CHROMOSOMAL BASIS OF HEREDITY Flashcards
WHAT IS CYTOGENETICS?
the study of inheritance in relation to the structure and function of chromosomes
WHAT IS THE SIZE AND LENGTH OF THE HUMAN GENOME?
SIZE =3b BP
LENGTH = 1.8m
WHAT IS THE COMPOSITION AND FUNCTION OF CHROMATIN?
complex of DNA and protein found in the nucleus of eukaryotic cells
STRUCTURE: (DNA, HISTONES = (differ in the ration of Arg, Lys; H1, H2A,
H2B, H3 a H4), NON-HISTONE PROTEINS = (neutral or slightly acidic)
FUNCTION: to package DNA molecules into a denser, more compact molecule to enable it to fit into the nucleus
HOW MUCH OF THE HUMAN GENOME IS TRANSCRIBED?
only 10 % of human genome is transcribed (including introns)
-the rest are repetitive sequences
WHAT IS THE TOTAL LENGTH OF DNA IN A CHROMOSOME?
2M
WHEN ARE CHROMOSOMES MOST VISIBILE?
Chromosomes are best visible in metaphase of mitotic division (because they are all
condensed)
EXPLAIN THE ORGANIZATION OF CHROMATIN IN INTERPHASE.
-consists of the nucleosome which is the basic building block of chromatin (DNA wrapped around an octamer of histone core) and linker
DNA (free or H1 associated DNA which connects 2 nucleosomes) (allows size of DNA to go down 7 fold)
-nucleosome further folded to produce a chromatin fiber (secondary Solenoid structure) (6 nucleosomes turn) which are coiled and condensed to form chromosomes (enables DNA condensation X40)
-Solenoid structure can be further packaged into loops (Laemmli loops) which are attached to a non-histone protein scaffold composed of Condensin I and ii
WHAT DO THE NUCLEOSOME AND SOLENOID STRUCTURES FORM?
interphase chromatin
WHAT IS THE FUNCTION OF TOPOISOMERASES?
temporally cut DNA, detangle it and then glue back together
EXPLAIN THE CONDENSATION OF CHROMATIN INTO CHROMOSOMES.
BEFORE MITOSIS: non-histone proteins form a skeleton, around which solenoid loops are wrapped
PROPHASE: as chromosomes line up, they are shortened to 1/3 000th of their length
METAPHASE: DNA continues to spiralize into chromatids, to 1/10.000 of their length
-non-histone protein scaffold with loops becomes coiled into the spiral structure of chromatids
-a chromatid is a copy of a chromosome, the sister chromatids are joined together by a single centromere
WHAT DETERMINES THE MORPHOLOGY OF THE CHROMOSOMES?
the position of the centromere
WHAT ARE THE 2 ARMS IN A CHROMOSOME?
one arm is shorter, marked p
one arm is longer, marked q
WHAT DO HUMAN CHROMOSOMES CONSIST OF?
-1 centromere
-2 arms
WHAT ARE THE 4 MORPHOLOGIES OF CHROMOSOMES?
METACENTRIC = centromere divides chromatids into equal length arms
SUBMETACENTRIC = centromere divides chromatids into unequal arm lengths
ACROCENTRIC = centromere is located near the end of chromatids so that a short arm is seen
TELOCENTRIC = centromere located at one end of chromatid
WHAT ARE CENTROMERES?
the compressed region or a part of elongated chromosomes
-it is the specialized DNA sequence in the chromosomes that links or holds together the pair of sister chromatids
HOW MANY DNA HELIXI DOES ONE CHROMATID CONTAIN?
1
WHAT TYPE OF CHROMOSOME MORPHOLOGY IS NOT FOUND IN HUMANS?
telocentric chromosomes – with no p arm, in mice
chromosomes have only one chromatid, only during S phase they synthesize the second
-chromosomes of D and G groups
WHAT IS THE KINETOCHORE?
3 layered structure in centromere – mediates the attachment of spindle microtubules
-made up of dynein, kinesin and tubulin
-only visible under electron microscope
WHAT IS CAUSED BY THE DYSFUNCTION OF THE CENTROMERE?
nondisjunction (error in the separation of
chromosomes and formation of abnormal gamete)
WHAT ARE TELOMERES?
region at the ends of chromosomes containing specific and repetitive sequences
TTAGGG/CCCTAA
-provide stability for chromosomes - protect the ends of chromosomes from deterioration or from fusion with neighboring chromosomes during the pairing of homologous chromosomes in meiosis
-associate with the nuclear membrane
WHAT ARE NUCLEOLUS ORGANISER REGIONS?
chromosomal regions crucial for the formation of the nucleus
-in humans they are located on the short arms of the acrocentric chromosomes 13, 14, 15, 21, 22
-these regions code for 5.8S, 18S, 28S ribosomal RNA
-contain tandemly repetitive ribosomal RNA gene clusters which vary in length (10 -100 copies)
WHAT IS CELL AGING SENSECENCE OF TELOMERES?
the length of telomeres changes during its lifespan
-following constant division, telomeres undergo progressive shortening until they no longer protect the chromosome and the cell can no longer divide and dies
-decrease in telomere length leads to a lack of cell division
WHAT IS THE HAYFLICK LIMIT?
describes how many times a cell can divide until telomeres lose their function
→ 40-60 times
WHAT IS TELOMERASE / TERMINAL TRANSFERASE?
an enzyme that copies telomeres
-it is a ribonucleoprotein that adds repeat sequences with the function of reverse transcriptase, synthesizes DNA via a template of RNA (RNA →DNA)
WHY DO SOMATIC CELLS AGE?
they do not make telomerase
WHERE IS TELOMERASE ABUNDANT?
stem, embryonic and cancer cells
WHAT DISEASES ARE ASSOCIATED WITH TELOMERE SHORTENING?
Werner syndrome and Ataxia telangiectasia – diseases linked with accelerated aging (accelerated shortening of telomere)
Dyskeratosis Congenita - failure of bone marrow to form and skin pigmentation
WHAT ARE DIPLOID CELLS?
pair of homologous chromosomes – one paternal (from sperm) and second of maternal origin (from egg)
-2 alleles per gene, one on each homologous chromosome
HOW MANY AUTOSOMES AND GONOSOMES?
22 autosomes pairs
1 gonosome pair
EXPLAIN EUCHROMATIN
-active, less condensed
-contains transcribing genes, structural genes, DNA, RNA, histones, non-histone proteins
-light under microscope
-stains lighter with banding techniques
-gene rich
-high GC content
EXPLAIN HETEROCHROMATIN
-strongly condensed
- late replication
-methylated (DNA) and hypoacetylated (histones)
- inactive transcription
- acetylation = histones loose positive charge -> untangling of heterochromatin -> active chromatin
- deacetylation = restoration of positive charge in histones -> condensation -> non-active chromatin
-dark under microscope
-attach to the nuclear envelope
-divided into constitutive and facultative
EXPLAIN HETEROCHROMATIN
-strongly condensed
- late replication
-methylated (DNA->cytosines = cpg islands) and hypoacetylated (histones = inactive trasncription)
- gene poor
-stains darker with banding techniques
- acetylation = histones loose positive charge -> untangling of heterochromatin -> active chromatin (HATs) (LYSINE TAILS OF HISTONES ACETYLATED MOST FREQUENTLY)
- deacetylation = restoration of positive charge in histones -> condensation -> non-active chromatin (HDACs)
-dark under microscope
-attach to the nuclear envelope
-divided into constitutive and facultative
-envelopes euchromatin
-protects active genes - heterochromatin on periphery, euchromatin inside (protected
EXPLAIN CONSTITUTIVE HETEROCHROMATIN.
-stable
-contains satellite DNA (short repeated tandemly sequences)
-polymorphic in size
-C bands
-located at the centromeres of all chromosomes and on 1, 9, 16, Y chromosomes
-little more spiralized than facultative heterochromatin and therefore replicate later
WHAT DISEASES DOES CONSTITUTIVE HETEROCHROMATIN CAUSE?
IMMUNODEFICIENCY, CENTROMERE INSTABILITY, FACIAL ABNORMALITIES (ICF):
-inherited in an autosomal recessive manner
-mutations in the gene encoding DNA methyltransferase 3b (DNMT3b)
-instability of the constitutive heterochromatin on chromosomes 1q, 16q.
RETT SYNDROME:
-neurodevelopmental condition
-low levels of norepinephrine in the brain
-new mutation within the MECP2 (global transcription repressor) gene which encode methyl cytosine binding protein, located on the X chromosome
EXPLAIN FACULTATIVE HETEROCHROMATIN.
-reversible, can change into euchromatin
-contains all genes which are not transcribed (some genes are structural)
-enriched with LINEs (long interspersed
nucleotide elements that can function as transposons)
-C bands -
DISCUSS X CHROMOSOME INACTIVATION / LYONIZATION
X-inactivation is controlled by the inactivation center on X chromosome
-Xic harbors a gene encoding a long non-coding RNA (lncRNA) called Xist which triggers X-Inactivation
-random process and occurs when an organism is roughly 1000-2000 cells big (therefore during early embryonic development)
-inactivation is random, only if one X chromosome is not abnormal
-structurally abnormal X – inactivation is non-random; late replication (end of S-phase).
In case of balanced reciprocal translocation between chromosome X and autosome, abnormal X is not preferentially inactivated
-not complete, not stable, reversible, - drives the production of mRNA of gene XIST on Xq
HOW DOES HETEROCHROMATIN PLAY A ROLE IN THE EPIGENETIC REGULATION OF GENE EXPRESSION?
-gene expression is influenced by mechanisms which do not interfere with the primary structure of DNA (gene active genes become methylated and inactive, even though their primary structure is unchanged, the cell behavior is different)
-these processes (methylation, acetylation etc) are the reasons why skeletal cells, muscle cells, neurons all have the same DNA but behave differently due to epigenetically altered genes
WHERE ARE THE GENES FOR RIBOSOMAL RNAS LOCATED?
satellite stalks of all acrocentrics
T/F: if a woman has a chromosomal abnormality with additional X chromosomes, all X chromosomes except one are inactive
true
DURING THE S PHASE IS HETEROCHROMATIN OR EUCHROMATIN REPLICATED FIRST?
euchromatin
IS INACTIVE X REACTIVATED BEFORE MEOISIS?
yes
WHAT DOES HISTONE ACETYLATION DO?
removes the positive charge from histones it is present in active chromatin
WHAT MORPHOLOGY DOES THE X CHROMOSOMES HAVE?
submetacentric
WHAT MORPHOLOGY DOES THE Y CHROMOSOME HAVE?
acrocentric
WHAT IS TOPOISOMERASE II?
a part of non histone proteins of chromatin
WHAT ARE RETROTRANSPOSONS?
type of genetic component that copy and paste themselves into different genomic locations via converting RNA back into DNA by reverse transcriptase of an RNA intermediate
WHAT IS THE FIBROUS CORONA ASSOCIATED WITH?
kinetochore (meshwork of fibres in the absent of microtubules
-contains many microtubule interacting proteins (CENP-E, dynein) and checkpoint proteins (Bub1, Mad2)
WHAT DETERMINES THE SEVERITY OF A CANCER?
the amount of hTERT - the catalytic protein domain of telomerase
-hTERT is dependent on the amount of tumor cells only
-if not tumor cells are not dividing = no telomerase activity
WHAT ARE THE NORMAL MALE AND FEMALE KARYOTYPES?
MALE: 46,XY
FEMALE: 46,XX
-G bands
