Wk4 The Genome And Its Transmission

Question 1

What is the human genome?

Answer 1

the genes & non-coding sequences on one copy of each chromosome)

• Is approximately 3,200 million base pairs long
• Is made up of the DNA sequences on:
• 1 chromosome from each of the 22 autosome pairs
• Both sex (X & Y) chromosomes

Question 2

Nucleus

Answer 2

• 3,200,000,000 DNA bases

- 24 different chromosomes (22 autosomes, X, Y)

Question 3

Mitochondria

Answer 3

• 16,600 DNA bases - 37 genes

Question 4

Human genome summary

Answer 4

• ~23 000 genes encode mRNA to make proteins
• Protein coding genes vary in size and internal organisation
(numbers of exons and introns)
• Genes unevenly distributed between & within chromosomes
• Several mRNAs can be transcribed from some genes
• Genes can make functional non-protein coding RNAs
• Genes can overlap on different strands of DNA molecule; some genes within introns of other genes

Question 5

Is all RNA translated into proteins?

Answer 5

No

22,000 genes encode untranslated RNAs:
rRNA and tRNA involved in translation of mRNA
Some RNA molecules modify other RNA molecules (snRNA; snoRNA)
microRNA (miRNA)
Bind to 3’UTR of mRNA and inhibit translation

Question 6

What is satellite DNA?

Answer 6

Human genome contains repeated sequences

Often occur in blocks of tandem repeats (1-10 nucleotides, or 100s)

Human genome contains repeated sequences
Mini satellite
Highly polymorphic up to 1000 copies in one block Common core sequence: GGGCAGGAXG: Also found at telomeres:

Micro satellite:
Small arrays of simple sequence repeats (e.g. CAG) Usually in intergenic / intronic (non-coding) DNA

Question 7

What is the role of multiply repeated sequences?

Answer 7

Telomeres
Allow replication to tip of chromosome
long tracts of repeats can be unstable = deletions immediately below
telomeres

Centromeres
Essential for segregation during cell division
Proteins involved in cell division bind to centromeric sequences

1000s of GGGCAGGAXG repeats (mini satellites) No known purpose
Can cause mispairing during cell division to give:
large scale duplication/deletion between homologous chromosomes translocation of DNA between non-homologous chromosomes

Question 8

What are chromosomes?

Answer 8

A chromosome is a single molecule of DNA
A specific stretch of DNA where the sequence contains genetic instructions is a gene
Genes are arranged one after the other along a chromosome, with stretches of non-coding DNA between them
Each chromosome has hundreds of genes

Mitotic chromosomes formed from tightly packed chromatin

Question 9

What forms can chromatin exist in?

Answer 9

Euchromatin:
Extended state throughout the nucleus
Genes are expressed depending on cell type / environment
Heterochromatin:
Can exist in active (decondensed) or inactive (condensed)
forms.
e.g. X-chromosome inactivation (“facultative heterochromatin”) Or can always be inactive and condensed
e.g. Centromeres (“constitutive heterochromatin”)

Question 10

What is the purpose of mitosis?

Answer 10

1. Cell division/organismal growth

2. Genetically identical products

Question 11

Process of replication

Answer 11

Interphase - duplicate DNA
Mitosis - divide DNA
Interphase - transmit to daughter cells

Question 12

Full cell cycle

Answer 12

Interphase
- nuclear envelope intact
- no chromosomes visible
Prophase
- chromosomes condense and become visible
- bipolar spindle develops
Prometaphase
- nuclear envelope dissolves
- chromosomes begin to migrate to equatorial plane (metaphase plate) and are seen to contain 2 chromatids
Metaphase
- chromosomes fully condensed and located at metaphase plate
Anaphase
- each centromere splits
- the two chromatids of each chromosome are pulled to opposite poles
Telophase
- chromosomes reach poles and start to decondense
- nuclear membrane reforms
- cytoplasm starts to divide
Cytokinesis
- cytoplasm division completed to give two daughter cells

Question 13

Where is the cell cycle regulated?

Answer 13

G2
Is all DNA replicated?
Is all DNA damage repaired?
Enter mitosis

Mitosis checkpoint
Are all chromosomes properly attached to the mitosis spindle?
Pull duplicated chromosomes apart

G1
Is environment favourable?
Enter S phase

Very highly regulated

Question 14

Why is meiosis necessary?

Answer 14

Function
Reduction division (23 chromosomes per gamete) Re-assortment of genes:
- independent segregation of chromosomes - crossing-over
Mechanism
Each homologue replicates to give two chromatids
Homologues pair
Exchange of material between non-sister chromatids e.g. crossing-over, recombination
Chiasmata (visible cytologically) are the physical manifestations of crossing-over

Question 15

How to generate genetic variation?

Answer 15

Independent assortment of maternal & paternal homologues at meiosis I

Re-assortment of genes by crossing-over

Question 16

What do chiasmata do?

Answer 16

Chiasmata hold the chromosomes together Typical bivalent has at least one chiasma Larger chromosomes show more

Question 17

What is the process of meiosis?

Answer 17

Leptotene
Chromosomes are unpaired fine threads consisting of two tightly bound sister chrimatids

Zygotene
Maternal and paternal homologs pair together to form bivalents

Pachytene
Chromosomes thicken
Crossing over occurs

Diplotene
Homologs separate but are held together by chiasmata
Crossovers can be counted and positions recorded

Diakinesis
Bivalents more contracted

Question 18

Number of cell divisions required to make sperm

Answer 18

Each spermatogonium in the testis at age 15 is the result of 30 previous cell divisions

This spermatogonium maintains the stock of spermatogonia and continues to divide

Number of cell divisions required to produce a human sperm cell is much greater than that required to produce an egg.

Question 19

Number of cell divisions required to make egg

Answer 19

22 cell divisions by 5 months gestation to make stock of 2.6 million oocytes

Each month one is ovulated

Meiosis I completed at ovulation

Meiosis II completed at fertilisation