Chromosomes

Question 1

Explain the features of a prokaryotic chromosome.

Answer 1

• No nucleus
• Genetic material in the cytoplasm in a region of bacteria called nucleoid
• Genetic material consists of 1 long circular thread of DNA molecule called genophore
• DNA is naked, no additional packing involved
• Additional circular DNA called plasmids

Question 2

What are plasmids?

Answer 2

• Small circular DNA molecules that contain only a few genes, capable of self-replication
• Plasmids present in prokaryotic cells, NOT present in eukaryotic cells
• They can be easily exchanged between prokaryotic cells via sex pili in a process called bacterial conjugation
• Used for gene manipulation in labs

Question 3

Explain the features of a eukaryotic chromosome.

Answer 3

• Genetic material consists of linear molecules of DNA that are associated with histone proteins
• DNA is wrapped around histones which results in nucleosomes
• Nucleosomes are tightly compacted structures which allow more efficient storage
• Linked nucleosomes form a string that coils to form a solenoid structure and later forms loops which are folded around proteins to form chromatin
• Chromatin will supercoil during cell division to form chromosomes
• Chromosomes only visible during mitosis and meiosis

Question 4

Explain the structure of chromosomes.

Answer 4

• Contain centromere which divides the chromosome into two sections of arms. The spindle fibers during cell division attach to this point.
• Chromosomes differ in size and position of the centromere
• Each chromosome will carry specific genes and at certain position (locus)

Question 5

How can the position of the locus be identified?

Answer 5

• First point of reference: number indicates the chromosome e.g. 7q31 (the 7)
• Second point of reference: letter indicates which arm the locus is positioned e.g. 7q31 (the q)
• Third point of reference: number corresponding to the G band location e.g. 7q31 (the 31) longitudinal position

Question 6

What are homologous chromosomes?

Answer 6

• The offspring inherits two copies of each chromosome from its parents
• These maternal and paternal chromosomes are homologous chromosomes
• They share the same structural features (size, centromere position) and the same sequence of genes at the same loci positions
• The genes may be the same, the alleles may be different

Question 7

Which level of classification during reproduction require diploid nuclei and which require haploid nuclei?

Answer 7

• Sexually reproducing organisms have 2 sets of chromosomes (diploid)
• Their gametes need to have half the number of chromosomes (haploid) for reproduction purposes
• After fertilization, the zygote contains a diploid nucleus

Question 8

What are diploid nuclei?

Answer 8

• Containing pairs of homologous chromosomes (2n)
• Two gene copies (alleles) for each trait
• All body cells in the organism will be diploid
• Diploid cells present in most animals and many plants
• Diploid cells created via mitosis

Question 9

What are haploid nuclei?

Answer 9

• Containing one set of chromosomes (n)
• Single gene copy (allele) for each trait
• All gametes in the organism will be haploid
• Present in bacteria (asexual) and fungi
• Haploid cells derived from diploid cells via meiosis

Question 10

What do sex chromosomes determine?

Answer 10

• Pair of sex chromosomes (heterosomes) determine the sex of the human
• Females XX (homologous), Males XY (non-homologous)

Question 11

How does the Y chromosome distinguish between male and female?

Answer 11

• Y chromosome contains genes for developing male sex characteristics
• In the absence of it, female sex organs will develop

Question 12

Why does the father determine the sex of the offspring?

Answer 12

• Since the male sperm either contains an X or Y chromosome, the growing embryo will develop into a girl or boy respectively
• Female gametes contain XX

Question 13

How are the remaining chromosomes in an organism called?

Answer 13

• Autosomes and determine all other characteristics

Question 14

What is a homologue?

Answer 14

• Each chromosome in a homologous pair
• Tetrad: structure consisting of two homologues

Question 15

What are karotypes?

Answer 15

• A property of a cell, the number and type of chromosomes present in the nucleus
• They are determined by harvesting cells and cell division is chemically induced
• Mitosis is halted when the chromosomes are fully condensed and spread apart
• The chromosomes are stained and photographed to generate a karyogram

Question 16

What do karyograms do?

Answer 16

• Shows the chromosomes of an organism in homologous pairs of decreasing length
• Sex chromosomes are shown last
• A photograph of the chromosomes

Question 17

For what and how are karyograms used?

Answer 17

• To determine the sex of unborn children, using the sex chromosomes
• Testing for chromosomal abnormalities
• Down syndrome can be tested, since the individual has 3 copies of chromosome 21
• When analysing a karyogram, check sex first then the autosomes (21 particular)

Question 18

How is the actual length of DNA molecules measured?

Answer 18

• Using the Cairn’s autoradiography technique
• Autoradiography allows radioactive molecules to take pictures of themselves
• Uses X-ray film to visualize the two-dimensional distribution of the substance
• The resulting image is an autoradiograph
• Chromosomes in uncoiled status, more accurate length measurement

Question 19

How does autoradiography work?

Answer 19

• The solution radioactive thymidine (tritiated thymidine) with H^3 is produced
• Bacteria cells are grown in a medium including the radioactive thymidine
• When the DNA of the bacteria is replicated, they become radioactive
• The cell walls and membranes are ruptured to release the cell contents
• They are placed on a photographic surface
• Surface covered in radioactively-sensitive emulsion containing AgBr
• Radiation from the tritiated thymidine converts the Ag+ of the AgBr into insoluble metal grains
• Excess silver bromide is washed away, silver grains appear as black dots
• Photographic film is developed and chromosomal DNA visualized under microscope

Question 20

Why is the chromosome number a characteristic feature of members of a particular species?

Answer 20

• Organisms with different diploid numbers are unlikely to be able to interbreed (cannot form homologous pairs of zygotes)
• Usually when different species do interbreed, their offspring are infertile
  E.g. horse (64) and donkey (62) produce mule (63)

Question 21

Be able to compare diploid chromosomes numbers between Homo sapiens, Pan troglodytes, Canis familiaris, Oryza sativa, Parascaris equorum.

Answer 21

Homo sapiens (humans): 46
Pan troglodytes (chimpanzee): 48
Canis familiaris (Dog): 78
Oryza sativa (rice): 24
Parascaris equorum (worm): 4

Question 22

Be able to compare the genome size between T4 phage, E. coil, D. melanogaster, Homo sapiens, P. japonica.

Answer 22

T4 phage (virus): 170,000 bp
E. coil (bacteria): 4.6 million bp
D. melanogaster (fruit fly): 130 million bp
Homo sapiens (humans): 3,2 billion bp
P. japonica (flower): 150 million bp

Genome size: bp - base pairs

Question 23

Explain the trend of genome sizes.

Answer 23

• Viruses and bacteria tend to have very small genomes
• Prokaryotes typically have smaller genomes than eukaryotes
• Sizes of plant genomes vary