Lecture 11 Flashcards
How many chromosome pairs are there?
46 chromosomes
23 pairs - 22 autosomes and X and Y
Recap - what makes up a chromosome (chromosome -> base pairs)
What does a chromosome look like after replication?
What do replicated chromosomes consist of?
Each replicated chromosome contains two identical DNA molecules
What is the sturcture of a chromatid?
Each chromatid contains one DNA molecule, and has p-arm and q-arm
remember - ‘p’ for petit (the smaller arm)
What is a centromere?
Centromere consists of repetitive sequences and links the chromatids
What are the names of all the chromosomes with different centromeres?
- Metacentric
- Submetacentric
- Acrocentric
- Telocentric (this is not found in humans - found in species like apes)
What is the other way the different chromosomes with their different position of centromere, be drawn?
What is another key part of chromosome structure?
Telomeres are repeated sequences (TTAGGG) at each end of each DNA molecule
- if there is 2 chromatids in the chromosome, there will be 4 telomeres
- all the white dots in the picture attached are telomeres
In what ways can chromosomes be analysed?
- Metaphase spreads
- Karyotyping
- Ideograms
Metaphase spreads (what does this require? when does it occur? what does this look like?)
- require actively dividing cells
- growth ‘arrested’ in metaphase (‘arrested’ with chemicals, condensed chromosomes)
- require staining of chromosomes
- results in banding patterns
- dark (AT-rich) / light (GC-rich)
What is karyotyping?
A type of chromosome analysis
Karyotyping is the process by which photographs of chromosomes are taken in order to determine the chromosome complement of an individual, including the number of chromosomes and any abnormalities.
What are ideograms?
Another type of chromosome analysis
Ideograms are diagrammatic or idealized representations of chromosomes, showing their relative size, homologous groups and cytogenetic landmarks.
Shows the binding patterns, as all the chromosomes are lined up with their centromeres.
Where else can ideograms be used?
Also used in programming
In kerotyping, how can chromosomes be grouped?
Can line chromosomes up according to:
- size (length)
- where the centromere is positioned
Label the chromatids in homologous chromosomes
Include:
- sister chromatids
- non-sister chromatids
Overview of chromosomes (include homologous chromosomes, alleles, chromatids)
- homologous chromosomes have the same DNA sequence; have the same genes located on them
- genes on homologous chromosomes may be different variants (alleles)
- sister chromatids have identical DNA, have the same alleles
- non-sister chromatids have the same genes, but this maybe different alleles (as a pair is from the mother, and a pair is from the father)
Where the chromosomes during interphase?
They are in their chromosome territories
Check point - What do I need to be able to do?
- Describe the relationship between DNA molecules, chromosomes and chromatids
- Outline and describe chromosome and chromatid structure
- Appreciate how metaphase spreads can be used in chromosome analysis
- Appreciate how chromosome banding patterns can be used in karyotypes, ideograms and online programmes
- Explain the difference between ‘gene’ and ‘allele’
Overview of mitosis (which types of cells, what does it produce, which part of the cell cycle?)
- cell division for somatic cells
- chromosome number stays the same - one round of replication and one round of division
- production of two identical daughter cells with same chromosome content (diploid 2n) as parental cell
- ~50 mitotic rounds during early development
- mitotic growth also necessary for some tissue epidermis, mucosae, bone marrow, spermatogonia
- M phase: - nuclear division (mitosis) - cellular division (cytokinesis)
Stages of mitosis
What do chromosomes look like in interphase (G1, S, G2)
What are the roles of centrosomes?
Centrosomes are involved in forming spindle fibres
What happens in prophase?
- Centrosomes are seperating during prophase (pair of centrioles move to either side of the nucleus - to the two poles of the cell)
- The spindle fibres are being created
- Nuclear membrane is disintegrating
What are kinetochore spindle fibres? (this is part of pro-metaphase)
What happens in prometaphase?
- Kinetochore and spindles are connected
What happens in metaphase?
- Condensed chromosomes line up along the metaphase plate (at the centre of the cell). This is called chromosome alignment/metaphase alignment.
What happens in anaphase?
- Spindle fibres contract and the chromatids are seperated. The moment the chromatids are seperated, we them chromosomes again (daughter choromosomes - they now have their own centromere)
What happens in telophase?
- Nuclear membrane develops again
- Cleavage furrow forms (getting ready to become two daughter cells)
Which step is this? (in the picture)
Early prophase
- Chromosome condensation
- Centrosome seperation
What stage is this?
Late prophase
What stage is this?
Prometaphase
- nuclear envelop breakdown
- mitotic spindle assembly
- chromosome capture
What stage is this?
Metaphase
- Bipolar chromosome attachment
- Chromosome or metaphase alignment
What stage is this?
Anaphase
Can see sister chromatid seperation
What stage is this?
Telophase
- nuclear envelope reformation
- chromosome decondensation
- spindle disassembly
Phase after telophase?
Cytokinesis
What happens in non-disjunction?
Non-disjunction = the failure of homologous chromosomes or sister chromatids to separate properly during cell division
- missegregation of chromosomes
- leads to aneuploidy (aneuploidy is the occurance of one or more extra or missing chromosomes)
What is aneuploidy?
The occurance of one or more extra or missing chromosome
Non-disjunction leads to aneuploidy
What happens is mitotic non-disjunction occurs at the very first post-zygotic division?
Leads to 47 chromosomes (look at pic below)
- 47 chromosomes: ALL cells have an extra chromosome. This particular cell line continues, it goes through mitosis and develops into a fetus. It is NOT a mosaic phenotype as all cells have the same number of chromosomes (even though they are a wrong number). If the extra chromosome (that all the cells have!) is chromosome 21, the offspring will have Down Syndrome.
- 45 chromosomes (missing the 46th chromosome), this is called monosomy (monosomy = when you have one chromosome that lacks its chromosome partner - it’s homologous chromosome). Monosomy cells often die as they are not viable for life (unless X or Y chromosomes). Therefore, this cell line will not continue, only the 47 chromosome cell line will continue.
Very first mitotic division = 47 chromosomes
What happens in mitotic non-disjunction during later post-zygotic division?
This is where non-disjunction occurs but later on. This means that some cells in the body will have the right number of chromosomes and other cells in the body will have an extra chromosome.
This leads to mosaicism, which is the presecne of two or more cell lines in an indiviudal (throughout the body or tissue limited). The degree of mosaicim is dependent on when the non-disjunction happens in early development. It may be throughout the boyd (e.g. if it occured in the second mitotic division) or it may only be in specific tissues.
Overview of meiosis (type of division, what happens, number of chromosomes)
• special cell division for germline cells
• chromosome number reduced - one round of replication and two rounds of division
• Replication, then Meiosis I and Meiosis II
• Production of four non-identical daughter cells with half the chromosome content of the parental cell
• diploid (2n) reduced to haploid (n)
• production of gametes (sperm and egg - these gametes are then ready for fertilisation:
sperm (gamete) + egg (gamete) -> zygote
What is the first stage is meiosis?
Meiosis 1:
- Prophase 1
1. The centrosomes seperate
2. Nuclear membrane disintegrates
3. Homologous pairs ‘find each other’. Then crossing over occurs.
Describe the process of crossing over in prophase 1
- Recombination
- Each crossover is called a chiasma, multiple cross-overs are called chiasmata
- Bivalent: homologous chromosomes form pairs
- Crossing over means the genetic material will be ‘mixed up’
What is the second stage of meiois (second ‘sub-stage’)
Metaphase 1
- Homologous pairs (after crossing over), then align on the metaphase plate. They from a tetrad. They are randomly assorted on the metaphase plate.
Meaning of a tetrad - means a group or set of four
Random assortment (when does this occur? how variations of the gametes is possible?)
2n where ‘n’ is the number of chromosome pairs
223 = 8,388,608 gametes (number of gamete variations that could be produced PER PARENT)
What is the 3rd ‘sub’ stage of meiois?
Anaphase 1:
5. Homologous pairs seperate to each pole
What is the 4th ‘sub-stage’ of meiosis?
Telophase 1
- Nuclear membrane forms (now 23 chromosomes in each nuclei)
What preparation do the daughter cells made in meiosis 1 do before meiosis 2?
The daughter cells centrosomes have duplicated, they are ready to divide again
What happens is prophase 2?
- Centrosomes seperate
- Nuclear membrane disintegrates
What happens in metaphase 2?
Chromosomes line up along the metaphase plate
What happens in anaphase 2?
Chromatids are seperated
What happens in telophase 2?
2 diploid daughter cells form 4 haploid daughter cells
Overview of meiosis 1
Homologous pairs of chromosomes line up and seperate
Overview of meiosis 2
Chromosomes line up and chromatids seperate
Daughter cells are non-identical
Spermatogenesis (Steps of this)
- Cell division to produce male gametes
…from 1 spermatocyte (2n) to 4 sperm (n), in approximately 60 days
Sperm is made from puberty to death in males
Spermatogonial stem cell -> primary spermatocyte
-> secondary spermatocyte -> early spermatid
Oogenesis (steps for this)
…from 1 oocyte (2n) to 1 egg (n) plus 3 polar bodies
born with all their eggs (!), 1 egg will mature each month (menstrual cycle)
What is meiotic nondisjunction?
• missegregation of chromosomes
• leads to aneuploidy in gametes…
• …which leads to monosomy (1 copy of a chromosome, instead of 2) and trisomy (3 copies of a chromosome, instead of 2) in the zygote
Overview of non-disjunction in meiosis 1 (anaphase 1) and also in meiosis 2 (anaphase 2), from the diagram explain which zygotes will survive and which zygotes are not-viable for life
If non-disjunction takes place in meiosis 1:
non of the offspring will be normal (either triosomy or monosomy)
If non-disjunction takes place in meiosis 2:
50% of the zygotes will be normal (the other 50% will be either triosomy or monosomy)
Triosomy = can nomrally survive, but offspring is not normal (e.g. if there is an extra chromosome 21, the offspring will have common downsyndrome) Monosomy = not viable for life (will die)
