are the three topics of reproduction?
Cell division, asexual reproduction, and sexual reproduction
What is cell division?
The process by which a cell doubles its organelles and cytoplasm, replicates its DNA, and then divides in two
What does this represent?
For unicellular organisms this means reproduction. For multicellular organisms it is a method of growth, development, and replacement of worn out cells
Do prokaryotes and eukaryotes differ in their means of cell division?
How to prokaryotes divide?
By binary fission
What is binary fission?
It is a simple form of asexual reproduction seen in prokaryotes
What happens during binary fission?
The single DNA molecule attaches to the plasma membrane during replication and duplicates, while the cell continues to grow in size. The cell membrane pinches inward, splitting the cell into two equal halves, with each daughter cell receiving a complete copy of the original chromosome
Is eukaryotic cell division more complicated than binary fission?
It is necessary for sperm survival
What do eukaryotic somatic cells or autosomal cells contain?
The diploid number of chromosomes
What is this designated by?
What is the diploid number in humans?
What is the haploid number?
How many chromosomes are inherited from each parent?
The life cycle of a eukaryotic cell can be broken down into how many stages?
4 distinct stages
What are they collectively known as?
The cell cycle
What are the four stages of the cell cycle?
G1, S, G2, and M
Which stages are the interphase stages?
G1, S, and G2
What does that imply?
It means they occur between cell divisions
What does the fourth stage, mitosis, include?
Cell division
What is the longest part of the cell cycle?
Interphase, by far
How much time does a cell spend in interphase?
At least 90% of the cycle
What happens during G1 stage?
This stage is one of growth
By how much does the cell grow during G1?
It doubles in size
What organelles are produced in G1?
Mitochondria, ribosomes, endoplasmic reticulum, and centrioles are produced
What is the restriction point in G1?
The point that after which it is committed to continue through the rest of the cell cycle and divide
Do all cells pass the restriction point?
What are some examples?
Skeletal muscle cells and nerve cells never pass this point
Where do they go instead?
They go to a nondividing phase sometimes called G0
What happens during S stage?
In the synthetic stage, each chromosome is replicated
Why is this done?
So that during division a complete copy of each genome can be distributed to both daughter cells
After replication what do the chromosomes consist of?
They each consist of two identical sister chromatids
What are the sister chromatids held together by?
A central region called the centromere
What are the ends of the chromosome called?
The telomeres
After this point, does the cell have 2N or N chromosomes?
It still has 2N
What does this imply?
Since each chromosome consists of two chromatids, cells entering G2 actually contain twice as much DNA as cells in G1
What happens during G2 stage?
The cell continues to grow in size, while assembly of new organelles and other cell structures continues
What is M stage?
It is the mitotic stage
What occurs during it?
This stage consists of mitosis and cytokinesis
What is mitosis?
The division and distribution of the cell’s DNA to its two daughter cells such that each cell receives a complete copy of the original genome
What is cytokinesis?
It refers to the division of cytoplasm that follows?
During interphase, is the nucleus membrane-bound and clearly visible, or invisible?
It is visible, and one or more nucleoli may be observed
Will individual chromosomes be visible under a light microscope? Why or why not?
They will not be because they are active and uncoiled
What is chromatin?
It is the DNA that appears granular
What happens as mitosis begins?
The chromosomes coil up, condense, and become visible under high-power microscopy.
What does this do?
The coiling facilitates their movement during the later stages of mitosis
What is chromosome movement dependent on?
It is dependent on certain cytoplasmic organelles
How are the centrioles involved in mitosis?
They are typically found in pairs, and they are cylindrical organelles located outside of the interphase nucleus.
What is this area referred to as?
The centrosome
During the first stage of mitosis, what do the centrioles do?
They have already replicated and they migrate to opposite poles of the cell
What happens then?
Then the sperm cell nucleus enters the ovum’s cytoplasm
What are the spindle fibers composed of?
They are composed of microtubules and associated proteins
What do the spindle fibers do?
They radiate outward from the centrioles
What does this form?
Structures called asters
What do the asters do?
They extend toward the center of the nucleus, forming the spindle apparatus
What is the movement of chromosomes toward opposite poles of the cell during the later stages of mitosis caused by?
It is caused by the shortening of the spindle apparatus
What are the four stages of mitosis?
Prophase, Metaphase, Anaphase, Telophase
What happens during prophase?
The chromosomes condense, the centrioles pairs separate and move toward opposite poles of the cell, and the spindle apparatus forms between them.
What happens to the nuclear membrane?
It dissolves, allowing spindle fibers to enter the nucleus
What happens to the nucleoli?
They become less distinct or disappear
What appears at the chromosome centromere?
What is attached to kinetochores?
Kinetochore fibers are attached to kinetochores
What happens during metaphase?
Centriole pairs are now at opposite poles of the cell.
What do the kinetochore fibers do during metaphase?
They interact with the fibers of the spindle apparatus to align the chromosomes
Where do they align?
They align at the metaphase plate
Where is the metaphase plate?
It is equidistant to the two poles of the spindle fibers
During anaphase, what happens?
The centromeres now split, so that each chromatid has its own distinct centromere
What does this allow?
It allows the sister chromatids to separate
What is the last part of the chromatids to separate?
The telomeres
Why are the sister chromatids pulled toward opposite poles of the cell?
They are pulled by the shortening of the kinetochore fibers
What happens during Telophase?
The spindle apparatus disappears
What forms around each set of chromosomes?
The nuclear membrane forms around them
What else reappears?
The nucleoli
At this point, do the chromosomes coil or uncoil?
They uncoil
What does this lead to?
It leads to them resuming their interphase form
Overall, what has happened?
Each of the two new nuclei has received a complete copy of the genome identical to the original genome and to each other
What is the last thing to occur?
When does cytokinesis occur?
Near the end of Telophase
How does it occur?
The cytoplasm divides into two daughter cells, each with a complete nucleus and its own set of organelles
What happens in animal cells here?
A cleavage furrow forms; the cell membrane indents along the equator of the cell and finally pinches through the cell, separating the two nuclei
How many times is a typical somatic cell programmed to divide?
20 to 50 times
What happens after it has divided 50 times?
It dies
What is asexual reproduction?
It is the production of offspring without fertilization
How are new organisms formed?
They are formed by the division of a single parent’s cell
Are the offspring different genetically?
No, they are identical genetically
What are the 4 types of asexual reproduction?
Binary fission, budding, regeneration, and parthenogenesis
Do eukaryotes ever use binary fission?
A very similar process occurs in some primitive eukaryotic cells
What is budding?
Budding is the replication of the nucleus followed by unequal cytokinesis
What occurs?
The cell membrane pinches inward to form a new cell that is smaller in size but genetically identical to the parent cell, and which subsequently grows to adult size
When does the new cell separate?
It may separate immediately from the parent or it may remain attached to it, develop as an outgrowth, and separate at a later stage
When does budding occur?
It occurs in hydra and yeast
What is regeneration?
It is the regrowth of a lost or injured body part
Replacement of cells occurs how?
Through mitosis
What are some examples of animals that regenerate?
Some lower animals such as hydra and starfish have extensive regenerative capabilities
For salamanders and tadpoles, what does the extent of regeneration depend on?
The nerve damage to the severed body part
What is regeneration limited to in adult birds and mammals?
It is usually limited to the healing of tissues
What is an exception?
Some internal organs such as the liver have considerable regenerative capabilities as long as part of the organ remains viable
What is parthenogenesis?
It is the development of an unfertilized egg into an adult organism
How does this process occur?
It occurs naturally in certain lower organisms
What is an example?
In most species of bees and ants, the males develop from unfertilized eggs, and several species of all-female, parthenogenetic salamander exist
Are the cells of a parthenogenetized organism diploid or haploid?
They are haploid and not diploid
What is sexual reproduction?
It differs from asexual reproduction in that there are two parents involved
What is the end result?
Genetically unique offspring
When does sexual reproduction occur?
Via the fusion of two gametes
What are gametes?
They are specialized sex cells produced by each parent
What is meiosis?
It is the process whereby these sex cells are produced
How is meiosis similar to mitosis?
It is similar in that a cell duplicates its chromosomes before undergoing the process
What is different about meiosis?
In meiosis, the haploid number is produced as opposed to the diploid number
How many divisions are there in mitosis?
There is one division
What does it result in?
Two diploid cells
How many divisions are there in meiosis?
There are two divisions
What do they result in?
They result in four haploid gametes
What do somatic cells undergo?
They undergo mitosis
What do gametocytes under?
They undergo meiosis
What happens during fertilization?
During fertilization, two haploid gametes fuse, restoring the diploid number
In meiosis, when are the gametocytes chromosomes replicated?
They are replicated during the S phase of the cell cycle
Is this the same as in mitosis?
When do the centrioles replicate in meiosis?
They replicate at some point during interphase
What does the first round of division in meiosis produce?
It produces two intermediate daughter cells
What does the second round of division meiosis produce?
It involves the separation of sister chromatids, resulting in four genetically distinct haploid gametes
During meiosis I, what happens during prophase I?
The chromatin condenses into chromosomes, the spindle apparatus forms, and the nucleoli and nuclear membrane disappear
What are homologous chromosomes?
They are chromosomes that code for the same traits, one inherited from each parent
What do they do in prophase I?
They come together and intertwine in a process called synapsis
How many chromatids are there in each synaptic pair of homologous chromosomes?
There are four chromatids
What are they often called?
A tetrad
What is crossing over?
It is when chromatids of homologous chromosomes break at corresponding points and exchange equivalent pieces of DNA
What do the chromosomes remain joined at?
Points called chiasmata
What does this signify?
The chiasmata signify where the crossing over occurred
What is crossing over also called?
Genetic recombination
What can genetic recombination unlink?
It can unlink linked genes, thereby increasing the variety of genetic combination that can be produced via gametogenesis
What does recombination among chromosomes result in?
It results in increased genetic diversity within a species
After recombination occurs, are sister chromatids identical?
No, they are no longer identical after recombination
What occurs during Metaphase I?
Homologous pairs (tetrads) align at the equatorial plate, and each pair attaches to a separate spindle fiber by its kinetochore
What happens during Anaphase I?
During Anaphase I, the homologous pairs separate and are pulled to opposite poles of the cell
What is this process called?
What does it account for?
It accounts for a fundamental Mendelian law
What happens during disjunction?
Each chromosomes of paternal origin separates from it homologue of maternal origin, and either chromosome can end up in either daughter cell
What does this result in?
It results in the distribution of homologous chromosomes with the two intermediate daughter cells being random with respect to parental origin
What does each daughter cell have?
A unique pool of alleles
What happens during Telophase I?
A nuclear membrane forms around each new nucleus
What does each chromosome consist of at this point?
Sister chromatids joined at the centromere
Does cytokinesis happen now?
What do they divide into?
Two daughter cells, each of which receives a nucleus containing the haploid number of chromosomes
What is interkinesis?
It is the period during which the chromosomes partially uncoil
What happens during Meiosis II?
It works in the same way as mitosis
Where are the gametes produced?
In the primary reproductive organs, or gonads
What are the male gonads called?
The testes
What two functional components do they contain?
They contain the seminiferous tubules and the interstitial cells or cells of Leydig
Where are sperm produced?
In the highly coiled seminiferous tubules
What nourishes the sperm?
The Sertoli cells
The interstitial cells are located where?
Between the seminiferous tubules
What do they secrete?
They secrete testosterone and other androgens
Where are the testes located?
They are located in the scrotum
What is the purpose of the scrotum?
To keep the testes temperature between 2-4C lower than body temperature
Where do the sperms pass from the seminiferous tubules into?
They pass into the coiled tubules of the epididymis
What happens in the epididymis?
They acquire motility, mature, and are stored until ejaculation
During the ejaculation where do they travel?
Through the vas deferens to the ejaculatory duct and then to the urethra
Where does the urethra pass through?
Through the penis and opens to the outside at its tip
In males, the urethra is a common passageway for what?
Excretion and reproduction
What are sperm mixed with they move along the reproductive tract?
They mix with seminal fluid
Where is seminal fluid produced?
The seminal vesicles, the prostate gland, and the bulbourethral glands
What do the paired seminal vesicles secrete?
A fructose-rich fluid that serves as an energy source for the highly active sperm
What does the prostate gland release?
An alkaline milky fluid that protects the sperm from the acidic environment of the female reproductive tract
What do the bulbourethral glands secrete?
A small amount of viscous fluid prior to ejaculation
What is the function of this fluid?
It is not known
What does seminal fluid do?
It aids sperm transport by lubricating the passageways through which the sperm will travel
Seminal fluid and sperm are called what together?
What is spermatogenesis?
It is sperm production
Where does it occur?
In ovarian follicles
What are spermatogonia?
They are diploid cells that differentiate into diploid cells called primary spermatocytes
What do primary spermatocytes do?
They undergo the first meiotic division to yield two haploid secondary spermatocytes of equal size
What does the second meiotic division produce?
It produces four haploid spermatids of equal size
After the spermatids undergo a series of changes, they become mature sperm or what?
What are they?
They are specialized for transporting the sperm nucleus to the egg
What is the egg also called?
The ovum
What does the head of a sperm consist of?
It consists almost entirely of the nucleus
What do the mitochondria in the sperm do?
They provide energy for locomotion
What is the acrosome?
It is derived from the Golgi apparatus, and it develops over the anterior half of the head
What does it contain?
It contains enzymes needed to penetrate the tough outer covering of the ovum
How many primary spermatocytes begin to undergo spermatogenesis per day?
3 million
How long does the maturation process take?
A total of 65-75 days
What are female gonads called?
The ovaries
What do they produce?
They produce ova and they secrete hormones
What hormones do they secrete?
They secrete estrogen and progesterone
Where are the ovaries found?
They are found in the abdominal cavity, below the digestive system
What do the ovaries consist of?
Thousands of follicles
What is a follicle?
It is a multi-layered sac of cells that contains nourishes and protects an immature ovum
It is actually the follicle cells that produce what?
What is the inner surface of the fallopian tube lined with?
It is lined with cilia that create currents that move the ovum into and along the tube
What is the uterus?
The site of fetal development
What is the lower narrow end of the uterus?
The cervix
What does the cervix connect with?
The vaginal canal
What is the vaginal canal?
The site of sperm deposition during intercourse and is also the passageway through which a baby is expelled during childbirth
What are the external female genitalia referred to as?
The vulva
Are the reproductive and excretory systems together or distinct in mammalian females?
They are distinct
What is oogenesis?
It is the production of female gametes
What are immature ova called?
Primary oocytes
Primary oocytes are haploid or diploid?
They are diploid cells that form by mitosis in the ovary
When one primary oocyte per month completes meiosis I, what does it yield?
Two daughter cells of unequal size
What are they called?
The large one is called a secondary oocyte, and there is a small cell known as a polar body
What happens during ovulation?
The secondary oocyte is expelled from the follicle during ovulation
When does meiosis II take place?
Only during fertilization
What is the oocyte cell membrane surrounded by?
Two layers of cells
What are they known as?
The inner layer is the zona pellucida and the outer layer is the corona radiate
When meiosis II is triggered by the sperm cell moving through these layers, what occurs?
Meiosis II produces two haploid cells, a mature ovum and another polar body
What is the mature ovum?
It is a large cell containing a lot of cytoplasm, RNA, organelles, and nutrients needed by a developing embryo
How often do women ovulate?
Once every four weeks
What are exceptions?
During pregnancy and usually lactation
When does menopause usually occur?
Between 45 and 50
What happens during menopause?
The ovaries become less sensitive to the hormones that stimulate follicle development (FSH and LH) and eventually they atrophy
What happens to the remaining follicles?
They disappear
What happens to estrogen and progesterone levels?
They greatly decline
What happens to ovulation?
It stops
When can an egg be fertilized?
It can be fertilized during the 12-24 hours following ovulation
Where does fertilization occur?
It occurs in the lateral, widest portion of the fallopian tube
How long are sperm viable?
They are viable and capable of fertilization for 1-2 following intercourse
What is the first barrier that a sperm must penetrate?
The outer layer of the ovum, the corona radiate
How does the sperm get through?
It secretes enzymes that aid in penetration
It secretes enzymes that aid in penetration
What does the acrosome do?
It is responsible for penetrating the zona pellucida
How does it work?
It releases enzymes that digest the inner layer, thereby allowing the sperm to come into direct contact with the ovum cell membrane
Once in contact with the membrane, what happens?
The sperm forms a tube-like structure called the acrosomal process
Where does it extend?
It extends to the cell membrane and penetrates it
What does this cause?
It initiates a series of reactions that result in the formation of the fertilization membrane
Now what?
It is now that the ovum completes meiosis II
What does the acrosomal reaction trigger?
It triggers the cortical reaction in the ovum, causing calcium ions to be released into the cytoplasm
What does the membrane do?
It is a hard layer that surrounds the ovum membrane and prevents multiple fertilizations
What else does the release of Ca2+ do?
It stimulates metabolic changes within the ovum, greatly increasing metabolic rate
When the sperm nucleus and ovum nucleus fuse, what results?
A diploid zygote
Now what occurs?
The first mitotic division of the zygote soon follows
What are monozygotic twins?
They are identical twins that result when a single zygote splits into two embryos
If the splitting occurs at the two-cell stage of development, what occurs?
The embryos will have separate chorions and separate placentas
What happens if it occurs in the blastula stage?
The embryos will have only one chorionic sac and will therefore share a placenta and possibly an amnion
What happens if the division is incomplete?
Siamese twins develop
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