BBS1005 Human Genetics, Reproduction …. - 03/06/22
Case 1 - Should we worry?
Gametogenesis, epigenetics, and mutations
DNA
Reviewing DNA (BBS1001)
Structure
A DNA molecule consists of two long polynucleotide chains. Each
of these chains is known as a DNA chain or a DNA strand.
These DNA strands are held together by relatively weak hydrogen
bonds between the base portions of the nucleotides. Because of
these bonds, all bases are on the inside of the double helix - the DNA is polymerized in the 5’-3’ direction since DNA polymerase
three-dimensional structure of DNA - and the sugar-phosphate can synthesize only in this direction. Because of the antiparallel
backbones are on the outside. orientation of the two DNA strands in the DNA double helix, it’s
The two ends of the DNA chain will be easily distinguishable, as harder to replicate the lagging strand than the leading strand.
one has a hole (3’ hydroxyl) and the other a knob (5’ phosphate) ○ Leading strand: DNA polymerase needs a primer, called RNA
at its terminus. primase, at the beginning to hook on to so that it can start
building the new DNA chain by adding the matching
The nucleotides in DNA are composed of the sugar deoxyribose
nucleotides.
attached to a single phosphate group (hence the name
○ Lagging strand: since this strand is 3’-5’, it has to be copied
deoxyribonucleic acid), and the base may be either adenine (A),
in a series of segments. Each segment starts with an RNA
Cytosine (C ), Guanine (G), or Thymine (T). In each case, a
primer. DNA polymerase then works backward along the
two-ring base (purine) is paired with a single-ring base
strand. These segments that are made are called Okazaki
(pyrimidine).
fragments. Another kind of DNA polymerase has to go over
The members of each base pair can fit together within the double
the Okazaki fragments to replace all the RNA primers. After
helix only if the two strands of the helix are antiparallel - that is,
this, the enzyme DNA ligase connects the Okazaki fragments.
only if the polarity of one strand is oriented opposite to that of the
other strand. Proofreading
Exonucleolytic proofreading
DNA replication DNA polymerase also performs the first
All organisms duplicate their DNA with extraordinary accuracy proofreading step just before a new
during the interphase ->before each cell division nucleotide is covalently added to the growing
(mitosis/meiosis). chain. This enzyme corrects a mismatched
nucleotide pair by means of a separate
DNA templating catalytic site. This 3’-5’ proofreading exonuclease clips off any
DNA templating is the mechanism the cell uses to copy the unpaired or mispaired residues at the primer terminus. DNA
nucleotide sequence of one DNA strand into a complementary polymerase can then replace the nucleotide right away, before
DNA sequence. continuing with DNA synthesis.
This process requires the separation of the DNA helix into two Strand-directed mismatch repair
template strands. The enzyme Helicase does this by slicing open Immediately after DNA synthesis, any remaining mispaired bases,
those loose hydrogen bonds between the base pairs. SSB proteins which slipped through the exonucleolytic proofreading, can be
(single-stranded binding proteins) bind to the DNA strands to detected and replaced in a process called mismatch repair.
keep them separated. And Topoisomerase keeps the DNA from
First, a protein complex (MutS) is recognized and binds to the
supercoiling.
mispaired base. A second complex
The point where the splitting starts is known as the replication fork.
(MutL) cuts the DNA near the mismatch,
These two strands are now separated into a leading strand and a
and more enzymes chop out the
lagging strand, which can now be used as templates to create
incorrect nucleotide and a surrounding
two complementary DNA strands.
patch of DNA. DNA polymerase then
replaces the missing section with
correct nucleotides, and DNA ligase
seals the gap.
, Alternative splicing is a process during gene expression that
DNA transcription allows a single gene to code for multiple proteins. In this process,
particular exons of a gene may be included within or excluded
Transcription is the first step in gene
from the final, processed mRNA produced from that gene. This
expression. It involves copying a gene’s
means that exons are joined in different combinations, leading to
DNA sequence to make an RNA molecule.
different alternative mRNA strands.
RNA molecules
There are 3 major differences between DNA and RNA:
DNA translation
○ RNA is a single-stranded molecule - no double helix here. After the DNA sequence of a gene is ‘rewritten’ in the form of RNA
○ The sugar in RNA is ribose, which has one more oxygen atom during transcription, translation takes place. During translation,
than deoxyribose. the mRNA is ‘decoded’ to build a protein that contains a specific
○ RNA does not contain thymine (T). Its fourth nucleotide is the series of amino acids.
base Uracil (U) In an mRNA, the instructions for building a
polypeptide are RNA nucleotides (A,U,C, and G)
Stages of transcription
read in groups of three. These groups of three
1. Initiation are called codons. One codon, AUG, specifies
RNA polymerase uses a single-stranded DNA template to the amino acid methionine and also acts as a
synthesize a complementary strand of RNA in the 5’-3’ start codon to signal the start of protein
direction. It binds to a sequence of DNA called the promoter, construction. The stop codons, UAA, UAG, and
found near the beginning of a gene. The general UGA, tell the cell when a polypeptide is
transcription factors1 (GTFs) also bind to the promotor. complete.
Once bound, RNA polymerase separates the DNA strands,
Transfer RNAs (tRNAs) are molecular ‘bridges’ that connect the
providing the single-stranded template needed for
mRNA codons to the amino acids they encode. One end of each
transcription.
tRNA has a sequence of three nucleotides called an anticodon,
2. Elongation which can bind to specific mRNA codons, since it is the
As the RNA polymerase reads the DNA complementary sequence. The other end of the tRNA carries the
template strand one base at a time, it amino acid specified by the codons.
builds an RNA molecule out of
complementary nucleotides. Ribosomes
The RNA transcript carries the same info as Ribosomes are the structures where
the non-template (coding) strand of DNA, but it contains the base polypeptides (proteins) are built. They
Uracil instead of Thymine.
are made up of protein and RNA. Each
3. Termination ribosome has two subunits, a large one
Sequences called terminators and a small one, which come together
signal that the RNA transcript is around an mRNA.
complete. Once they are The ribosome has three binding sites for tRNA: A, P, and E sites. A
transcribed, they cause the and P sites span both the ribosome subunits, whereas the E site
transcript to be released from the (exit site) resides in the large ribosomal subunit.
RNA polymerase.
Stages of translation
Extra processing 1. Initiation
In bacteria, RNA transcripts can act as messenger RNAs (mRNAs) The ribosome assembles around the mRNA to be read and
right away. In eukaryotes, the transcript of a protein-coding gene the first tRNA (carrying methionine) binds to the specific
is called a pre-mRNA and must go through extra processing mRNA codon (AUG). This setup, called the initiation complex,
before it can direct translation. is needed in order for translation to get started.
Eukaryotic pre-mRNAs must have their ends modified, by addition 2. Elongation
of a 5’ cap (at the beginning) and 3’ poly-A tail (at the end). This is the stage where the amino acid chain gets longer. The
mRNA is read one codon at a time, and the amino acid
Many eukaryotic pre-mRNAs undergo
matching each codon is added to a growing protein chain.
splicing. In this process, parts of the
3. Termination
pre-mRNA (called introns) are
This is the stage in which the finished polypeptide chain is
chopped out, and the remaining
released. It begins when a stop codon enters the ribosome,
pieces (the exons) are stuck back
triggering a series of events that
together.
separate the chain from its tRNA
These end modifications increase the stability of the mRNA, while
and allow it to drift out of the
splicing gives the mRNA its correct sequence.
ribosome.
1
General transcription factors activate transcription of genetic info from DNA to mRNA
, Prometaphase
At the end of the prophase, the membrane around the nucleus in
Gametogenesis
the cell dissolves away, releasing the chromosomes.
The process (cell division/meiosis) & differences between gender The (early) mitotic spindle, consisting of the microtubules and
other proteins, extends across the cell between the centrioles as
Gametogenesis is the formation of male and female gametes, they move to the opposite poles of the cell
which takes place through meiosis. It converts primordial germ
cells to mature male (spermatozoa) and female (oocytes) Metaphase
gametes by spermatogenesis and oogenesis. The chromosomes line up neatly end-to-end along the center
(equator) of the cell.
Gametogenesis consists of four phases: The centrioles are now at opposite poles of the cell with the
1. The migration of germ cells (identical in males and females) mitotic spindle fibers extending from them. The mitotic spindle
2. The increase of primordial germ cells via mitosis fibers attach to each of the chromatids.
3. The reduction of chromosomes number via meiosis
4. The maturation of the eggs and spermatozoa Anaphase
The chromatids are pulled apart by the mitotic spindle. The
mitotic spindle pulls one chromatid to one pole and the other
Migration of germ cells
chromatid to the opposite pole.
Primordial germ cells (PGC) are the precursors of the gametes.
They exit from the yolk sac into the hindgut and migrate to the Telophase
primordia of the gonads. At each pole of the cell, a full set of chromosomes gather together.
A membrane forms around each set of chromosomes to create
two new nuclei.
Mitosis
The single-cell then pinches in the middle to form two separate
When PGC reach the gonads, they start a phase of rapid mitotic daughter cells each containing a full set of chromosomes within a
proliferation. nucleus. This process is known as cytokinesis.
- Oogonia go through a period of intense mitotic activity in
the ovary from the 2nd to the 5th month of pregnancy.
Meiosis
- Spermatogonia mitosis starts early in the embryonic
testes. Male germ cells maintain the ability to divide Meiosis, on the other hand, is used for just one purpose in the
throughout postnatal life. human body: the production of gametes. Its goal is to make
Mitosis is a process where a single cell daughter cells with exactly half as many chromosomes as the
divides into two identical daughter starting cell.
cells (cell division). The major purpose In other words, meiosis in humans is a division process that takes
of mitosis is for growth and to replace us from a diploid cell3 - one with two sets of chromosomes - to
worn-out cells. Mitosis is divided into haploid cells - ones with a single set of chromosomes. When a
six phases: sperm and egg join in fertilization, the two haploid sets of
chromosomes form a complete diploid set: a new genome.
Interphase (90% of cell cycle)
Meiosis is more complex than mitosis. It still needs to separate the
The DNA in the cell is copied in preparation for cell division (DNA
sister chromatids and it must separate homologous
templating), this results in two identical full sets of chromosomes.
chromosomes, the similar but nonidentical chromosome pairs an
Outside of the nucleus are two centrosomes, each containing a organism receives from its two parents.
pair of centrioles, these structures are critical for the process of
These goals are accomplished in meiosis using a two-step
cell division.
division process. Homolog pairs separate during a first round of
During interphase, microtubules extend from these centromeres. cell division, called meiosis I. Sister chromatids separate during a
second round, called meiosis II.
Prophase
Since cell division occurs twice during meiosis, one starting cell
The chromosomes condense into X-shaped
can produce four gametes. In each round of division, cells go
structures that can be easily seen under a
through four stages: prophase, metaphase, anaphase, and
microscope. Each chromosome is composed
telophase.
of two chromatids2, containing identical
genetic info.
The chromosomes pair up so that both
copies of chromosome 1 are together, both
copies of chromosome 2 are together, and so
on.
2
A chromosome can consist of 1 chromatid or 2. So it is possible to have 92 chromatids
3
(46 chromosomes), but also 46 chromatids (46 chromosoms) All body cells are diploid (n=46) except for the gametes, which are haploid (n=23)
, Meiosis I Meiosis II
Before entering meiosis I, a cell must first go through interphase. Cells move from meiosis I to meiosis II without copying their DNA.
As in mitosis, the cell grows during G1 Meiosis II is a shorter and simpler process than meiosis I, and you
phase, copies all of chromosomes can think of it as ‘mitosis for haploid cells’.
during S phase, and prepares for The cells that enter meiosis II are the ones made in meiosis I. They
division during G2 phase. are haploid - have just one chromosome from each homologue
During prophase I, the chromosomes pair - but their chromosomes still consist of two chromatids.
begin to condense (just as in mitosis), In meiosis II, the sister chromatids separate, making haploid cells
but they also pair up. Each chromosome carefully aligns with its with non-duplicated chromosomes.
homologue partner so that the two match up at corresponding
positions along their full length (see image4). During prophase II, chromosomes condense and the nuclear
This process, in which homologous envelope breaks down, if needed. The centrosomes move apart,
chromosomes trade parts, is called the spindle forms between them, and the spindle microtubules
crossing over. It’s helped along by a begin to capture chromosomes.
protein structure called the The two sister chromatids of each chromosome are captured by
synaptonemal complex that holds the homologues microtubules from opposite spindle poles. In metaphase II, the
together. The chromosomes would actually be chromosomes line up individually along the metaphase plate.
positioned one on top of the other throughout
In anaphase II, the sister chromatids separate and are pulled
crossing over (see image on the right).
towards opposite poles of the cell.
Chiasmata are cross-shaped structures where In telophase II, nuclear membranes form around each set of
homologues are linked together. Chismata keep the homologues chromosomes, and the chromosomes decondense. Cytokinesis
connected to each other after the synaptonemal complex breaks splits the chromosome sets into new cells, forming the final
down. products of meiosis: four haploid cells in which each chromosome
has just one chromatid. In humans, the products are sperm or egg
Metaphase I : After crossing over, the spindle begins to capture cells.
chromosomes and move them towards the center of the cell, just
as in mitosis. But during meiosis, each chromosome attaches to
microtubules from just one pole of the spindle, and the two
homologues of a pair bind to microtubules from opposite poles.
So during metaphase I, homologue pairs -not individual
chromosomes - line up at the metaphase plate for separation.
The orientation of each pare is random at the metaphase plate
(e.g. in the diagram below, the pink version of the big chromosome and purple version
of little chromosome happen to be positioned towards the same pole and go into the
same cell. But the orientation could have equally well been flipped, so that both purple
chromosomes went into the cell together).
Numerical abnormalities
Normally, during meiosis, the chromosome from the mother
(orange) will be separated from the chromosome from the father
(blue). After this, the sister chromatids get separated.
However, nondisjunctions can
happen during the 1st or 2nd
meiotic division. When it happens
during the first division, it is called
a monosomy. When it happens
during the second division, it is
In anaphase I, the homologous are pulled apart and move apart
called a trisomy. -> trisomy 21 =
to the opposite ends of the cell. The chromatids of each
down syndrome
chromosome, however, remain attached to one another and don’t
come apart.
Finally, in telophase I, the chromosomes arrive at opposite poles of
Differences between spermatogenesis and oogenesis
the cell and cytokinesis occurs, forming two haploid daughter One marked difference between the human male and female is
cells. that there are many more germline cell divisions in the life history
of a sperm relative to that of an egg. Furthermore, the difference
increases with the age at which the sperm is produced.
4
The capital and lowercase letters are alleles for each gene.
