Meiosis Flashcards
What is the control centre for all cellular activities?
The nucleus
What happens just before cell division?
The chromatin network contracts and becomes visible as individual chromosomes
What does each chromosome consist of?
DNA molecules wrapped around proteins
What makes up genes?
DNA molecules
Do all organisms have the same number of chromosomes?
No
How many chromosomes do humans have?
46
What happens during DNA replication?
single-stranded chromosomes replicate to become double stranded
What does a double-stranded chromosome consist of?
two chromatids joined by a centromere
What joins chromatids?
a centromere
What does replication ensure?
that all daughter cells contain the same hereditary information as the parent cells
When are chromosomes arrached in pairs called homologous chromosomes?
just before cell division
What is similar between each chromosome of the homologous pair?
They are the same length and contain the same genes
How many chromatids does one pair of homologous chromosomes contain?
4
What are autosomes?
The are chromosomes that control features in males and females
What are gonosomes?
They are sex chromosomes that determine the sex of the organism.
How many types of gonosomes are there?
2 (X & Y)
What are some features that could be controlled by autosomes?
eye colour
blood group
What is meiosis?
A special type of cell division necessary for sexual reproduction
What type of cell does the process of meiosis begin with?
one diploid (2n) cell
What does diploid imply?
That it is a cell containing two copies of each chromosome
One copy from the organism’s father; one copy from the organism’s mother
Define karyotype
The number and appearance of chromosomes in the nucleus of a cell
What does the process of meiosis produce?
four haploid cells
These are all genetically different from each other
Define fertilisation
When one of the haploid male gametes fuses with one of the haploid female gametes
What does fertilisation produce?
a diploid zygote
What describes the complete set of chromosomes in a species or in an individual organism?
Karyotype
Where does meiosis occur?
In the gonads
What are the gonads?
Reproductive organs (testes and ovaries)
What are the cells produced by meiosis called?
gametes
What can the process of meiosis also be called?
gametogenesis
What are the male gametes called?
spermatozoa (sperm cells)
What process are sperm cells produced by?
spermatogenesis
What are female gametes called?
ova (egg cells)
What process are egg cells produced by?
oogenesis
What is the order of phases in meiosis?
IPMAT Interphase Prophase Metaphase Anaphase Telophase
What are the gametes of plants called?
spores
In plants, where does meiosis occur in males?
In the anther
What does meiosis produce in male plants?
pollen grains
In plants, where does meiosis occur in females?
In the ovary
What does meiosis produce in female plants?
Ovule
What does paternal mean?
originating from the male parent
What type of cell does meiosis begin with?
one diploid cell
What does meiosis follow in the cell cycle?
Interphase
Why does DNA replication take place?
To double the genetic material
In which phase of the cell cycle does DNA replication take place?
Interphase
When do the chromosomes duplicate?
During DNA replication
What does DNA replication produce?
An identical copy of each maternal and paternal chromosome
What are copies produced by DNA replication called?
homologous chromosomes
What is each chromosome made up of?
two chromatids joined by a centromere
What happens in prophase I?
Nuclear membrane starts to disappear
Chromatin network becomes visible as chromosomes
Chromosomes arrange themselves into homologous pairs
Crossing over occurs
What does crossing over involve?
Homologous chromosomes lie next to each other
Touch at points along the chromatids called chiasma
Where they touch, there is an exchange of genetic material
What are the points where genetic material is exchanged called?
chiasma
In what stage of meiosis do chromosomes arrange themselves into homologous pairs?
Prophase I
In what stage of meiosis does the nuclear membrane start to disappear?
Prophase I
What can meiosis I be referred to as?
A reduction division
What is the main thing that is happening in meiosis I?
homologous chromosomes are getting separated resulting in two separate daughter cells
What is a does each daughter cell (resulting from meiosis I) contain?
Half the number of chromosomes found the original cell (this is the haploid number)
What happens in metaphase I?
A spindle thread forms
homologous pairs randomly arrange along equator
centromeres attach to spindle threads
What do the spindle threads consist of?
protoplasmic threads
What are the things that form the spindle threads?
centrioles
What happens in anaphase I?
Spindle threads contract
Whole chromosomes move towards the poles
What happens in telophase I?
Chromosomes uncoil and lengthen spindle disappears nuclear membrane reforms cytoplasm divides two new cells genetically different (due to crossing over)
Are the cells produced at the end of meiosis I haploid or diploid?
two haploid daughter cells form
Basically, what happens in meiosis II?
The chromatids of the chromosomes now separate from each other, creating four haploid daughter cells
What happens in prophase II?
chromosomes are visible as two chromatids
centromere splits between the sister chromatids
What forms when the sister chromatids split?
Daughter chromosomes
What happens in metaphase II?
the daughter chromosomes randomly arrange along the equator
centromere of each daughter chromosome attaches to the spindle threads
What happens in anaphase II?
spindle threads contract
centromere splits allowing spindle threads to pull daughter chromosomes apart
What happens in telophase II?
the chromosomes uncoil and lengthen
spindle disappears
cytoplasm divides
nuclear envelope (membrane) forms
What does meiosis ultimately result in?
four haploid daughter cells
that are genetically different from each other
Why is meiosis important?
formation of gametes
counteracts doubling effect of fertilisation (due to haploid cells being created)
introduces genetic variation
What are the reasons for genetic variation in meiosis?
crossing over
random arrangement of chromosomes during metaphase
Define gametogenesis
the process whereby gametes are formed
What are the two different types of gametogenesis called?
spermatogenesis and oogenesis
Where does oogenesis take place?
In a cell inside a follicle in a ovary
Where does spermatogenesis take place?
In a germinal epithelial cell in testes
What does spermatogenesis result in?
4 haploid spermatids
What does oogenesis result in?
1 mature ovum
What does a reduction of the chromosome number prevent in the process of reproduction?
Prevents the chromosome number from doubling when fertilisation occurs
What does crossing over result in?
The exchange of genetic material between the paternal and maternal chromosome of each homologous pair
What does crossing over ensure?
That all gametes of both parents contain a unique combination of genes
What happens when different gametes of the same parents combine during fertilisation?
the resultant offspring will al contain a unique karyotype, ensuring variation in the offspring
Explain random separation of chromosomes
when each homologous chromosome pair is randomly arranged along the equatorial line
What does random arrangement ensure?
that the combination of genetic material received by each gamete is random
What are chromosomal mutations?
When chromosomal abnormalities occur when the process of meiosis does not occur properly
What happens to chromosomal mutations as women increase in age?
As women increase in age, the frequency of chromosomal mutation increases.
What do chromosomal mutations result in?
a change in the structure or distribution of one or more of the chromosomes.
(I.e. there has been a change to the cell’s karyotype)
What is non-disjunction?
When a chromosome is lost or gained when they do not separate correctly during meiosis
When does non-disjuction occur in terms of the cell cycle?
During meiosis I or meiosis II
What happens if one pair of chromosomes fail to separate?
Non-disjuction occurs
One cell receive two copies of that chromosome while the other does not receive any
What is aneuploidy
a type of non-disjunction
when gametes have have one extra or one less chromosome
What is polyploidy?
When gametes have an entire extra set of chromosomes
E.g. 3n
When does does trisomy occur?
When an individual has three of the same chromosome in a cell instead of two.
This condition is usually fatal or results in abnormal physical and mental characteristics.
When does Down’s syndrone occur?
When chromosome 21 pair does not separate during metaphase I of meiosis.
This results in a gamete that contain two #21 chromosomes
What happens due to trisomy of chromosome 21?
This is the cause of Down’s Syndrome in humans
How many chromosomes does a human with Down’s Syndrome have
47 (instead of 46)
What are some similarities between mitosis and meiosis?
DNA replication takes place
The nucleus divides
The cytoplasm divides
New cells are formed
Where does mitosis occur?
In somatic cells
How many times does the nucleus divide in mitosis?
once
How many times does the nucleus divide in meiosis?
twice
Do homologous chromosomes pair in the process of mitosis?
No
Do homologous chromosomes pair in the process of meiosis?
Yes
Does the exchange of genetic material occur in the process of mitosis?
No
Do homologous chromosomes lie on the equator during metaphase of mitosis or do homologous chromosomes lie on the equator?
single chromosomes
Does the centromere divide during anaphase in mitosis?
Yes
