zygotic gene activation

gene reg sem 2

What did Ethel Harvey’s sea urchin experiments show about early embryo development?

optical centrifuge to separate nuclei from sea urchin embryos

• embryos made only of cytoplasm could still undergo early cleavage divisions

• early stages of dev are not driven by zygotic nuclear DNA but by maternal cytoplasmic factors esp maternal RNAs and proteins

what drives the earliest cleavage divisions after fertilisation

early cleavage is driven mainly by maternal RNAs and proteins alr stored in the egg cytoplasm before fertilisation

Why does the embryo eventually need zygotic gene expression?

Maternal products are limited and become diluted/degraded. Later development requires the embryo to start transcribing its own genes for patterning, cell fate decisions, gastrulation, and multicellularity.

What is zygotic gene activation,

ZGA is the process where control of development switches from maternal gene products to embryo/zygotic gene expression.

What is the maternal-to-zygotic transition?

It is the broader developmental transition where maternal mRNAs/proteins are cleared or reduced, while zygotic transcription increases and the embryo begins controlling its own development.

What types of genes are often activated early in ZGA?

Early ZGA genes often encode transcription factors and microRNAs that help clear maternal mRNAs and prepare the embryo for later development.

What are anamniotes and how do they develop?

Anamniotes include bony fish and amphibians. They usually lay eggs externally in water and often have fast early development.

What are amniotes and how do they develop?

Amniotes include mammals, birds, and reptiles. They lay fertilised eggs on land or retain embryos internally and usually show slower early development.

What are the key features of fast-developing embryos such as frogs and fish?

They have rapid, synchronous cleavage divisions, usually lack G1 and G2 growth phases early on, undergo many quick S/M cycles, and show major ZGA around the mid-blastula transition.

What is the mid-blastula transition, or MBT?

MBT is the stage when rapid cleavage divisions slow down, embryos regain longer cell-cycle phases and checkpoints, and large-scale zygotic transcription begins.

When does MBT occur in fast-developing embryos?

In fish, around the 10th cell cycle; in frogs, around the 12th cell cycle.

What are the key features of slow-developing embryos such as mouse and human?

Their early cleavage divisions take much longer, often 18–36 hours per division. ZGA occurs in waves over several cell cycles rather than as one sudden large burst.

How does ZGA timing differ between fast and slow developers?

In fast developers, major ZGA is linked to the MBT after many rapid divisions.

In mammals, ZGA is phased in gradually over early cleavage stages

What is compaction?

Compaction is when early embryonic cells increase adhesion and pack tightly together, helping establish the first differences between inside and outside cells.

Outer cells

form trophectoderm

inner cells

the inner cell mass.

What does the trophectoderm become?

The trophectoderm contributes to extraembryonic tissues, especially the placenta.

Why are ZGA and cell fate decisions linked?

ZGA activates gene expression programmes that help cells respond to position and signalling cues, allowing the embryo to begin making distinct cell types.

What are the three main models/mechanisms controlling ZGA timing?

• nuclear-to-cytoplasmic ratio,

• maternally deposited transcription factors,

• chromatin dynamics.

What is the nuclear-to-cytoplasmic ratio model?

This model proposes that ZGA begins when the ratio of nuclear/genomic material to cytoplasm reaches a threshold.

How does cleavage increase the nuclear-to-cytoplasmic ratio?

The embryo’s total cytoplasm does not grow much, but the number of nuclei increases with each division. This increases the amount of DNA relative to cytoplasm.

How can the nuclear-to-cytoplasmic ratio trigger ZGA?

As DNA increases relative to cytoplasm, maternal repressors such as histones or other inhibitory factors become diluted/titrated.

Less repression makes chromatin more accessible and allows transcription to begin.

What is the histone titration model?

Maternal histones can bind DNA and repress transcription

As DNA content increases during cleavage, available histones become spread more thinly, allowing genes to become more accessible.

What role do maternally deposited transcription factors play in ZGA?

Maternal transcription factors are stored in the egg and can bind zygotic DNA after fertilisation to activate the first wave of embryonic transcription.

How can transcription factors create waves of ZGA?

A maternal transcription factor activates early zygotic genes, including genes encoding new transcription factors. These then activate later target genes, creating a cascade.

How do maternal RNAs contribute to transcription factor accumulation?

Maternal RNAs can be translated after fertilisation to produce proteins, including transcription factors, that help activate the zygotic genome.

Why must chromatin change before ZGA?

DNA must become accessible to transcription factors and RNA polymerase. If chromatin is too closed, genes cannot be activated.

What happens to DNA methylation after fertilisation?

Paternally derived DNA demethylates rapidly after fertilisation. Maternal DNA also loses methylation, but more slowly.

Why is DNA demethylation important after fertilisation?

It helps reset the genome and makes developmental genes more capable of being activated.

What is H3K4me3 and why is it important?

H3K4me3 is a histone methylation mark often associated with active or poised promoters. In embryos, it appears around gene promoters and is linked to increased transcriptional accessibility.

What happens to H3K4me3 during early embryo development?

Peaks of H3K4me3 become stronger/more defined as development progresses, helping mark promoters for activation.

How are retroviral transposon sequences linked to early gene activation?

Some retroviral/transposon-derived sequences are associated with promoter activity and can contribute regulatory elements that help drive early embryonic transcription.

What are TADs?

TADs are topologically associating domains: regions of DNA that physically interact more frequently with each other than with regions outside the domain.

What does chromatin organisation look like at the 1-cell and 2-cell stages?

It is relatively flat and poorly organised, meaning there are fewer strong long-range chromatin interactions.

What happens to TADs later in development?

: More structured triangular interaction patterns appear, showing active 3D genome organisation and increased DNA region interactions.