Topic 3 Flashcards
What is a brief explanation of meiosis?
- 1 diploid cell to 4 haploid
- Meiosis 1 and 2
- Described as reduction division as chromosome number is halved
What is the definition of diploid cell?
2 chromosomes of each type
What is the definition of haploid cell?
1 chromosome of each type
What feature of meiosis allows there to be a sexual life cycle?
Chromosome numbers are halved
What is a sexual life cycle?
The differences between the chromosomes of offspring and parents
Why does the chromosome number need to be halved at some point of the life cycle?
Fertilisation occurs and it doubles the chromosome number each time
When can meiosis only occur in animals?
Making of gametes
The orientation of pairs of homologus chromosomes are what?
Random
How many poles is each chromosome attached to?
1
Does the orientation of one bivalent impact the other?
NO
Are 2 homologus chromosomes in a bivalent attached to the same pole?
no
What halves the chromosome number?
Separation of homologus chromosomes in meiosis 1
What is disjunction?
The separation of homologus chromosomes
How does disjunction occur?
1 chromosome from each bivalent moves to opposite poles of the cell which halves the chromosome number
What three processes promote genetic variation?
Crossing over, fertilisation and random orientation of bivalents
How does crossing over promote genetic diversity?
Allows linked genes to produce new allele combinations and increases the number of allele combinations generated by meiosis
How does random orientation of bivalents promote genetic diversity?
- Promotes genetic variation among genes that are on different chromosomes
- n of allele combinations = 2 to the power of n ( n = haploid number)
- For every bivalent the number of combinations doubles
Why does fertilisation promote genetic diversity?
It is the start of new life, allows alleles from 2 different individuals to be combined into 1, combination of alleles are unlikely to have been seen before
What is non- disjunction?
When pairs of homologus chromosomes fail to separate
When the chromosomes fail to separate where would they move to?
Both to one pole and neither to the other pole
What is trisomy 21?
- When there are 3 of chromosome 21
- lead to hearing loss, heart and vision disorders
- down syndrome
What else can there be an abnormal number of?
Sex chromosomes
- Klinefelter’s syndrome = XXY
- Turner’s syndrome = X
What is the correlation between maternal age and non disjunction?
Strong positive correlation with the risk of chromosome abnormalities increasing after the age of 30
DNA replication
- during interphase
- so that all the chromosomes consist of 2 sister chromatids
We know that DNA replication is accurate because two chromatids are intitally genetically what?
Identical
What happens to the chromatids during meiosis?
- 1 diploid nucleus where each consists of 2 chromatids divides twice to produce 4 haploid nuclei which consist of 1 chromatid
What is it called when homologus chromosomes pair up?
Bivalent
What is the process in which the homologus chromosomes pair up called?
Synapsis
After the chromosomes have paired up what happens?
Crossing over
Describe the process of crossing over
- A junction is created where one chromatid in each of the homologus chromosomes breaks off and rejoins with another
- occurs anywhere along the chromosome
- there is a mutual exchange of genes as crossing over occurs at the same position in each chromatid
What are the two ways that you can maintain cells from a fetus?
-amniocentesis ( passing a needle from the mothers abdomen wall and using it to withdraw a sample of amniotic fluid)
-chronic vilus sampling ( sampling tool that enters through the vagina and obtains cells from the chronion)
CHRONION = MEMBRANE THAT THE PLACENTA DEVELOPS IN
Stages of meiosis
PMAT 1 + PMAT 2
Prophase 1
- 2n chromosomes
- homologus chromosomes pair up
- crossing over occurs
Metapahse 1
- spindle fibres move the chromosomes to the equator of the cell
- orientation is random
Anaphase 1
- the homologus pairs are separated ( one chromosome of each pair moves to each pole)
Teleophase 1
- chromosomes uncoil
- chromosomes go from diploid to haploid
- cytokinsesis occurs
Prophase 2
- chromosomes condense and become more visible
Metaphase 2
- chromosomes line up at the equator
Anaphase 2
- chromosomes separate and the chromatids are pulled to opposite poles
Teleophase 2
- chromatids reach opposite poles
- nuclear envelope forms
- cytokinesis occurs
Bacterial chromosomes
- Prokaryotes that have 1 chromosome consisting of a circular DNA molecule
- Naked DNA
- 1 copy of a gene
- Two copies after replication
Features of plasmids
- Small and circular
- Contain genes that may be useful to the cell but not genes needed for basic processes ( like genes for antibiotic resistance)
- may be multiple copies
Why can there be multiple copes of plasmids?
They are not replicated the same time as the chromsomes
How can plasmids be spread through a population?
The copies are transferred from one cell to another
How can plasmids cross the species barrier?
Plasmid released when a prokaryotic cell dies and is absorbed the cell of another species
