Genome Glossary

Question 1

Genome

Answer 1

The genome of a species is the complete complement of DNA sequence that constitutes the genetic blueprint for all inherited characteristics of that organism. In humans the most recent assembly of the genome is ~3,253,848,404 bases

Question 2

Chromosome

Answer 2

Chromosomes are packages of DNA that enable stable transmission of the genome through cell division. Bacteria have single circular chromosomes. All mammals have multiple linear chromosomes although the number and size of these is variable.
-Each mammalian chromosome has a single linear molecule of DNA that interacts with many multimeric proteins and RNA molecules to produce complex higher order structures that are required for cellular function or replication.

Question 3

Packing ratio

Answer 3

The degree to which DNA is condensed is expressed as its packing ratio (length of native DNA / length after condensation)

Question 4

Nucleosome

Answer 4

The winding of DNA around a protein core to produce a “bead-like” structure
The coiling of beads in a helical structure called the 30 nm fiber that is found in both interphase chromatin and mitotic chromosomes

Question 5

How does the packing ratio change?

Answer 5

• 6= nucleosome
• 40= The coiling of beads in a helical structure called the 30 nm fiber that is found in both interphase chromatin and mitotic chromosomes
• 1000= fiber is then organised into topologically associated domains (loops and scaffolds) in interphase
• 7000-10,000= mitotic phase of cell division, observed by light microscopy

Question 6

Centromere

Answer 6

condensed regions visible on mitotic chromosome that are responsible for the accurate segregation of the replicated chromosome during cell division

Question 7

Kinetochore

Answer 7

Within the centromere region the location of microtubule attachment is called the kinetochore and is composed of both DNA and protein.

Question 8

Telomere

Answer 8

Telomeres provide terminal stability to the chromosome and ensure its survival. The ends of broken chromosomes are sticky, whereas the normal end is not sticky, suggesting the ends of chromosomes have unique features. Telomeres contains tandemly repeated sequences - TTAGGGTTAGGGTTAGGG i.e. (TTAGGG)n. This repeat is added to chromosomes by a complex enzyme called telomerase. The lengths of telomeres appear to be under genetic control and may represent a genetic clock.

Question 9

Ploidy

Answer 9

Ploidy refers to the numbers of copies of the genome are present in each cell. In almost all humans the nucleus of each cell contains 46 chromosomes.

Question 10

Euploidy

Answer 10

the normal balanced state of ploidy in an organism

Question 11

Aneuploidy

Answer 11

Aneuploidy describes the deviation of the euploid state.

result of errors in meiosis resulting in either a numerical or structural chromosomal abnormality

Question 12

M1 non-disjunction

Answer 12

failure of homologous chromosome pair separation

Question 13

M2 non-disjunction

Answer 13

failure of sister chromatid separation

Question 14

Constitutional abnormality

Answer 14

If the abnormality is seen in every cell it is called constitutional

Question 15

Mosaic abnormality

Answer 15

If it is seem in only a proportion of cells it is known as mosaic

Question 16

Aneuploidy by structural chromosome abnormalities

Answer 16

loss or gain of contiguous segments of genomic DNA, called deletions and duplications respectively

Question 17

Non-homologous end joining

Answer 17

The boundaries of NHEJ structural chromosome anomalies are essentially random whereas NAHR targets specific regions of the genome.

Question 18

Haploinsufficiency

Answer 18

the situation where one copy of normally diploid gene is insufficient to allow development to proceed normally or health or homoeostasis to be maintained. It is most commonly used to describe diseases where the phenotype associated with deletion of the entire gene is identical, or at least very similar, to that associated with an intragenic loss-of-function (aka null) mutation affecting one copy of the gene.

Question 19

Balanced structural chromosome anomalies

Answer 19

Structural chromosome anomalies that change the order of sequence in the genome without altering the copy number
almost always the result of NHEJ during meiosis

Question 20

Inversions

Answer 20

If the change in order occurs within a chromosome (balanced structural chromosome anomalies)

Question 21

Balanced reciprocal translocations

Answer 21

If they occur between non homologous chromosomes

Question 22

Robertsonian translocations

Answer 22

arise from NAHR fusing two of the five chromosomes with very similar, highly repetitive sequences on their short arms; robertsonian translocations affect chromosomes 13, 14, 15, 21 or 22.