Case 2 Down's Syndrome Flashcards by Harriet Phelan

What specialised proteins associate with DNA to form chromatin?

Histones

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What are the 2 overall stages of mitosis?

Interphase

Mitotic phase

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What occurs during interphase? (3 stages)

G1 - Cell growth, organelles copied, molecular building blocks created

S - Cell synthesis, complete copy of DNA in nucleus, centrosome duplicated

G2 - More cell growth, synthesis of proteins and organelles, re-organisation to prep for mitosis

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What are the stages of mitosis?

Prophase
Metaphase
Anaphase
Telophase
Cytokinesis

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What occurs during Prophase?

Chromosomes condense
Mitotic spindle forms
Nucleolus disappears

Prometaphase > chromosomes become very compact and are released from nuclear envelope
Microtubules bind to chromosomes at kinetochore

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Where is the kinetochore found?

The centromere of each sister chromatid

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What occurs during Metaphase?

Spindle captures chromosomes
Chromosomes up at metaphase plate
Kinetochores attached to microtubules from opposite spindle poles

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What is the spindle checkpoint?

During metaphase, cell ensures all chromosomes are attached properly to kinetochores before proceeding to Anaphase
Cell will not move on if there is a problem at this stage until it is fixed

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What occurs during Anaphase?

Sister chromatids separate from one another
Pulled to opposite sides of cell
Motor proteins drive cell elongation

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What occurs during Telophase?

Mitotic spindle broken down
Two nuclei form
Nuclear membranes and nucleolus reappear
Chromosomes de-condense

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What occurs during Cytokinesis?

Division of cytoplasm to form 2 new cells

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What is cleavage furrow (cytokinesis)?

The pinch crease that separates the two cells

Carried out by the protein ACTIN

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What is the difference between haploid and diploid cells?

Haploid (N) = half the number of chromosomes and are only in gametes
Diploid (2N) = full set of chromosomes (46 in humans)

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How does Meiosis differ from Mitosis? (4 examples)

MEIOSIS IS:

4 daughter cells produced instead of 2
Haploid cells
Two step process
Germ cell division

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What is Meiosis 1 sometimes called and why?

Reduction division - it halves the chromosome number from 92 to 46

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What is synapsis?

In prophase 1: homologous chromosome pairs align opposite each other

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What occurs during crossing over of chromosomes?

Each BIVALENT becomes tightly coiled, recombination occurs at points known as CHIASMATA where homologous regions of DNA are exchanged between chromatids

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What is formed after Telophase 1 of Meiosis?

Two new daughter gametes HAPLOID chromosomes

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What does Meiosis achieve and how?

Ensures HAPLOID number of chromosomes in daughter cells

Generates genetic diversity via random assortment and crossing over

What is the process of DNA replication

Replication starts at origin of replication
DNA Helicase unzips/unwinds
DNA Polymerase binds to exposed bases
On leading strand - replication is continuous
On lagging strand - replication occurs in Okazaki fragments
Okazaki fragments sealed by DNA Ligase
Sliding clamp holds DNA Polymerase in place
Topoisomerase prevents DNA ahead of fork getting too tightly wound
DNA Ligase replaces any nicks in sequence

What does it mean by DNA replication is semi-conservative?

Each strand in double helix acts as template for new, complementary strand

What does DNA Polymerase do?

Makes new DNA strand, requires a template and a primer, runs 5’ to 3’

What does DNA Ligase do?

Seals Okazaki fragments

What is a karyotype? When can it be taken?

A picture of an individual’s chromosomes

When cells are undergoing METAPHASE

What is polyploidy?

When cells have chromosomes in multiples of N higher than 2N (diploid) e.g. 3N = 69 chromosomes = TRIPLOID

What is aneuploidy?

A variation in the number of chromosomes that is not in multiples of N e.g. An extra copy of chromosome 21 (Trisomy 21) for Down's Syndrome

What is monosomy?

A loss of a copy of that chromosome

What is trisomy?

An extra copy of a [articular chromosome

Are monosomies ever viable?

For monosomy of chromosomes 1 - 22, ALL complete monosomies are lethal Partial monosomies may be observed in unbalanced translocations Monosomy of the X chromosome results in Turner Syndrome (XO)

What can cause aneuploidy?

Non-disjunction - failure of chromosomes/sister chromatids to separate during anaphase Anaphase lag - delayed movement of chromosomes after separation at anaphase

What is a balanced/unbalanced structural chromosomal abnormality?

Balanced: no loss of genetic material Unbalanced: incorrect amount of chromosomal material

What structural abnormalities can be found in a single chromosome?

Deletion Inversion Duplication Isochromosome

What structural abnormalities can be found across two chromosomes?

Insertions | Translocations

What are the two types of translocations?

Reciprocal - breakage of at least two chromosomes with exchange of fragments Robertsonian - breakage of two acrocentric chromosomes at or close to their centromeres with subsequent fusion of their long arms (centric fusion)

Which chromosomes are involved in Robertsonian translocations?

13, 14, 15, 21, 22

What does colcichine do? When is it used?

Poisons mitotic spindle - used during metaphase to identify a karyotype

What is the FISH method?

Fluorescent In-Situ Hybridisation - uses fluorescent probes to bind to a complementary sequence in DNA, able to obtain a karyotype this way

What are the risks involved for a women aged >35 becoming pregnant?

``` Fewer viable oocytes Higher risk of gestational diabetes Higher risk of pre-eclampsia and eclampsia Higher risk of chromosomal abnormalities Higher risk of twins/triplets ```

What is the risk for mothers over aged 45 of having a baby with Down's Syndrome?

1:33

What puts a parent more at risk for having a baby with Down's Syndrome?

Being over aged 35 | Having a previous baby with DS AND carrying the chromosomal translocation for it

What are the three different types of Down's Syndrome?

Trisomy 21 - extra copy of chromosome 21 Translocation type - Robertsonian translocation (2 copies of chromosome 21 but some genetic material from 21 has translocated to another chromosome) Mosaic type - Non-disjunction in meiosis but not all cells have trisomy 21 (VERY RARE)

What other health complications may occur in someone with Down's Syndrome?

``` Congenital heart defects Learning disabilities Immune disorders Obesity Spinal problems Leukaemia Endocrine problems Seizures Dental problems Hearing and vision loss ```

What common congenital heart defects are observed in people with Down's Syndrome?

Endocardial Cushion Defect (L>R shunting of blood through heart Ventricular Septal Defect (failure to close intraventricular foramen) Secundum Atrial Septal Defect (failure to close embryonic foramen)

What is a healthy BMI?

18.5 - 24.9

How does a high BMI affect fertility and pregnancy?

BMI > 30 can reduce fertility by up to 50% Maternal diabetes High blood pressure (pre-eclampsia)