Bài giảng Lý thuyết (cơ sở) di truyền học nhiễm sắc thể

LY THUYET (CO SO)
DI TRUYEN HOC NHIEM SAC THE
PGS.TS. Dinh Doan

Long


Overview of today’s topics:
1st theme. DNA replication and repair
1.
2.
3.

The end replication problem and telomerase
How cells repair replication mistakes
Other types of DNA repair

2"¢ theme. Meiosis and Genetic Variability
1.

Vocabulary of meiosis

2.

The process of meiosis

3.

Mistake in melosis and sex determination

3'4 theme. Sex linkage and pedigrees

1.
2.
3.

How to know you are dealing with one or two genes
X-linked inheritance
Human pedigrees
—

———

a

J
-

DDL@VNU-SMP


How is new DNA

synthesized?

Matt Meselson & Frank Stahl, 1958
They wanted to distinguish between 3 different models
for how DNA might be copied

each gives rise to a new

PHOT Phước,

EPDM

Hybrid

¿

š

Generation 1

15N

Generation 2

Hybrid

!4N

Hybrid

đại
ý

14N

After
2 generations:
1/2 low-density DNA
1/2 intermediate-density DNA

| he

double-stranded structure

i=

Generation 0

PDX

complementary strand

gives rise to a new

Dispersive
Double-stranded DNA
replicated over short
stretches

ar Pm

The strands separate and

Conservative
Double-stranded DNA

2

Semi-conservative

—

a

After 2 generations:
1/4 high-density DNA
3/4 low-density DNA

After 2 generations:
All intermediate-density DNA

DDL@VNU-SMP


Meselson-Stahl:

Top of centrifuge

tube (lower density)

The results

"S7

~ ˆ

14N _
Hybrid —
15N

Bottom of centrifuge
tube (higher density)

===
—— ^”

——..

wu
—-

__—.._—
*š

After 2 generations:
1/2 low-density DNA

1/2 intermediate-density

DNA

Ị
6

NI

„

2

Generation

After generation

1:

DNA all of intermediate density

After generation 2:

Y2 DNA of low density
Y2 DNA of intermediate density

This result was only consistent with semi-conservative replication,
in which each parental strand is used to synthesize a
complementary strand

II

DDL@VNU-SMP


Let's take a closer look at what happens during DNA synthesis
Deoxynucleoside

triphosphates are

added to a template.

0

DNA polymerase

BẾP

catalyzes this

:

is the enzyme that
reaction.

annealed
primer

(A, G, C, or T)

|

.

Y

o:

:

/ growing end of DNA
= _/template strand

DDL@VNU-SMP


DNA Is always synthesized in the 5’ > 3’ direction

C-

=

a

2
O
Sas
Oo
(
oO
®
ứ)
=
O
>
O

O

How can DNA be
synthesized in both
directions at the
same time?

B.

=.

°
2
z
>

—

©

a
a


The Replication Fork:
DNA synthesis starts at an origin and proceeds in both directions
(a) A chromosome being replicated

(b) Bacterial chromosomes have a single point of origin.

_

§

go

pOPY,

"eg

3

>

Đ

:2 —~ §
Bạn
3

§

“2À

ay

se

\

IVP

Mp

%a

2

dy

> Old DNA

4

—New DNA

\. \ Replication/ /
proceeds in

Origin of

both directions

replication

Replication

(c) Eukaryotic chromosomes have multiple points of origin.

09000%0%016z

oo

i

| L7

0%0%.0%63

oe

`

| 5,

mms...»

....

30%:0%40%63
Old DNA

`New DNA

`

Replication proceeds

directions from

each

fork

—_

=o

—=Ää

B.

ald

Roles

®mŠ...-›}`

KVOVIVO

in both

starting noint

> But what about the other strand?

II

5

DDL@VNU-SMP


Solution:

DNA synthesis is

continuous on one
strand (the leading
strand) but
discontinuous on
the lagging strand.

3 T
5

Nx
/

Newly

iy

Template strands

r

Unwinding—

and replication

synthesized DNA

Lagging strand

Okazaki fragments
53

Discontinuous

SK
5:3

s

On

yes

2

Leading strand

Continuous

DNA synthesis


Bi-directional DNA synthesis starting at an origin of replication
Okazaki
fragments

|
Origin

i

Directionof
fork movement

=

\

`

¬...

"

Direction of
fork movement

—

⁄” Lagging HE
Leading

Laggino PA
_—

Origin

Question:

ae

\

:

Okazak|

fragments

Does point ‘A’ represent a 5’ or 3’ end of DNA?
II

DDL@VNU-SMP


Leading strand synthesis:

Primase synthesizes RNA primer

3

Topoisomerase relieves twisting forces

PY

5’

PUP VIVIVIVIVIVHU

Helicase opens double helix
Single-strand DNA-binding proteins (SSBP) stabilize single strands

1. DNA is opened, unwound, and primed.
Sliding clamp holds DNA polymerase in place
fi ‘j

đd

Si eo

RNA primer

Leading strand

5

,

“a

DNA polymerase Ill works in 5— 3’ direction, synthesizing leading strand

TRE

La vs. Àve.es KAA

Th
a at
l/®›
>/0)9?ao002020
00eh 00

0000%0044d048 5
,P

—|

2. Synthesis of leading strand begins.

DDL@VNU-SMP


Lagging strand synthesis:
5

|
1. Primase synthesizes
RNA primer.

S

RNA |
sư

aNSOULE i

per
<<

44 4

wed

Fay

:

SSBPs

`Primase

™ Helicase

Okazaki fragment

2. DNA polymerase III

works in 5'-> 3’ direction,

3

synthesizing first Okazaki

fragment of lagging stand.

3. Primase and DNA

polymerase lll synthesize

another Okazaki fragment.

5

+

|

=

-

¬

ne

HH

\. _ Sliding

A &Ê.

“Me

Tin? Qn?
Vin 51?
eT a

5

ns

Vin? Unc NeDaee

3

~ Topoisomerase

:

HEL vovovouess

=

~>

clamp

DNA polymerase III

Okazaki fragment

UAL mm
5 ee

mm

Okazaki fragment

,
`

:

(%,
1
1

a

oweib

ii

4

LÝ

T

EN”

1,1% «7

NUR

A

“

a

7

5

` 4/

=>

DDL@VNU-SMP


Lagging strand synthesis (continued):

35 mmmmm
TT

có—
— „ ĐNA polymerasel
a

m_

fe

`

Seah.
ey

pe

8. ae

&” 3:

”

C2

*

«

O1

4. DNA polymerase | removes ribonucleotides of primer, replaces them with
deoxyribonucléotides in 5
3' direction.

3

- DNA ligase

°

|

fi.

So

X

brews

5’

5. DNA ligase closes gap in sugar-phosphate backbone.

DDL@VNU-SMP


The trombone model explains how leading and lagging strand
synthesis can be coordinated
ran
strand
><
DNA polymerase

y-complex
sliding ~_
clamp

gì

OI

sae.
T-protein

$

>

=

1)

7
laqơinn

—.`
To

A
iy

“_

.

NOT

`4
strend

DMÀ.gbbndrä

SSB bound

%

SERN
2
“

Link to web animation of DNA replication:
/>

DNA replication in eukaryotes...same logic,
different protein names
_— RPA

L5

⁄

Lagging strand

Pol a+
Primase

5'

3—

&S

ea

LẠ .&..á

..
LAS
`
_`

8722
3'

MCM2-7

Pal«

Helicase

i

PCNA
Leading strand
http:/www.nature_comí/cr/ioumal/v18/n1/mages/cr20084f2
Ipg


What happens when the replication fork reaches the end of
a linear chromosome?

4

PUP

coer

x

DNA polymerase
Sliding clameÂ

WI WCOWOUCPUCIUO WYN

Sener

%

Lagging strand

SSBPs

a

`

ơ=.`/

Helicase

1. Helicase unwinds end of DNA helix (at end of chromosome).

ADEN

EN

NNN

NN
RNA primer

2. DNA polymerase completes the leading strand. Primase synthesizes RNA primer at end

of lagging strand.

|

3

5

'


What happens when the replication fork reaches the end of
a linear chromosome?

Aly qh lu rh | A

rT Wd

ly

Ply ql”

li if

\

WP DD DD WP WP DP Ts

t Ni f} lad

AN

ohh

bd

\„⁄

z

oe

aT

Last Okazaki fragment

3. DNA polymerase synthesizes the last Okazaki fragment in lagging strand.

WAU

LANL NINN

DP DP DI DIDI DP DIDI DI DIDI DIDI

Tes
Unreplicated end

4. No DNA synthesis occurs after primer is removed (no free 3’ end for DNA polymerase);
chromosome is shortened.


Many cells use telomerase to solve the end replication problem
The ends of chromosomes
(telomeres) have many copies of
a TTAGGG repeated sequence.
»

Telomerase is an enzyme that uses
an RNA template to add nucleotides
to the ends of chromosomes

Telomeres

extended by

telomerase

can

then be replicated

DDL@VNU-SMP

Telomeres and Telomerase: 2009 Nobel Prize
Elizabeth Blackburn, Carol Greider, Jack Szostak

In the late 1980’s, Calvin Harvey showed that telomere
shortening can cause cells to exit the cell cycle and
become senescent (the “telomere clock” hypothesis).
Telomerase is expressed in germline cells but is turned
off in most somatic cells.
Therefore, it is thought that most somatic cells have a
limited number of divisions that they can undergo.
Can aging be reversed by stimulating expression of telomerase in
key stem cell populations?

II

DDL@VNU-SMP


What happens when mistakes are made during replication?
- Based on the physical chemistry of the hydrogen
bond, one would predict an error
rate of 1 in every
`
10,000 -100,000 bases polymerized.

m'ef^®ke
“~~

- The E. coli bacterium has 4,639,000 base pairs.
A 1 in 100,000 error rate would result in about 92 incorrect bases
introduced with every round of replication.
> Unacceptable! Many of these would result in mutations
that have a negative impact on the cell.

- Humans have over 3 billion base pairs in their genomes.
prevents mistakes from being made during replication?

II

What

DDL@VNU-SMP


One source of mistakes: A small percentage of the time
bases will take on a rare tautomeric form
Normal
a

~

bases
amino

Base tautomers
Imino

This tautomer of C

will base pair with A

keto

enol

ee

_ee

This tautomer of G

will base pair with T

™ H-bond donor
™* H-bond acceptor

When this occurs on the incoming dNTP during replication, DNA polymerase
will incorporate an incorrect base at the 3’ end of the growing chain

II

5

DDL@VNU-SMP