Topic 13

Question 1

Chromosomes

Answer 1

Structures that package DNA

Question 2

Kinetochore

Answer 2

A structure made of protein that surrounds the centromere

It forms the attachment point for the spindle fibres that are necessary for cell division

Question 3

Telomeres

Answer 3

Chromosome ends that consist of DNA made up of many thousand repeats of short sequences of base pairs. Telomeres prevent chromosomes sticking together and enable compete replication of chromosomes to occur.

Question 4

Chromosomes in Humans

Answer 4

46 chromosomes in every somatic or germ cell
All somatic and germ cell carry the same number of chromosomes and therefore the same genes
23 chromosomes are found in the gametes (egg and sperm)

Question 5

How are human karyotypes used?

Answer 5

Karyotypes are used to assist in the analysis of the chromosomes that are present in cells.

Question 6

Arrangement of chromosomes in a karyotype

Answer 6

The chromosome images are organised in a pattern according to an international convention
The homologous chromosomes are arranged in order from largest to smallest with sex chromosomes on the end.
Such an arrangement enables any abnormalities in either number or structure of the chromosomes to be quickly identified.

Question 7

What is a karyotype?

Answer 7

An organised picture of chromosomes produced by cutting out and organising pictures of stained chromosomes of somatic cells photographed during the metaphase stage of mitosis.

Question 8

Autosomes

Answer 8

The 22 matched pairs of chromosomes present in both males and females
they are matched according to:
- banding pattern
- centromere position
- length
 - presence of satellites

Question 9

Sex chromosomes

Answer 9

One pair which determines the sex of an individual

Question 10

Homologous chromosomes

Answer 10

Two similar chromosomes, one paternal and one maternal, that are matched according to chromosome length, position of centromere, banding pattern and presence of satellites.

Question 11

Non-homologous chromosomes

Answer 11

Non-matching chromosomes

Question 12

Cytogeneticists

Answer 12

Scientists who specialise in the study of human karyotypes

Question 13

During meiosis, mistakes can occur which may cause changes in…

Answer 13

1. The total number of chromosomes

2. Chromosome structure or arrangement

Question 14

Aneuploidy

Answer 14

Chromosome number is more or less than the normal diploi or haploid cell.
Sometimes during meiosis the normal separation of homologous chromosomes does not occur during meiosis 1 or sister chromatids fail to separate during meiosis 2.
Gametes end up with more or less than the normal number of chromosomes.
This type of error is called non-disjunction

Question 15

Non-disjunction of homologous chromosomes or sister chromatids can result in…

Answer 15

• Gametes that lack one member of a homologous pair. If a gamete is involved in fertilisation but is missing an autosome, the zygote is not viable.
• Gametes that contain 2 chromosomes of the homologous pair
• When a gamete with the wrong number of chromosomes is fertilised, the resulting zygote will also have an abnormal amount of chromosomes, which has detrimental effects on the development and results in a syndrome.
• For most chromosomal abnormalities, death occurs and baby is miscarried.

Question 16

Note: Sex chromosomes in non-disjunction

Answer 16

X and Y act like homologous chromosomes during meiosis. They pair but crossing over does not happen between them. Each chromosome then segregates to opposite poles during anaphase 1. Failure to segregate can result in gametes with no sex chromosomes or gametes with 2 sex chromosomes.

Question 17

Disomy

Answer 17

In normal cells, chromosomes appear as homologous pairs

Question 18

Examples: aneuploidy in autosomes

Answer 18

Down syndrome, trisomy in chromosome 21

Edward syndrome , trisomy in chromosome 18

Question 19

Examples: aneuploidy in sex chromosomes

Answer 19

Klinefelter Syndrome, XXY

Turner Syndrome, XO

Question 20

Changes to chromosome structure or arrangement

Answer 20

Deletion
Inversion
Translocation
Duplication

Question 21

Deletion

Answer 21

A break occurs at two points of the chromosome
the middle piece of the chromosome falls out
the two ends rejoin and the chromosome is deficient in some genes

Question 22

Inversion

Answer 22

the middle piece of the chromosome falls out
it rotates 180* and then rejoins
no loss of genetic material
genes are in reverse order for this segment of the chromosome

Question 23

Translocation

Answer 23

involves the movement of a group of genes between different chromosomes
a piece of one chromosome breaks off and joins onto another chromosome
When the genes are passed onto gametes, some will receive extra genes while some will be deficient

Question 24

Duplication

Answer 24

A segment is lost from one chromosome and is added to its homologue
the chromosome with the segment removed is deficient in genes

Question 25

Examples: changes in chromosome structure and arrangement

Answer 25

Cri du chat syndrome- deletion of a small portion of chromosome 5 Cancer- inversion, breaking off, rejoining of segments.

Question 26

Describe the kind of information that karyotyping provides about an individual

Answer 26

Karyotyping is used to assist in the analysis of chromosomes present in cells. Shows all homologous pairs of autosomes and sex chromosomes - tells us the gender and outlines any chromosomal abnormalities.

Question 27

What kind of genetic information is not available from karyotype analysis?

Answer 27

Point mutations, frame shift mutations, nucleotide sequences, types of genes.

Question 28

Outline a technique that may be used in conjunction with karyotyping that allows geneticists to arrange and interpret chromosomes with greater accuracy

Answer 28

DNA sequencing- looking at the sequence of bases encoded in DNA

Question 29

Hybrid

Answer 29

Is an offspring produced as a result of a mating between two different species. The chromosomes are not in homologous pairs but the 2 sets of genes are sufficiently similar that a hybrid organism is produced.

Question 30

What happens during meiosis in hybrids?

Answer 30

Some or all of the chromosomes will not pair up as they are not homologous with another chromosome. The chromosome pairs or single chromosomes will then segregate independently of other chromosomes.

Question 31

Viable gametes in hybrids

Answer 31

Viable gametes contain one complete set of chromosomes. To obtain viable gametes in a geep, all the goat chromosomes must segregate to one pole and all the sheep chromosomes to the other. The gamete produced will contain a complete set of chromosomes- either the goat's or the sheep's, and therefore only contains one set of genes. The chance of this happening in a geep is very small so the likely result will be that the gametes will not contain one complete set of chromosomes. The gametes will be genetically unbalanced and therefore not viable. The organism is said to be infertile.

Question 32

Gene

Answer 32

The unit of heredity, made up of a segment of DNA that codes for the production of a particular protein.

Question 33

Allele

Answer 33

An alternative form of a gene

Question 34

Locus, Loci

Answer 34

The place on a chromosome where a gene is found

Question 35

Linked Genes

Answer 35

Genes that are carried on the same chromosome

Question 36

What factors determine the sex of an offspring in mammals

Answer 36

The sex of a human foetus can be predicted before birth on the basis of the sex chromosomes present- XX=female XY=male

Question 37

What factors determine the sex of an offspring in birds?

Answer 37

The sex of a bird can be predicted before birth on the basis of the sex chromosomes present. Arrangement is different to that in mammals. ZZ=male WZ=female

Question 38

What factors determine the sex of an offspring in reptiles?

Answer 38

In some reptiles, the sex of an offspring depends on the incubation temperature of the eggs. This is called Environmental sex determination. Male- above 31*C, female- below 27*C, 29*C= equal amount of each sex. The WZ/ZZ genetic system of sex determination is also seen in some reptiles (snakes, lizards)