Chromosomes and Gametes

Question 1

What are the fundamental principles of evolution?

Answer 1

• The ability to reproduce
• Variation

Each new generation in turn reproduces or dies out – selection of the fittest

Question 2

What is a karyotype?

Answer 2

The number and appearance of human chromosomes

Question 3

Describe the structure of a chromosome

Answer 3

Each chromosome has a constriction point called the centrosome

Divides chromosome into 2 sections / “arms” – short (p) and long (q)

Question 4

What is the significance of the centromere location?

Answer 4

Gives a chromosome its characteristic shape

- used to describe location of the gene

Question 5

What are the 3 roles of functional DNA?

Answer 5

• replicate
• separate its 2 copies at mitosis
• maintain itself between generations

Question 6

What are the DNA requirements for sexual reproduction?

Answer 6

Each parent passes on one allele to each offspring

If alleles are heterozygous, trait phenotype either dominant / recessive

Question 7

When do copy number variants occur?

Answer 7

Copy number variants (CNV) occur if more than one, three or more copies of alleles

Question 8

Briefly outline transcription and translation if a gene

Answer 8

1. Promoter + coding seq. transcribed into gene product
2. Introns removed from exon by splicing
3. mRNA exported out of nucleus
4. Translated into proteins in ribosomes by complexes of tRNA & proteins
5. Proteins folded into unique 3D structure - determines function

Question 9

How can a single gene become tissue specific?

Answer 9

Same gene can be tissue specific by having alternative promoters

Question 10

Give an example of a tissue specific gene

Answer 10

CYP19A1 (encodes aromatase)

uses different promoters in breast, ovary and brain

Aromatase converts androgens into oestrogen

Question 11

What is the role of some non-coding regions in genes?

Answer 11

Most genes contain non-coding regions that regulate gene expression

Question 12

How does one gene produce multiple gene products?

Answer 12

Via Alternative splicing of exons

- Products are known as Isoforms

Question 13

How can a protein be modified after production?

Answer 13

Post-translational modification - e.g phosphorylation

Glycosylation

• adding on carbohydrates to protein
• making protein more stable and soluble
• LH/FSH is modified like this

Question 14

How are hormones modified for action?

Answer 14

Often secreted as “prohormones” and are enzymatically processed to form active hormones
eg. prepro GnRH

Question 15

Give an example of alternative splicing producing isoforms

Answer 15

3 alternatively spliced variants of human FSHR found in testicular tissue
– possible association with spermatogenic defects

Question 16

What is the purpose of LH/FSH glycosylation?

Answer 16

Different no. of glycosylated elements added on to gonadotropins to form variations of the hormones.

Different glycosylated elements have different activities and will form different isoforms throughout reproductive life.

Question 17

What is the significance of DNA in reproduction?

Answer 17

DNA needs to be able to be passed through generations

Question 18

How do most cells in the body reproduce?

Answer 18

Most cells and many organisms replicate by doubling DNA and dividing to give 2 identical progeny or clones - asexual reproduction

Question 19

What is the process by which human cells reproduce?

Answer 19

Mitosis:
Fusion of haploid cells (gametes) to create unique progeny.
Uniqueness brought about by crossing over and independent sorting of chromosomes

Question 20

Describe the asexual mitosis of somatic cells

Answer 20

1. Homologous chromosomes
2. DNA replicated
3. Sister chromatids form
4. BIvalent chromosomes line up between 2 mitotic
   spindles
5. Undergo two cell divisions
6. gametes formed

Question 21

What is the product of somatic cell division?

Answer 21

Somatic / diploid cells replicate by simple cell division

• give identical progeny
• limited no. of divisions,
  e. g hepatocytes, pancreas, skin cells

Question 22

What are the advantages of diploidy?

Answer 22

Prevents accumulation of genetic mutations

Increased genetic diversity

Maintenance occurs due to advantage of genetic variability

Variation in off-spring → survival of the fittest? More able to evolve + adapt to changing environment

Question 23

Why is allelic variation not always bad?

Answer 23

Mitosis can cause allelic variation - but just as many good as bad

If 2 good ones occurred then both would proliferate
- out-survive others

Question 24

Where are X and Y chromosomes thought to originate from?

Answer 24

Differentiated from pair of identical chromosomes (autosomes) ~300 million years ago

Ancestral mammals developed variation which made it male - gradually becoming Y and the other the X (sex locus (Y) = male)

Question 25

How many working genes are found on each X and Y chromosome?

Answer 25

X chromosome → 1000 working genes

Y chromosome → 86 working genes

Question 26

What is a gamete?

Answer 26

A haploid cell specialised for sexual fusion (egg + sperm)

Question 27

How many chromosomes in a haploid cell?

Answer 27

23 in total

Question 28

How do gametes reproduce?

Answer 28

Unlike other cells gametes go through cycles of diploidy & haploidy

Undergo cycles of mitosis to increase numbers
Then undergo meiosis
Then combine at fertilisation

Question 29

How are gametes formed?

Answer 29

From germ line cells: primordial germ cells that migrate into gonad and differentiate to either male or female gametes

Question 30

What is the process producing oocytes?

Answer 30

oogenesis (incorporated as part of folliculogenesis)

Question 31

What is the process producing sperm?

Answer 31

spermatogenesis

Question 32

Which phase of the cell cycle do chromsosomes duplicate?

Answer 32

Chromosomes replicate during S-phase of cell cycle

- still attached at centromere

Question 33

What is a chromatid?

Answer 33

Each copy of replicated chromososme

‘sister’ as they’re identical

Question 34

What is interphase?

Answer 34

Period of cell cycle between cell divisions - Not a “resting period”

Question 35

What processes occurs during interphase?

Answer 35

Cell carries out functions and grows

DNA and organelles duplicate

Question 36

Describe the structure of genetic material during interphase

Answer 36

Genetic material in long, thin threads called chromatin.

Twist randomly around one another like tangled strands of yarn.

Question 37

What are the 4 stages of mitosis?

Answer 37

Prophase; Metaphase, Anaphase & Telophase

Question 38

What occurs during prophase of the cell cycle?

Answer 38

nuclear & cytoplasmic changes.

Question 39

Outline the nuclear changes that occur in prophase

Answer 39

Chromatin condenses
Forms chromosomes - DNA wraps around histones

DNA loops + twists to form tightly compacted structure

Condensed DNA can’t replicate - gene activity shut down

Sister chromatids easier to separate without breaking

Nuclear membrane also begins to break down.

Question 40

What are the cytoplasmic changes of prophase?

Answer 40

Mitotic spindle forms - made of microtubules associated with centrioles

Centrioles duplicated during interphase, move away from each other toward opposite poles in prophase